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Title: The Working of Steel
Annealing, Heat Treating and Hardening of Carbon and Alloy Steel
Author: Fred H. Colvin
A. Juthe
Release Date: January 4, 2007 [EBook #20282]
Language: English
*** START OF THIS PROJECT GUTENBERG EBOOK THE WORKING OF STEEL ***
Produced by Robert J. Hall
THE
WORKING OF STEEL
ANNEALING, HEAT TREATING
AND
HARDENING OF CARBON AND ALLOY STEEL
BY
FRED H. COLVIN
Member American Society of Mechanical Engineers and Franklin Institute;
Editor of the _American Machinist_, Author of "_Machine Shop
Arithmetic_," "_Machine Shop Calculations_," "_American Machinists'
Hand Book_."
AND
K. A. JUTHE, M.E.
Chief Engineer, American Metallurgical Corp. Member American Society
Mechanical Engineers, American Society Testing Materials, Heat
Treatment Association, Etc.
SECOND EDITION
THIRD IMPRESSION
McGRAW-HILL BOOK COMPANY, Inc.
NEW YORK: 370 SEVENTH AVENUE
LONDON: 6 & 8 BOUVERIE ST., E. C. 4
PREFACE TO SECOND EDITION
Advantage has been taken of a reprinting to revise, extensively,
the portions of the book relating to the modern science of
metallography. Considerable of the matter relating to the influence
of chemical composition upon the properties of alloy steels has
been rewritten. Furthermore, opportunity has been taken to include
some brief notes on methods of physical testing--whereby the
metallurgist judges of the excellence of his metal in advance of
its actual performance in service.
NEW YORK, N. Y.,
_August, 1922._
PREFACE TO FIRST EDITION
The ever increasing uses of steel in all industries and the necessity
of securing the best results with the material used, make a knowledge
of the proper working of steel more important than ever before.
For it is not alone the quality of the steel itself or the alloys
used in its composition, but the proper working or treatment of
the steel which determines whether or not the best possible use
has been made of it.
With this in mind, the authors have drawn, not only from their
own experience but from the best sources available, information
as to the most approved methods of working the various kinds of
steel now in commercial use. These include low carbon, high carbon
and alloy steels of various kinds, and from a variety of industries.
The automotive field has done much to develop not only new alloys but
efficient methods of working them and has been drawn on liberally
so as to show the best practice. The practice in government arsenals
on steels used in fire arms is also given.
While not intended as a treatise on steel making or metallurgy in
any sense, it has seemed best to include a little information as
to the making of different steels and to give considerable general
information which it is believed will be helpful to those who desire
to become familiar with the most modern methods of working steel.
It is with the hope that this volume, which has endeavored to give
due credit to all sources of information, may prove of value to
its readers and through them to the industry at large.
_July_, 1921.
THE AUTHORS.
CONTENTS
PREFACE
INTRODUCTION
CHAPTER
I. STEEL MAKING
II. COMPOSITION AND PROPERTIES OF STEELS
III. ALLOYS AND THEIR EFFECT UPON STEEL
IV. APPLICATION OF LIBERTY ENGINE MATERIALS TO THE AUTOMOTIVE INDUSTRY
V. THE FORGING OF STEEL
VI. ANNEALING
VII. CASE-HARDENING OR SURFACE-CARBURIZING
VIII. HEAT TREATMENT OF STEEL
IX. HARDENING CARBON STEEL FOR TOOLS
X. HIGH SPEED STEEL
XI. FURNACES
XII. PYROMETRY AND PYROMETERS
APPENDIX
INDEX
INTRODUCTION
THE ABC OF IRON AND STEEL
In spite of all that has been written about iron and steel there
are many hazy notions in the minds of many mechanics regarding
them. It is not always clear as to just what makes the difference
between iron and steel. We know that high-carbon steel makes a
better cutting tool than low-carbon steel. And yet carbon alone
does not make all the difference because we know that cast iron
has more carbon than tool steel and yet it does not make a good
cutting tool.
Pig iron or cast iron has from 3 to 5 per cent carbon, while good
tool steel rarely has more than 1-1/4 per cent of carbon, yet one
is soft and has a coarse grain, while the other has a fine grain
and can be hardened by heating and dipping in water. Most of the
carbon in cast iron is in a form like graphite, which is almost pure
carbon, and is therefore called graphitic carbon. The resemblance
can be seen by noting how cast-iron borings blacken the hands just
as does graphite, while steel turnings do not have the same effect.
The difference is due to the fact that the carbon in steel is not
in a graphitic form as well as because it is present in smaller
quantities.
In making steel in the old way the cast iron was melted and the
carbon and other impurities burned out of it, the melted iron being
stirred or "puddled," meanwhile. The resulting puddled iron, also
known as wrought iron, is very low in carbon; it is tough, and on
being broken appears to be made up of a bundle of long fibers.
Then the iron was heated to redness for several days in material
containing carbon (charcoal) until it absorbed the desired amount,
which made it steel, just as case-hardening iron or steel adds
carbon to the outer surface of the metal. The carbon absorbed by
the iron does not take on a graphitic form, however, as in the
case of cast iron, but enters into a chemical compound with the
iron, a hard brittle substance called "cementite" by metallurgists.
In fact, the difference between the hard, brittle cementite and
the soft, greasy graphite, accounts for many of the differences
between steel and gray cast iron. Wrought iron, which has very
little carbon of any sort in it, is fairly soft and tough. The
properties of wrought iron are the properties of pure iron. As
more and more carbon is introduced into the iron, it combines with
the iron and distributes itself throughout the metal in extremely
small crystals of cementite, and this brittle, hard substance lends
more and more hardness and strength to the steel, at the expense
of the original toughness of the iron. As more and more carbon is
contained in the alloy--for steel is a true alloy--it begins to
appear as graphite, and its properties counteract the remaining
brittle cementite. Eventually, in gray cast iron, we have properties
which would be expected of wrought iron, whose tough metallic texture
was shot through with flakes of slippery, weak graphite.
But to return to the methods of making steel tools in use 100 years
ago.
The iron bars, after heating in charcoal, were broken and the carbon
content judged by the fracture. Those which had been in the hottest
part of the furnace would have the deepest "case" and highest carbon.
So when the steel was graded, and separated into different piles,
a few bars of like kind were broken into short lengths, melted
in fire-clay crucibles at an intense white heat, cast carefully
into iron molds, and the resulting ingot forged into bars under
a crude trip hammer. This melting practice is still in use for
crucible steel, and will be described further on page 4.
THE WORKING OF STEEL
ANNEALING, HEAT TREATING AND HARDENING
OF
CARBON AND ALLOY STEEL
CHAPTER I
STEEL MAKING
There are four processes now used for the manufacture of steel.
These are: The Bessemer, Open Hearth, Crucible and Electric Furnace
Methods.
BESSEMER PROCESS
The bessemer process consists of charging molten pig iron into
a huge, brick-lined pot called the bessemer converter, and then
in blowing a current of air through holes in the bottom of the
vessel into the liquid metal.
The air blast burns the white hot metal, and the temperature increases.
The action is exactly similar to what happens in a fire box under
forced draft. And in both cases some parts of the material burn
easier and more quickly than others. Thus it is that some of the
impurities in the pig iron--including the carbon--burn first, and
if the blast is shut off when they are gone but little of the iron
is destroyed. Unfortunately sulphur, one of the most dangerous
impurities, is not expelled in the process.
A bessemer converter is shown in Fig. 1, while Fig. 2 shows the
details of its construction. This shows how the air blast is forced
in from one side, through the trunnion, and up through the metal.
Where the steel is finished the converter is tilted, or swung on
its trunnions, the blast turned off, and the steel poured out of
the top.
OPEN HEARTH PROCESS
The open hearth furnace consists of a big brick room with a low
arched roof. It is charged with pig iron and scrap through doors
in the side walls.
[Illustration: FIG. 1.--A typical Bessemer converter.]
Through openings at one end of the furnace come hot air and gas,
which burn in the furnace, producing sufficient heat to melt the
charge and refine it of its impurities. Lime and other nonmetallic
substances are put in the furnace. These melt, forming a "slag"
which floats on the metal and aids materially in the refining
operations.
In the bessemer process air is forced _through_ the metal. In the
open-hearth furnace the metal is protected from the flaming gases
by a slag covering. Therefore it is reasonable to suppose that
the final product will not contain so much gas.
[Illustration: FIG. 2.--Action of Bessemer converter.]
[Illustration: FIG. 3.--Regenerative open hearth furnace.]
A diagram of a modern regenerative furnace is shown in Fig. 3.
Air and gas enter the hearth through chambers loosely packed with
hot fire brick, burn, and exit to the chimney through another pair
of chambers, giving to them some of the heat which would otherwise
waste. The direction is reversed about every twenty minutes by
changing the position of the dampers.
CRUCIBLE STEEL
Crucible steel is still made by melting material in a clay or graphite
crucible. Each crucible contains about 40 lb. of best puddled iron,
40 lb. of clean "mill scrap"--ends trimmed from tool steel bars--and
sufficient rich alloys and charcoal to make the mixture conform to
the desired chemical analysis. The crucible is covered, lowered
into a melting hole (Fig. 4) and entirely surrounded by burning
coke. In about four hours the metal is converted into a quiet white
hot liquid. Several crucibles are then pulled out of the hole, and
their contents carefully poured into a metal mold, forming an ingot.
[Illustration: FIG. 4.--Typical crucible furnace.]
If modern high-speed steel is being made, the ingots are taken
out of the molds while still red hot and placed in a furnace which
keeps them at this temperature for some hours, an operation known
as annealing. After slow cooling any surface defects are ground
out. Ingots are then reheated to forging temperature, hammered
down into "billets" of about one-quarter size, and 10 to 20 per
cent of the length cut from the top. After reheating the billets
are hammered or rolled into bars of desired size. Finished bars are
packed with a little charcoal into large pipes, the ends sealed,
and annealed for two or three days. After careful inspection and
testing the steel is ready for market.
THE ELECTRIC PROCESS
The fourth method of manufacturing steel is by the electric furnace.
These furnaces are of various sizes and designs; their size may be
sufficient for only 100 lb. of metal--on the other hand electric
furnaces for making armor-plate steel will hold 40 tons of steel.
Designs vary widely according to the electrical principles used.
A popular furnace is the 6-ton Heroult furnace illustrated in Fig. 5.
It is seen to be a squat kettle, made of heavy sheet steel, with
a dished bottom and mounted so it can be tilted forward slightly
and completely drained. This kettle is lined with special fire
brick which will withstand most intense heat and resist the cutting
action of hot metal and slag. For a roof, a low dome of fire brick
is provided. The shell and lining is pierced in front for a pouring
spout, and on either side by doors, through which the raw material
is charged.
Two or three carbon "electrodes"--18-in. cylinders of specially
prepared coke or graphite--extend through holes in the roof. Electrical
connections are made to the upper ends, and a very high current
sent through them. This causes tremendous arcs to form between
the lower ends of the electrodes and the metal below, and these
electric arcs are the only source of heat in this style of furnace.
Electric furnaces can be used to do the same work as is done in
crucible furnaces--that is to say, merely melt a charge of carefully
selected pure raw materials. On the other hand it can be used to
produce very high-grade steel from cheap and impure metal, when
it acts more like an open-hearth furnace. It can push the refining
even further than the latter furnace does, for two reasons: first
the bath is not swept continuously by a flaming mass of gases;
second, the temperature can be run up higher, enabling the operator
to make up slags which are difficult to melt but very useful to
remove small traces of impurities from the metal.
Electric furnaces are widely used, not only in the iron industry,
but in brass, copper and aluminum works. It is a useful melter of
cold metal for making castings. It can be used to convert iron
into steel or vice versa. Its most useful sphere, however, is as a
refiner of metal, wherein it takes either cold steel or molten steel
from open hearth or bessemer furnaces, and gives it the finishing
touches.
[Illustration: FIG. 5.--"Slagging off" an electric furnace.]
[Illustration: FIG. 6.--Pouring the ingots.]
As an illustration of the furnace reactions that take place the
following schedule is given, showing the various stages in the
making of a heat of electric steel. The steel to be made was a
high-carbon chrome steel used for balls for ball bearings:
6-TON HEROULT FURNACE
11:50 A.M.--Material charged:
Boiler plate 5,980 lb.
Stampings 5,991 lb.
-----------
11,971 lb.
Limestone 700 lb.
12:29 P.M.--Completed charging (current switched on).
3:20 P.M.--Charge melted down.
Preliminary analysis under black slag.
Analysis:
Carbon Silicon Sulphur Phosphorus Manganese
0.06 0.014 0.032 0.009 0.08
Note the practical elimination of phosphorus.
3:40 P.M.--The oxidizing (black) slag is now poured and skimmed off as
clean as possible to prevent rephosphorizing and to permit of adding
carburizing materials. For this purpose carbon is added in the form
of powdered coke, ground electrodes or other forms of pure carbon.
The deoxidizing slag is now formed by additions of lime, coke and
fluorspar (and for some analyses ferrosilicon). The slag changes
from black to white as the metallic oxides are reduced by these
deoxidizing additions and the reduced metals return to the bath.
A good finishing slag is creamy white, porous and viscous. After
the slag becomes white, some time is necessary for the absorption
of the sulphur in the bath by the slag.
The white slag disintegrates to a powder when exposed to the atmosphere
and has a pronounced odor of acetylene when wet.
Further additions of recarburizing material are added as needed to
meet the analysis. The further reactions are shown by the following:
3:40 P.M.--Recarburizing material added:
130 lb. ground electrodes.
25 lb. ferromanganese.
Analysis:
Carbon Silicon Sulphur Phosphorus Manganese
0.76 0.011 0.030 0.008 0.26
To form white slag there was added:
225 lb. lime.
75 lb. powdered coke.
55 lb. fluorspar.
4:50 P.M.--
Analysis:
Carbon Silicon Sulphur Phosphorus Manganese
0.75 0.014 0.012 0.008 0.28
Note reduction of the sulphur content.
During the white-slag period the following alloying additions were
made:
500 lb. pig iron.
80 lb. ferrosilicon.
9 lb. ferromanganese.
146 lb. 6 per cent carbon ferrochrome.
The furnace was rotated forward to an inclined position and the
charge poured into the ladle, from which in turn it was poured
into molds.
5:40 P.M.--Heat poured.
Analysis:
Carbon Silicon Sulphur Phosphorus Manganese Chromium
0.97 0.25 0.014 0.013 0.33 0.70
Ingot weight poured 94.0 per cent
Scull 2.7 per cent
Loss 3.3 per cent
Total current consumption for the heat, 4,700 kW.-hr. or 710 kw.-hr.
per ton.
Electric steel, in fact, all fine steel, should be cast in big-end-up
molds with refractory hot tops to prevent any possibility of pipage
in the body of the ingot. In the further processing of the ingot,
whether in the rolling mill or forge, special precautions should
be taken in the heating, in the reduction of the metal and in the
cooling.
No attempt is made to compare the relative merits of open hearth
and electric steel; results in service, day in and day out, have,
however, thoroughly established the desirability of electric steel.
Ten years of experience indicate that electric steel is equal to
crucible steel and superior to open hearth.
The rare purity of the heat derived from the electric are, combined
with definite control of the slag in a neutral atmosphere, explains
in part the superiority of electric steel. Commenting on this recently
Dr. H. M. Howe stated that "in the open hearth process you have such
atmosphere and slag conditions as you can get, and in the electric
you have such atmosphere and slag conditions as you desire."
Another type of electric furnace is shown in Figs. 7 and 8. This
is the Ludlum furnace, the illustrations showing a 10-ton size.
Figure 7 shows it in normal, or melting position, while in Fig.
8 it is tilted for pouring. In melting, the electrodes first rest
on the charge of material in the furnace. After the current is
turned on they eat their way through, nearly to the bottom. By
this time there is a pool of molten metal beneath the electrode
and the charge is melted from the bottom up so that the roof is
not exposed to the high temperature radiating from the open arc.
The electrodes in this furnace are of graphite, 9 in. in diameter
and the current consumed is about 500 kw.-hr. per ton.
[Illustration: FIG. 7.--Ludlum electric furnace.]
[Illustration: FIG. S.--The furnace tilted for pouring.]
One of the things which sometimes confuse regarding the contents
of steel is the fact that the percentage of carbon and the other
alloys are usually designated in different ways. Carbon is usually
designated by "points" and the other alloys by percentages. The
point is one ten-thousandth while 1 per cent is one one-hundredth
of the whole. In other words, "one hundred point carbon" is steel
containing 1 per cent carbon. Twenty point carbon, such as is used
for carbonizing purposes is 0.20 per cent. Tool steel varies from
one hundred to one hundred and fifty points carbon, or from 1.00
to 1.50 per cent.
Nickel, chromium, etc., are always given in per cent, as a 3.5
per cent nickel, which means exactly what it says--3-1/2 parts in
100. Bearing this difference in mind all confusion will be avoided.
CLASSIFICATIONS OF STEEL
Among makers and sellers, carbon tool-steels are classed by "grade"
and "temper." The word grade is qualified by many adjectives of
more or less cryptic meaning, but in general they aim to denote
the process and care with which the steel is made.
_Temper_ of a steel refers to the carbon content. This should preferably
be noted by "points," as just explained; but unfortunately, a 53-point
steel (containing 0.53 per cent carbon) may locally be called something
like "No. 3 temper."
A widely used method of classifying steels was originated by the
Society of Automotive Engineers. Each specification is represented
by a number of 4 digits, the first figure indicating the class, the
second figure the approximate percentage of predominant alloying
element, and the last two the average carbon content in points.
Plain carbon steels are class 1, nickel steels are class 2,
nickel-chromium steels are class 3, chromium steels are class 5,
chromium-vanadium steels are class 6, and silico-manganese steels
are class 9. Thus by this system, steel 2340 would be a 3 per cent
nickel steel with 0.40 per cent carbon; or steel 1025 would be a
0.25 plain carbon steel.
Steel makers have no uniform classification for the various kinds
of steel or steels used for different purposes. The following list
shows the names used by some of the well-known makers:
Air-hardening steel Chrome-vanadium steel
Alloy steel Circular saw plates
Automobile steel Coal auger steel
Awl steel Coal mining pick or cutter steel
Axe and hatchet steel Coal wedge steel
Band knife steel Cone steel
Band saw steel Crucible cast steel
Butcher saw steel Crucible machinery steel
Chisel steel Cutlery steel
Chrome-nickel steel Drawing die steel (Wortle)
Drill rod steel Patent, bush or hammer steel
Facing and welding steel Pick steel
Fork steel Pivot steel
Gin saw steel Plane bit steel
Granite wedge steel Quarry steel
Gun barrel steel Razor steel
Hack saw steel Roll turning steel
High-speed tool steel Saw steel
Hot-rolled sheet steel Scythe steel
Lathe spindle steel Shear knife steel
Lawn mower knife steel Silico-manganese steel
Machine knife steel Spindle steel
Magnet steel Spring steel
Mining drill steel Tool holder steel
Nail die shapes Vanadium tool steel
Nickel-chrome steel Vanadium-chrome steel
Paper knife steel Wortle steel
Passing to the tonnage specifications, the following table from
Tiemann's excellent pocket book on "Iron and Steel," will give
an approximate idea of the ordinary designations now in use:
Approximate
Grades carbon range Common uses
Extra soft 0.08-0.18 Pipe, chain and other welding purposes;
(dead soft) case-hardening purposes; rivets; pressing
and stamping purposes.
Structural (soft) 0.15-0.25 Structural plates, shapes and bars for
(medium) bridges, buildings, cars, locomotives;
boiler (flange) steel; drop forgings; bolts.
Medium 0.20-0.35 Structural purposes (ships); shafting;
automobile parts; drop forgings.
Medium hard 0.35-0.60 Locomotive and similar large forgings; car
axles; rails.
Hard 0.60-0.85 Wrought steel wheels for steam and electric
railway service; locomotive tires; rails;
tools, such as sledges, hammers, pick points,
crowbars, etc.
Spring 0.85-1.05 Automobile and other vehicle springs; tools,
such as hot and cold chisels, rock drills
and shear blades.
Spring 0.90-1.15 Railway springs; general machine shop tools.
CHAPTER II
COMPOSITION AND PROPERTIES OF STEEL
It is a remarkable fact that one can look through a dozen text
books on metallurgy and not find a definition of the word "steel."
Some of them describe the properties of many other irons and then
allow you to guess that everything else is steel. If it was difficult
a hundred years ago to give a good definition of the term when the
metal was made by only one or two processes, it is doubly difficult
now, since the introduction of so many new operations and furnaces.
We are in better shape to know what steel is than our forefathers.
They went through certain operations and they got a soft malleable,
weldable metal which would not harden; this they called iron. Certain
other operations gave them something which looked very much like
iron, but which would harden after quenching from a red heat. This
was steel. Not knowing the essential difference between the two,
they must distinguish by the process of manufacture. To-day we
can make either variety by several methods, and can convert either
into the other at will, back and forth as often as we wish; so
we are able to distinguish between the two more logically.
We know that iron is a chemical element--the chemists write it
Fe for short, after the Latin word "ferrum," meaning iron--it is
one of those substances which cannot be separated into anything
else but itself. It can be made to join with other elements; for
instance, it joins with the oxygen in the air and forms scale or
rust, substances known to the chemist as iron oxide. But the same
metal iron can be recovered from that rust by abstracting the oxygen;
having recovered the iron nothing else can be extracted but iron;
_iron is elemental_.
We can get relatively pure iron from various minerals and artificial
substances, and when we get it we always have a magnetic metal,
almost infusible, ductile, fairly strong, tough, something which
can be hardened slightly by hammering but which cannot be hardened
by quenching. It has certain chemical properties, which need not be
described, which allow a skilled chemist to distinguish it without
difficulty and unerringly from the other known elements--nearly
100 of them.
Carbon is another chemical element, written C for short, which is
widely distributed through nature. Carbon also readily combines
with oxygen and other chemical elements, so that it is rarely found
pure; its most familiar form is soot, although the rarer graphite and
most rare diamond are also forms of quite pure carbon. It can also
be readily separated from its multitude of compounds (vegetation,
coal, limestone, petroleum) by the chemist.
With the rise of knowledge of scientific chemistry, it was quickly
found that the essential difference between iron and steel was that
the latter was _iron plus carbon_. Consequently it is an alloy,
and the definition which modern metallurgists accept is this:
"Steel is an iron-carbon alloy containing less than about 2 per
cent carbon."
Of course there are other elements contained in commercial steel,
and these elements are especially important in modern "alloy steels,"
but carbon is the element which changes a soft metal into one which
may be hardened, and strengthened by quenching. In fact, carbon,
of itself, without heat treatment, strengthens iron at the expense
of ductility (as noted by the percentage elongation an 8-in. bar
will stretch before breaking). This is shown by the following table:
--------------------------------------------------------------------------
| | |Elastic |Ultimate|Percentage.
Class by use. | Class by | Per cent | limit |strength|elongation
| hardness. | carbon. |lb. per |lb. per |in 8 inches.
| | |sq. in. |sq. in. |
------------------|-----------|------------|--------|--------|------------
Boiler rivet steel|Dead soft |0.08 to 0.15| 25,000 | 50,000 | 30
Struc. rivet steel|Soft |0.15 to 0.22| 30,000 | 55,000 | 30
Boiler plate steel|Soft |0.08 to 0.10| 30,000 | 60,000 | 25
Structural steel |Medium |0.18 to 0.30| 35,000 | 65,000 | 25
Machinery steel |Hard |0.35 to 0.60| 40,000 | 75,000 | 20
Rail steel |Hard |0.35 to 0.55| 40,000 | 75,000 | 15
Spring steel |High carbon|1.00 to 1.50| 60,000 |125,000 | 10
Tool steel |High carbon|0.90 to 1.50| 80,000 |150,000 | 5
--------------------------------------------------------------------------
Just why a soft material like carbon (graphite), when added to
another soft material like iron, should make the iron harder, has
been quite a mystery, and one which has caused a tremendous amount
of study. The mutual interactions of these two elements in various
proportions and at various temperatures will be discussed at greater
length later, especially in Chap. VIII, p. 105. But we may anticipate
by saying that some of the iron unites with all the carbon to form a
new substance, very hard, a carbide which has been called "cementite."
The compound always contains iron and carbon in the proportions
of three atoms of iron to one atom of carbon; chemists note this
fact in shorthand by the symbol Fe3C (a definite chemical compound
of three atoms of iron to one of carbon). Many of the properties
of steel, as they vary with carbon content, can be linked up with
the increasing amount of this hard carbide cementite, distributed
in very fine particles through the softer iron.
SULPHUR is another element (symbol S) which is always found in
steel in small quantities. Some sulphur is contained in the ore
from which the iron is smelted; more sulphur is introduced by the
coke and fuel used. Sulphur is very difficult to get rid of in
steel making; in fact the resulting metal usually contains a little
more than the raw materials used. Only the electric furnace is
able to produce the necessary heat and slags required to eliminate
sulphur, and as a matter of fact the sulphur does not go until
several other impurities have been eliminated. Consequently, an
electric steel with extremely low sulphur (0.02 per cent) is by
that same token a well-made metal.
Sulphur is of most trouble to rolling and forging operations when
conducted at a red heat. It makes steel tender and brittle at that
temperature--a condition known to the workmen as "red-short." It
seems to have little or no effect upon the physical properties
of cold steel--at least as revealed by the ordinary testing
machines--consequently many specifications do not set any limit
on sulphur, resting on the idea that if sulphur is low enough not
to cause trouble to the manufacturer during rolling, it will not
cause the user any trouble.
Tool steel and other fine steels should be very low in sulphur,
preferably not higher than 0.03 per cent. Higher sulphur steels
(0.06 per cent, and even up to 0.10 per cent) have given very good
service for machine parts, but in general a high sulphur steel
is a suspicious steel. Screw stock is purposely made with up to
0.12 per cent sulphur and a like amount of phosphorus so it will
cut freely.
Manganese counteracts the detrimental effect of sulphur when present
in the steel to an amount at least five times the sulphur content.
PHOSPHORUS is an element (symbol P) which enters the metal from
the ore. It remains in the steel when made by the so-called acid
process, but it can be easily eliminated down to 0.06 per cent
in the basic process. In fact the discovery of the basic process
was necessary before the huge iron deposits of Belgium and the
Franco-German border could be used. These ores contain several
per cent phosphorus, and made a very brittle steel ("cold short")
until basic furnaces were used. Basic furnaces allow the formation
of a slag high in lime, which takes practically all the phosphorus
out of the metal. Not only is the resulting metal usable, but the
slag makes a very excellent fertilizer, and is in good demand.
SILICON is a very widespread element (symbol Si), being an essential
constituent of nearly all the rocks of the earth. It is similar to
carbon in many of its chemical properties; for instance it burns
very readily in oxygen, and consequently native silicon is unknown--it
is always found in combination with one or more other elements.
When it bums, each atom of silicon unites with two atoms of oxygen
to form a compound known to chemists as silica (SiO2), and to the
small boy as "sand" and "agate."
Iron ore (an oxide of iron) contains more or less sand and dirt
mixed in it when it is mined, and not only the iron oxide but also
some of the silicon oxide is robbed of its oxygen by the smelting
process. Pig iron--the product of the blast furnace--therefore
contains from 1 to 3 per cent of silicon, and some silicon remains
in the metal after it has been purified and converted into steel.
However, silicon, as noted above, burns very readily in oxygen,
and this property is of good use in steel making. At the end of
the steel-making process the metal contains more or less oxygen,
which must be removed. This is sometimes done (especially in the
so-called acid process) by adding a small amount of silicon to
the hot metal just before it leaves the furnace, and stirring it
in. It thereupon abstracts oxygen from the metal wherever it finds
it, changing to silica (SiO2) which rises and floats on the surface
of the cleaned metal. Most of the silicon remaining in the metal
is an excess over that which is required to remove the dangerous
oxygen, and the final analysis of many steels show enough silicon
(from 0.20 to 0.40) to make sure that this step in the manufacture
has been properly done.
MANGANESE is a metal much like iron. Its chemical symbol is Mn. It
is somewhat more active than iron in many chemical changes--notably
it has what is apparently a stronger attraction for oxygen and
sulphur than has iron. Therefore the metal is used (especially in
the so-called basic process) to free the molten steel of oxygen,
acting in a manner similar to silicon, as explained above. The
compound of manganese and oxygen is readily eliminated from the
metal. Sufficient excess of elemental manganese should remain so
that the purchaser may be sure that the iron has been properly
"deoxidized," and to render harmless the traces of sulphur present.
No damage is done by the presence of a little manganese in steel,
quite the reverse. Consequently it is common to find steels containing
from 0.3 to 1.5 per cent.
ALLOYING ELEMENTS.--Commercial steels of even the simplest types
are therefore primarily alloys of iron and carbon. Impurities and
their "remedies" are always present: sulphur, phosphorus, silicon
and manganese--to say nothing of oxygen, nitrogen and carbon oxide
gases, about which we know very little. It has been found that other
metals, if added to well-made steel, produce definite improvements
in certain directions, and these "alloy steels" have found much
use in the last ten years. Alloy steels, in addition to the
above-mentioned elements, may commonly contain one or more of the
following, in varying amounts: Nickel (Ni), Chromium (Cr), Vanadium
(Va), Tungsten (W), Molybdenum (Mo). These steels will be discussed
at more length in Chapters III and IV.
PROPERTIES OF STEEL
Steels are known by certain tests. Early tests were more or less
crude, and depended upon the ability of the workman to judge the
"grain" exhibited by a freshly broken piece of steel. The cold-bend
test was also very useful--a small bar was bent flat upon itself,
and the stretched fibers examined for any sign of break. Harder
stiff steels were supported at the ends and the amount of central
load they would support before fracture, or the amount of permanent
set they would acquire at a given load noted. Files were also used
to test the hardness of very hard steel.
These tests are still used to a considerable extent, especially in
works where the progress of an operation can be kept under close
watch in this way, the product being periodically examined by more
precise methods. The chief furnace-man, or "melter," in a steel
plant, judges the course of the refining process by casting small
test ingots from time to time, breaking them and examining the
fracture. Cutlery manufacturers use the bend test to judge the
temper of blades. File testing of case-hardened parts is very common.
However there is need of standardized methods which depend less
upon the individual skill of the operator, and which will yield
results comparable to others made by different men at different
places and on different steels. Hence has grown up the art of testing
materials.
TENSILE PROPERTIES
Strength of a metal is usually expressed in the number of pounds
a 1-in. bar will support just before breaking, a term called the
"ultimate strength." It has been found that the shape of the test
bar and its method of loading has some effect upon the results,
so it is now usual to turn a rod 5-1/2 in. long down to 0.505 in.
in diameter for a central length of 2-3/8 in., ending the turn
with 1/2-in. fillets. The area of the bar equals 0.2 sq. in., so
the load it bears at rupture multiplied by 5 will represent the
"ultimate strength" in pounds per square inch.
Such a test bar is stretched apart in a machine like that shown
in Fig. 9. The upper end of the bar is held in wedged jaws by the
top cross-head, and the lower end grasped by the movable head.
The latter is moved up and down by three long screws, driven at
the same speed, which pass through threads cut in the corners of
the cross-head. When the test piece is fixed in position the motor
which drives the machine is given a few turns, which by proper
gearing pulls the cross-head down with a certain pull. This pull
is transmitted to the upper cross-head by the test bar, and can
be weighed on the scale arm, acting through a system of links and
levers.
Thus the load may be increased as rapidly as desirable, always
kept balanced by the weighing mechanism, and the load at fracture
may be read directly from the scale beam.
This same test piece may give other information. If light punch
marks are made, 2 in. apart, before the test is begun, the broken
ends may be clamped together, and the distance between punch marks
measured. If it now measures 3 in. the stretch has been 1 in. in 2,
or 50 per cent. This figure is known as the elongation at fracture,
or briefly, the "elongation," and is generally taken to be a measure
of ductility.
When steel shows any elongation, it also contracts in area at the
same time. Often this contraction is sharply localized at the fracture;
the piece is said to "neck." A figure for contraction in area is
also of much interest as an indication of toughness; the diameter
at fracture is measured, a corresponding area taken out from a
table of circles, subtracted from the original area (0.200 sq.
in.) and the difference divided by 0.2 to get the percentage
contraction.
[Illustration: FIG. 9.--Olsen testing machine.]
Quite often it is desired to discover the elastic limit of the
steel, in fact this is of more use to the designer than the ultimate
strength. The elastic limit is usually very close to the load where
the metal takes on a permanent set. That is to say, if a delicate
caliper ("extensometer," so called) be fixed to the side of the
test specimen, it would show the piece to be somewhat longer under
load than when free. Furthermore, if the load had not yet reached the
yield point, and were released at any time, the piece would return
to its original length. However, if the load had been excessive, and
then relieved, the extensometer would no longer read exactly 2.0
in., but something more.
Soft steels "give" very quickly at the yield point. In fact, if
the testing machine is running slowly, it takes some time for the
lower head to catch up with the stretching steel. Consequently at
the yield point, the top head is suddenly but only temporarily
relieved of load, and the scale beam drops. In commercial practice,
the yield point is therefore determined by the "drop of the beam."
For more precise work the calipers are read at intervals of 500 or
1,000 lb. load, and a curve plotted from these results, a curve
which runs straight up to the elastic limit, but there bends off.
A tensile test therefore gives four properties of great usefulness:
The yield point, the ultimate strength, the elongation and the
contraction. Compression tests are seldom made, since the action
of metal in compression and in tension is closely allied, and the
designer is usually satisfied with the latter.
IMPACT TESTS
Impact tests are of considerable importance as an indication of
how a metal will perform under shock. Some engineers think that
the tensile test, which is one made under slow loading, should
therefore be supplemented by another showing what will happen if
the load is applied almost instantaneously. This test, however, has
not been standardized, and depends to a considerable extent upon
the type of machine, but more especially the size of the specimen
and the way it is "nicked." The machine is generally a swinging
heavy pendulum. It falls a certain height, strikes the sample at
the lowest point, and swings on past. The difference between the
downward and upward swing is a measure of the energy it took to
break the test piece.
FATIGUE TESTS
It has been known for fifty years that a beam or rod would fail
at a relatively low stress if only repeated often enough. It has
been found, however, that each material possesses a limiting stress,
or endurance limit, within which it is safe, no matter how often
the loading occurs. That limiting stress for all steels so far
investigated causes fracture below 10 million reversals. In other
words, a steel which will not break before 10,000,000 reversals
can confidently be expected to endure 100,000,000, and doubtless
into the billions.
About the only way to test one piece such a large number of times
is to fashion it into a beam, load it, and then turn the beam in
its supports. Thus the stress in the outer fibers of the bar varies
from a maximum stretch through zero to a maximum compression, and
back again. A simple machine of this sort is shown in Fig. 10,
where _B_ and _E_ are bearings, _A_ the test piece, turned slightly
down in the center, _C_ and _D_ ball bearings supporting a load
_W_. _K_ is a pulley for driving the machine and _N_ is a counter.
[Illustration: FIG. 10.--Sketch of rotating beam machine for measuring
endurance of metal.]
HARDNESS TESTING
The word "hardness" is used to express various properties of metals,
and is measured in as many different ways.
"Scratch hardness" is used by the geologist, who has constructed
"Moh's scale" as follows:
Talc has a hardness of 1
Rock Salt has a hardness of 2
Calcite has a hardness of 3
Fluorite has a hardness of 4
Apatite has a hardness of 5
Feldspar has a hardness of 6
Quartz has a hardness of 7
Topaz has a hardness of 8
Corundum has a hardness of 9
Diamond has a hardness of 10
A mineral will scratch all those above it in the series, and will
be scratched by those below. A weighted diamond cone drawn slowly
over a surface will leave a path the width of which (measured by
a microscope) varies inversely as the scratch hardness.
"Cutting hardness" is measured by a standardized drilling machine,
and has a limited application in machine-shop practice.
"Rebounding hardness" is commonly measured by the Shore scleroscope,
illustrated in Fig. 11. A small steel hammer, 1/4 in. in diameter,
3/4 in. in length, and weighing about 1/12 oz. is dropped a distance
of 10 in. upon the test piece. The height of rebound in arbitrary
units represents the hardness numeral.
[Illustration: FIG. 11.--Shore scleroscope.]
Should the hammer have a hard flat surface and drop on steel so hard
that no impression were made, it would rebound about 90 per cent
of the fall. The point, however, consists of a slightly spherical,
blunt diamond nose 0.02 in. in diameter, which will indent the steel
to a certain extent. The work required to make the indentation
is taken from the energy of the falling body; the rebound will
absorb the balance, and the hammer will now rise from the same
steel a distance equal to about 75 per cent of the fall. A permanent
impression is left upon the test piece because the impact will
develop a force of several hundred thousand pounds per square inch
under the tiny diamond-pointed hammer head, stressing the test
piece at this point of contact much beyond its ultimate strength.
The rebound is thus dependent upon the indentation hardness, for
the reason that the less the indentation, the more energy will
reappear in the rebound; also, the less the indentation, the harder
the material. Consequently, the harder the material, the more the
rebound.
"Indentation hardness" is a measure of a material's resistance
to penetration and deformation. The standard testing machine is
the Brinell, Fig. 12. A hardened steel ball, 10 mm. in diameter,
is forced into the test piece with a pressure of 3,000 kg. (3-1/3
tons). The resulting indentation is then measured.
[Illustration: FIG. 12.--Hydraulic testing machine. (Brinell
principle.)]
While under load, the steel ball in a Brinell machine naturally
flattens somewhat. The indentation left behind in the test piece is
a duplicate of the surface which made it, and is usually regarded
as being the segment of a sphere of somewhat larger radius than
the ball. The radius of curvature of this spherical indentation
will vary slightly with the load and the depth of indentation.
The Brinell hardness numeral is the quotient found by dividing the
test pressure in kilograms by the spherical area of the indentation.
The denominator, as before, will vary according to the size of the
sphere, the hardness of the sphere and the load. These items have
been standardized, and the following table has been constructed
so that if the diameter of the identation produced by a load of
3,000 kg. be measured the hardness numeral is found directly.
TABLE FOR BRINELL BALL TEST
------------------------------------------------------------------------
Diameter of Ball | Hardness Number | Diameter of Ball | Hardness Number
Impression, mm. | for a Load of | Impression, mm. | for a Load of
| 3,000 kg. | | 3,000 kg.
-----------------|-----------------|------------------|-----------------
2.0 | 946 | 4.5 | 179
2.1 | 857 | 4.6 | 170
2.2 | 782 | 4 7 | 163
2.3 | 713 | 4.8 | 156
2.4 | 652 | 4.9 | 149
2.5 | 600 | 5.0 | 143
| | |
2.6 | 555 | 5.1 | 137
2.7 | 512 | 5.2 | 131
2.8 | 477 | 5.3 | 126
2.9 | 444 | 5.4 | 121
3.0 | 418 | 5.5 | 116
| | |
3.1 | 387 | 5.6 | 112
3.2 | 364 | 5.7 | 107
3.3 | 340 | 5.8 | 103
3.4 | 321 | 5.9 | 99
3.5 | 302 | 6.0 | 95
| | |
3.6 | 286 | 6.1 | 92
3.7 | 269 | 6.2 | 89
3.8 | 255 | 6.3 | 86
3.9 | 241 | 6.4 | 83
4.0 | 228 | 6.5 | 80
| | |
4.1 | 217 | 6.6 | 77
4.2 | 207 | 6.7 | 74
4.3 | 196 | 6.8 | 71.5
4.4 | 187 | 6.9 | 69
------------------------------------------------------------------------
CHAPTER III
ALLOYS AND THEIR EFFECT UPON STEEL
In view of the fact that alloy steels are coming into a great deal
of prominence, it would be well for the users of these steels to
fully appreciate the effects of the alloys upon the various grades
of steel. We have endeavored to summarize the effect of these alloys
so that the users can appreciate their effect, without having to
study a metallurgical treatise and then, perhaps, not get the crux
of the matter.
NICKEL
Nickel may be considered as the toughest among the non-rare alloys
now used in steel manufacture. Originally nickel was added to give
increased strength and toughness over that obtained with the ordinary
rolled structural steel and little attempt was made to utilize its
great possibilities so far as heat treatment was concerned.
The difficulties experienced have been a tendency towards laminated
structure during manufacture and great liability to seam, both
arising from improper melting practice. When extra care is exercised
in the manufacture, particularly in the melting and rolling, many
of these difficulties can be overcome.
The electric steel furnace, of modern construction, is a very important
step forward in the melting of nickel steel; neither the crucible
process nor basic or acid open-hearth furnaces give such good results.
Great care must be exercised in reheating the billet for rolling
so that the steel is correctly soaked. The rolling must not be
forced; too big reduction per pass should not be indulged in, as
this sets up a tendency towards seams.
Nickel steel has remarkably good mechanical qualities when suitably
heat-treated, and it is preeminently adapted for case-hardening. It
is not difficult to machine low-nickel steel, consequently it is
in great favor where easy machining properties are of importance.
Nickel influences the strength and ductility of steel by being
dissolved directly in the iron or ferrite; in this respect differing
from chromium, tungsten and vanadium. The addition of each 1 per
cent nickel up to 5 per cent will cause an approximate increase of
from 4,000 to 6,000 lb. per square inch in the tensile strength and
elastic limit over the corresponding steel and without any decrease
in ductility. The static strength of nickel steel is affected to
some degree by the percentage of carbon; for instance, steel with
0.25 per cent carbon and 3.5 per cent nickel has a tensile strength,
in its normal state, equal to a straight carbon steel of 0.5 per
cent with a proportionately greater elastic limit and retaining
all the advantages of the ductility of the lower carbon.
To bring out the full qualities of nickel it must be heat-treated,
otherwise there is no object in using nickel as an alloy with carbon
steel as the additional cost is not justified by increased strength.
Nickel has a peculiar effect upon the critical ranges of steel,
the critical range being lowered by the percentage of nickel; in
this respect it is similar to manganese.
Nickel can be alloyed with steel in various percentages, each percentage
having a very definite effect on the microstructure. For instance, a
steel with 0.2 per cent carbon and 2 per cent nickel has a pearlitic
structure but the grain is much finer than if the straight carbon
were used. With the same carbon content and say 5 per cent nickel,
the structure would still be pearlitic, but much finer and denser,
therefore capable of withstanding shock, and having greater dynamic
strength. With about 0.2 per cent carbon and 8 per cent nickel, the
steel is nearing the stage between pearlite and martensite, and
the structure is extremely fine, the ferrite and pearlite having
a very pronounced tendency to mimic a purely martensite structure.
Steel with 0.2 per cent carbon and 15 per cent nickel is entirely
martensite. Higher percentages of nickel change the martensitic
structure to austenite, the steel then being non-magnetic. The
higher percentages, that is 30 to 35 per cent nickel, are used
for valve seats, valve heads, and valve stems, as the alloy is a
poor conductor of heat and is particularly free from any tendency
towards corrosion or pitting from the action of waste gases of
the internal-combustion engine.
Nickel steels having 3-1/2 per cent nickel and 0.15 to 0.20 per
cent carbon are excellent for case-hardening purposes, giving hard
surfaces and tough interiors.
To obtain the full effect of nickel as an alloy, it is essential
that the correct percentage of carbon be used. High nickel and
low carbon will not be more efficient than lower nickel and higher
carbon, but the cost will be much greater. Generally speaking,
heat-treated nickel alloy steels are about two to three times stronger
than the same steel annealed. This point is very important as many
instances have been found where nickel steel is incorrectly used,
being employed when in the annealed or normal state.
CHROMIUM
Chromium when alloyed with steel, has the characteristic function
of opposing the disintegration and reconstruction of cementite.
This is demonstrated by the changes in the critical ranges of this
alloy steel taking place slowly; in other words, it has a tendency
to raise the _Ac_ range (decalescent points) and lower the _Ar_
range (recalescent points). Chromium steels are therefore capable
of great hardness, due to the rapid cooling being able to retard
the decomposition of the austenite.
The great hardness of chromium steels is also due to the formation
of double carbides of chromium and iron. This condition is not
removed when the steel is slightly tempered or drawn. This additional
hardness is also obtained without causing undue brittleness such as
would be obtained by any increase of carbon. The degree of hardness
of the lower-chrome steels is dependent upon the carbon content,
as chromium alone will not harden iron.
The toughness so noticeable in this steel is the result of the
fineness of structure; in this instance, the action is similar
to that of nickel, and the tensile strength and elastic limit is
therefore increased without any loss of ductility. We then have
the desirable condition of tough hardness, making chrome steels
extremely valuable for all purposes requiring great resistance
to wear, and in higher-chrome contents resistance to corrosion.
All chromium-alloy steels offer great resistance to corrosion and
erosion. In view of this, it is surprising that chromium steels
are not more largely used for structural steel work and for all
purposes where the steel has to withstand the corroding action
of air and liquids. Bridges, ships, steel building, etc., would
offer greater resistance to deterioration through rust if the
chromium-alloy steels were employed.
Prolonged heating and high temperatures have a very bad effect upon
chromium steels. In this respect they differ from nickel steels,
which are not so affected by prolonged heating, but chromium steels
will stand higher temperatures than nickel steels when the period
is short.
Chromium steels, due to their admirable property of increased hardness,
without the loss of ductility, make very excellent chisels and
impact tools of all types, although for die blocks they do not give
such good results as can be obtained from other alloy combinations.
For ball bearing steels, where intense hardness with great toughness
and ready recovery from temporary deflection is required, chromium
as an alloy offers the best solution.
Two per cent chromium steels; due to their very hard tough surface,
are largely used for armor-piercing projectiles, cold rolls, crushers,
drawing dies, etc.
The normal structure of chromium steels, with a very low carbon
content is roughly pearlitic up to 7 per cent, and martensitic
from 8 to 20 per cent; therefore, the greatest application is in
the pearlitic zone or the lower percentages.
NICKEL-CHROMIUM
A combination of the characteristics of nickel and the characteristics
of chromium, as described, should obviously give a very excellent
steel as the nickel particularly affects the ferrite of the steel
and the chromium the carbon. From this combination, we are able to
get a very strong ferrite matrix and a very hard tough cementite.
The strength of a strictly pearlitic steel over a pure iron is due
to the pearlitic being a layer arrangement of cementite running
parallel to that of a pure iron layer in each individual grain. The
ferrite _i.e._, the iron is increased in strength by the resistance
offered by the cementite which is the simple iron-carbon combination
known to metallurgists as Fe3C. The cementite, although adding
to the tensile strength, is very brittle and the strength of the
pearlite is the combination of the ferrite and cementite. In the
event of the cementite being strengthened, as in the case of strictly
chromium steels, an increased tensile strength is readily obtained
without loss of ductility and if the ferrite is strengthened then
the tensile strength and ductility of the metal is still further
improved.
Nickel-chromium alloy represents one of the best combinations available
at the present time. The nickel intensifies the physical characteristics
of the chromium and the chromium has a similar effect on the nickel.
For case-hardening, nickel-chromium steels seem to give very excellent
results. The carbon is very rapidly taken up in this combination,
and for that reason is rather preferable to the straight nickel steel.
With the mutually intensifying action of chromium and nickel there
is a most suitable ratio for these two alloys, and it has been found
that roughly 2-1/2 parts of nickel to about 1 part of chromium
gives the best results. Therefore, we have the standard types of
3.5 per cent nickel with 1.5 per cent chromium to 1.5 per cent
nickel with 0.6 per cent chromium and the various intermediate
types. This ratio, however, does not give the whole story of
nickel-chromium combinations, and many surprising results have
been obtained with these alloys when other percentage combinations
have been employed.
VANADIUM
Vanadium has a very marked effect upon alloy steels rich in chromium,
carbon, or manganese. Vanadium itself, when combined with steel very
low in carbon, is not so noticeably beneficial as in the same carbon
steel higher in manganese, but if a small quantity of chromium
is added, then the vanadium has a very marked effect in increasing
the impact strength of the alloy. It would seem that vanadium has
the effect of intensifying the action of chromium and manganese, or
that vanadium is intensified by the action of chromium or manganese.
Vanadium has the peculiar property of readily entering into solution
with ferrite. If vanadium contained is considerable it also combines
with the carbon, forming carbides. The ductility of carbon-vanadium
steels is therefore increased, likewise the ductility of chrome-vanadium
steels.
The full effect of vanadium is not felt unless the temperatures to
which the steel is heated for hardening are raised considerably.
It is therefore necessary that a certain amount of "soaking" takes
place, so as to get the necessary equalization. This is true of all
alloys which contain complex carbides, i.e., compounds of carbon,
iron and one or more elements.
Chrome-vanadium steels also are highly favored for case hardening.
When used under alternating stresses it appears to have superior
endurance. It would appear that the intensification of the properties
due to chromium and manganese in the alloy steel accounts for this
peculiar phenomenon.
Vanadium is also a very excellent scavenger for either removing
the harmful gases, or causing them to enter into solution with the
metal in such a way as to largely obviate their harmful effects.
Chrome-vanadium steels have been claimed, by many steel manufacturers
and users, to be preferable to nickel-chrome steels. While not
wishing to pass judgment on this, it should be borne in mind that
the chrome-vanadium steel, which is tested, is generally compared
with a very low nickel-chromium alloy steel (the price factor entering
into the situation), but equally good results can be obtained by
nickel-chromium steels of suitable analysis.
Where price is the leading factor, there are many cases where a
stronger steel can be obtained from the chrome and vanadium than
the nickel-chrome. It will be safe to say that each of these two
systems of alloys have their own particular fields and chrome-vanadium
steel should not be regarded as the sole solution for all problems,
neither should nickel-chromium.
MANGANESE
Manganese adds considerably to the tensile strength of steel, but
this is dependent on the carbon content. High carbon materially adds
to the brittleness, whereas low-carbon, pearlitic-manganese steels
are very tough and ductile and are not at all brittle, providing the
heat-treating is correct. Manganese steel is very susceptible to
high temperatures and prolonged heating.
In low-carbon pearlitic steels, manganese is more effective in
increasing ultimate strength than is nickel; that is to say, a
0.45 carbon steel with 1.25 per cent manganese is as strong as a
0.45 carbon steel with 1.5 per cent nickel. The former steel is
much used for rifle barrels, and in the heat-treated condition will
give 80,000 to 90,000 lb. per square inch elastic limit, 115,000 to
125,000 lb. per square inch tensile strength, 23 per cent elongation,
and 55 per cent reduction in area.
Manganese when added to steel has the effect of lowering the critical
range; 1 per cent manganese will lower the upper critical point
60°F. The action of manganese is very similar to that of nickel
in this respect, only twice as powerful. As an instance, 1 per
cent nickel would have the effect of lowering the upper critical
range from 25 to 30°F.
Low-carbon pearlitic-manganese steel, heat-treated, will give dynamic
strength which cannot be equaled by low-priced and necessarily
low-content nickel steels. In many instances, it is preferable to use
high-grade manganese steel, rather than low-content nickel steel.
High-manganese steels or austenite manganese steels are used for a
variety of purposes where great resistance to abrasion is required,
the percentage of manganese being from 11 to 14 per cent, and carbon
1 to 1.5 per cent. This steel is practically valueless unless
heat-treated; that is, heated to about yellow red and quenched
in ice water. The structure is then austenite and the air-cooled
structure of this steel is martensite. Therefore this steel has to
be heated and very rapidly cooled to obtain the ductile austenite
structure.
Manganese between 2 and 7 per cent is a very brittle material when
the carbon is about 1 per cent or higher and is, therefore, quite
valueless. Below 2 per cent manganese steel low in carbon is very
ductile and tough steel.
The high-content manganese steels are known as the "Hadfield manganese
steels," having been developed by Sir Robert Hadfield. Small additions
of chrome up to 1 per cent increase the elastic limit of low-carbon
pearlitic-manganese steels without affecting the steel in its resistance
to shock, but materially decrease the percentage of elongation.
Vanadium added to low-carbon pearlitic manganese steel has a very
marked effect, increasing greatly the dynamic strength and changing
slightly the susceptibility of this steel to heat treatments, giving
a greater margin for the hardening temperature. Manganese steel
with added vanadium is most efficient when heat-treated.
TUNGSTEN
Tungsten, as an alloy in steel, has been known and used for a long
time. The celebrated and ancient damascus steel being a form of
tungsten-alloy steel. Tungsten and its effects, however, did not
become generally realized until Robert Mushet experimented and
developed his famous mushet steel and the many improvement made
since that date go to prove how little Mushet himself understood
the peculiar effects of tungsten as an alloy.
Tungsten acts on steel in a similar manner to carbon, that is,
it increases its hardness, but is much less effective than carbon
in this respect. If the percentage of tungsten and manganese is
high, the steel will be hard after cooling in the air. This is
impossible in a carbon steel. It was this combination that Mushet
used in his well-known "air-hardening" steel.
The principal use of tungsten is in high-speed tool steel, but
here a high percentage of manganese is distinctly detrimental,
making the steel liable to fire crack, very brittle and weak in
the body, less easily forged and annealed. Manganese should be
kept low and a high percentage of chromium used instead.
Tools of tungsten-chromium steels, when hardened, retain their
hardness, even when heated to a dark cherry red by the friction of
the cutting or the heat arising from the chips. This characteristic
led to the term "red-hardness," and it is this property that has
made possible the use of very high cutting speeds in tools made
of the tungsten-chromium alloy, that is, "high-speed" steel.
Tungsten steels containing up to 6 per cent do not have the property
of red hardness any more than does carbon tool steel, providing
the manganese or chromium is low.
When chromium is alloyed with tungsten, a very definite red-hardness
is noticed with a great increase of cutting efficiency. The maximum
red-hardness seems to be had with steels containing 18 per cent
tungsten, 5.5 per cent chromium and 0.70 per cent carbon.
Very little is known of the actual function of tungsten, although
a vast amount of experimental work has been done. It is possible
that when the effect of tungsten with iron-carbon alloys is better
known, a greater improvement can be expected from these steels.
Tungsten has been tried and is still used by some steel manufacturers
for making punches, chisels, and other impact tools. It has also
been used for springs, and has given very good results, although
other less expensive alloys give equally good results, and are
in some instances, better.
Tungsten is largely used in permanent magnets. In this, its action
is not well understood. In fact, the reason why steel becomes a
permanent magnet is not at all understood. Theories have been evolved,
but all are open to serious questioning. The principal effect of
tungsten, as conceded by leading authorities, is that it distinctly
retards separation of the iron-carbon solution, removing the lowest
recalescent point down to atmospheric temperature.
A peculiar property of tungsten steels is that if a heating temperature
of 1,750°F. is not exceeded, the cooling curves indicate but one
critical point at about 1,350°F. But when the heating temperature
is raised above 1,850°F., this critical point is nearly if not
quite suppressed, while a lower critical point appears and grows
enormously in intensity at a temperature between 660 and 750°F.
The change in the critical ranges, which is produced by heating
tungsten steels to over 1,850°F., is the real cause of the red-hard
properties of these alloys. Its real nature is not understood,
and there is no direct evidence to show what actually happens at
these high temperatures.
It may readily be understood that an alloy containing four essential
elements, namely: iron, carbon, tungsten and chromium, is one whose
study presents problems of extreme complexity. It is possible that
complex carbides may be formed, as in chromium steels, and that
compounds between iron and tungsten exist. Behavior of these
combinations on heating and cooling must be better known before
we are able to explain many peculiarities of tungsten steels.
MOLYBDENUM
Molybdenum steels have been made commercially for twenty-five years,
but they have not been widely exploited until since the war. Very
large resources of molybdenum have been developed in America, and
the mining companies who are equipped to produce the metal are
very active in advertising the advantages of molybdenum steels.
It was early found that 1 part molybdenum was the equivalent of from
2 to 2-1/2 parts of tungsten in tool steels, and magnet steels. It
fell into disrepute as an alloy for high-speed tool steel, however,
because it was found that the molybdenum was driven out of the
surface of the tool during forging and heat treating.
Within the last few years it has been found that the presence of
less than 1 per cent of molybdenum greatly enhances certain properties
of heat-treated carbon and alloy steels used for automobiles and
high-grade machinery.
In general, molybdenum when added to an alloy steel, increases the
figure for reduction of area, which is considered a good measure
of "toughness." Molybdenum steels are also relatively insensible
to variations in heat treatment; that is to say, a
chromium-nickel-molybdenum steel after quenching in oil from 1,450°F.
may be drawn at any temperature between 900 and 1,100°F. with
substantially the same result (static tensile properties and hardness).
SILICON
Silicon prevents, to a large extent, defects such as gas bubbles
or blow holes forming while steel is solidifying. In fact, steel
after it has been melted and before it has been refined, is "wild"
and "gassy." That is to say, if it would be cast into molds it
would froth up, and boil all over the floor. A judicious amount
of silicon added to the metal just before pouring, prevents this
action--in the words of the steel maker, silicon "kills" the steel.
If about 1.75 per cent metallic silicon remains in a 0.65 carbon
steel, it makes excellent springs.
PHOSPHORUS
Phosphorus is one of the impurities in steel, and it has been the
object of steel makers for years to eliminate it. On cheap grades
of steel, not subject to any abnormal strain or stress, 0.1 per
cent phosphorus is not objectionable. High phosphorus makes steel
"cold short," i.e., brittle when cold or moderately warm.
SULPHUR
Sulphur is another impurity and high sulphur is even a greater
detriment to steel than phosphorus. High sulphur up to 0.09 per
cent helps machining properties, but has a tendency to make the
steel "hot short," i.e., subject to opening up cracks and seams
at forging or rolling heats. Sulphur should never exceed 0.06 per
cent nor phosphorus 0.08 per cent.
Steel used for tool purposes should have as low phosphorus and sulphur
contents as possible, not over 0.02 per cent.
We can sum up the various factors something as follows for ready
reference.
The ingredient Its effect
Iron The basis of steel
Carbon The determinative
Sulphur A strength sapper
Phosphorus The weak link
Oxygen A strength destroyer
Manganese For strength
Nickel For strength and toughness
Tungsten Hardener and heat resister
Chromium For resisting shocks
Vanadium Purifier and fatigue resister
Silicon Impurity and hardener
Titanium Removes nitrogen and oxygen
Molybdenum Hardener and heat resister
Aluminum Kills or deoxidizes steel
PROPERTIES OF ALLOY STEELS
The following table shows the percentages of carbon, manganese,
nickel, chromium and vanadium in typical steel alloys for engineering
purposes. It also gives the elastic limit, tensile strength, elongation
and reduction of area of the various alloys, all being given the same
heat treatment with a drawing temperature of 1,100°F. (600°C.). The
specimens were one inch rounds machined after heat treatment.
Tungsten is not shown in the table because it is seldom used in
engineering construction steels and then usually in combination
with chromium. Tungsten is used principally for the magnets of
magnetos, to some extent in the manufacture of hacksaws, and for
special tool steels.
TABLE I.--PROPERTIES OF ALLOY STEELS
------------------------------------------------------------------------------
\Manganese,/ \Chromium,/ |Elastic|Tensile |Elongation|Reduction
Carbon,\ per /Nickel,\ per /Vanadium,|limit, |Strength,|in 2 in., | of area,
per | cent | per | cent |per cent |lb. per|lb. per |per cent | per cent
cent | | cent | | |sq. in.|sq. in. | |
-------|------|-------|------|---------|-------|---------|----------|---------
0.27 | 0.55 | | | | 49,000| 80,000 | 30 | 65
0.27 | 0.47 | | | 0.26 | 66,000| 98,000 | 25 | 52
0.36 | 0.42 | | | | 58,000| 90,000 | 27 | 60
0.34 | 0.87 | | | 0.13 | 82,500| 103,000 | 22 | 57
0.45 | 0.50 | | | | 65,000| 96,000 | 22 | 52
0.43 | 0.60 | | | 0.32 | 96,000| 122,000 | 21 | 52
0.47 | 0.90 | | | 0.15 |102,000| 127,500 | 23 | 58
0.30 | 0.60 | 3.40 | | | 75,000| 105,000 | 25 | 67
0.33 | 0.63 | 3.60 | | 0.25 |118,000| 142,000 | 17 | 57
0.30 | 0.49 | 3.60 | 1.70 | |119,000| 149,500 | 21 | 60
0.25 | 0.47 | 3.47 | 1.60 | 0.15 |139,000| 170,000 | 18 | 53
0.25 | 0.50 | 2.00 | 1.00 | |102,000| 124,000 | 25 | 70
0.38 | 0.30 | 2.08 | 1.16 | |120,000| 134,000 | 20 | 57
0.42 | 0.22 | 2.14 | 1.27 | 0.26 |145,000| 161,500 | 16 | 53
0.36 | 0.61 | 1.46 | 0.64 | |117,600| 132,500 | 16 | 58
0.36 | 0.50 | 1.30 | 0.75 | 0.16 |140,000| 157,500 | 17 | 54
0.30 | 0.50 | | 0.80 | | 90,000| 105,000 | 20 | 50
0.23 | 0.58 | | 0.82 | 0.17 |106,000| 124,000 | 21 | 66
0.26 | 0.48 | | 0.92 | 0.20 |112,000| 137,000 | 20 | 61
0.35 | 0.64 | | 1.03 | 0.22 |132,500| 149,500 | 16 | 54
0.50 | 0.92 | | 1.02 | 0.20 |170,000| 186,000 | 15 | 45
------------------------------------------------------------------------------
NON-SHRINKING, OIL-HARDENING STEELS
Certain steels have a very low rate of expansion and contraction
in hardening and are very desirable for test plugs, gages, punches
and dies, for milling cutters, taps, reamers, hard steel bushings
and similar work.
It is recommended that for forging these steels it be heated slowly
and uniformly to a bright red, but not in a direct flame or blast.
Harden at a dull red heat, about 1,300°F. A clean coal or coke
fire, or a good muffle-gas furnace will give best results. Fish
oil is good for quenching although in some cases warm water will
give excellent results. The steel should be kept moving in the bath
until perfectly cold. Heated and cooled in this way the steel is
very tough, takes a good cutting edge and has very little expansion
or contraction which makes it desirable for long taps where the
accuracy of lead is important.
The composition of these steels is as follows:
Per cent
Manganese 1.40 to 1.60
Carbon 0.80 to 0.90
Vanadium 0.20 to 0.25
[Illustration: FIG. 13.--Effect of copper in steel.]
EFFECT OF A SMALL AMOUNT OF COPPER IN MEDIUM-CARBON STEEL
This shows the result of tests by C. R. Hayward and A. B. Johnston
on two types of steel: one containing 0.30 per cent carbon, 0.012
per cent phosphorus, and 0.860 per cent copper, and the other 0.365
per cent carbon, 0.053 per cent phosphorus, and 0.030 per cent
copper. The accompanying chart in Fig. 13 shows that high-copper
steel has decided superiority in tensile strength, yield point and
ultimate strength, while the ductility is practically the same.
Hardness tests by both methods show high-copper steel to be harder
than low-copper, and the Charpy shock tests show high-copper steel
also superior to low-copper. The tests confirm those made by Stead,
showing that the behavior of copper steel resembles that of nickel
steel. The high-copper steels show finer grain than the low-copper.
The quenched and drawn specimens of high-copper steel were found
to be slightly more martensitic.
HIGH-CHROMIUM OR RUST-PROOF STEEL
High-chromium, or what is called stainless steel containing from
11 to 14 per cent chromium, was originally developed for cutlery
purposes, but has in the past few years been used to a considerable
extent for exhaust valves in airplane engines because of its resistance
to scaling at high temperatures.
Percentage
Carbon 0.20 to 0.40
Manganese, not to exceed 0.50
Phosphorus, not to exceed 0.035
Sulphur, not to exceed 0.035
Chromium 11.50 to 14.00
Silicon, not to exceed 0.30
The steel should be heated slowly and forged at a temperature above
1,750°F. preferably between 1,800 and 2,200°F. If forged at temperatures
between 1,650 and 1,750°F. there is considerable danger of rupturing
the steel because of its hardness at red heat. Owing to the
air-hardening property of the steel, the drop-forgings should be
trimmed while hot. Thin forgings should be reheated to redness
before trimming, as otherwise they are liable to crack.
The forgings will be hard if they are allowed to cool in air. This
hardness varies over a range of from 250 to 500 Brinell, depending
on the original forging temperature.
ANNEALING can be done by heating to temperatures ranging from 1,290
to 1,380°F. and cooling in air or quenching in water or oil. After
this treatment the forgings will have a hardness of about 200 Brinell
and a tensile strength of 100,000 to 112,000 lb. per square inch.
If softer forgings are desired they can be heated to a temperature
of from 1,560 to 1,650°F. and cooled very slowly. Although softer
the forgings will not machine as smoothly as when annealed at the
lower temperature.
HARDENING.--The forgings can be hardened by cooling in still air
or quenching in oil or water from a temperature between 1,650 and
1,750°F.
The physical properties do not vary greatly when the carbon is
within the range of composition given, or when the steel is hardened
and tempered in air, oil, or water.
When used for valves the following specification of physical properties
have been used:
Yield point, pounds per square inch 70,000
Tensile strength, pounds per square inch 90,000
Elongation in 2 in., per cent 18
Reduction of area, per cent 50
The usual heat treatment is to quench in oil from 1,650°F. and
temper or draw at 1,100 to 1,200°F. One valve manufacturer stated
that valves of this steel are hardened by heating the previously
annealed valves to 1,650°F. and cooling in still air. This treatment
gives a scleroscope hardness of about 50.
In addition to use in valves this steel should prove very satisfactory
for shafting for water-pumps and other automobile parts subject to
objectionable corrosion.
TABLE 2.--COMPARISON OF PHYSICAL PROPERTIES FOR HIGH-CHROMIUM
STEELS OF DIFFERENT CARBON CONTENT --------------------------------------------------------------------------
| C 0.20 | C 0.27 | C 0.50
| Mn 0.45 | Mn 0.50 |
| Cr 12.56 | Cr 12.24 | Cr 14.84
-----------------------------------------|----------|----------|----------
Quenched in oil from degrees Fahrenheit | 1,600 | 1,600 | 1,650
Tempered at degrees Fahrenheit | 1,160 | 1,080 | 1,100
Yield point, pounds per square inch | 78,300 | 75,000 | 91,616
Tensile strength, pounds per square inch | 104,600 | 104,250 | 123,648
Elongation in 2 in., per cent | 25.0 | 23.5 | 14.5
Reduction of area, per cent | 52.5 | 51.4 | 33.5
--------------------------------------------------------------------------
TABLE 3.--COMPARISON OF PHYSICAL PROPERTIES BETWEEN AIR, OIL AND
WATER-HARDENED STEEL HAVING CHEMICAL ANALYSIS IN
PERCENTAGE OF
-------------------------------------------------------------------------
Carbon 0.24
Manganese 0.30
Phosphorus 0.035
Sulphur 0.035
Chromium 12.85
Silicon 0.20
-------------------------------------------------------------------------
| Hardened | | Elastic | Tensile | |
Hardening| from, | Tempered | limit, |strength,|Elongation|Reduction
medium | degrees |at, degrees| per lb. |lb. Per | in 2 in. |of area,
|Fahrenheit|Fahrenheit | sq. in. | sq. in. | per cent |per cent
---------|----------|-----------|---------|---------|----------|---------
| | 930 | 158,815 | 192,415 | 13.0 | 40.5
| | 1,100 | 99,680 | 120,065 | 21.0 | 59.2
Air | 1,650 | 1,300 | 70,785 | 101,250 | 26.0 | 64.6
| | 1,380 | 66,080 | 98,335 | 28.0 | 63.6
| | 1,470 | 70,785 | 96,990 | 27.0 | 64.7
---------|----------|-----------|---------|---------|----------|---------
| | 930 | 163,070 | 202,720 | 8.0 | 18.2
Oil | 1,650 | 1,100 | 88,255 | 116,480 | 20.0 | 56.9
| | 1,300 | 77,950 | 105,505 | 25.5 | 63.8
| | 1,380 | 88,255 | 98,785 | 27.0 | 66.3
---------|----------|-----------|---------|---------|----------|---------
| | 930 | 158,815 | 202,050 | 12.0 | 34.2
Water | 1,650 | 1,100 | 90,270 | 120,735 | 22.0 | 59.8
| | 1,300 | 66,080 | 102,590 | 25.8 | 64.8
| | 1,380 | 67,200 | 97,890 | 27.0 | 65.2
-------------------------------------------------------------------------
This steel can be drawn into wire, rolled into sheets and strips
and drawn into seamless tubes.
CORROSION.--This steel like any other steel when distorted by cold
working is more sensitive to corrosion and will rust. Rough cut
surfaces will rust. Surfaces finished with a fine cut are less
liable to rust. Ground and polished surfaces are practically immune
to rust.
When chromium content is increased to 16 to 18 per cent and silicon
is added, from 2 to 4 per cent, this steel becomes rust proof in
its raw state, as soon as the outside surface is removed. It does
not need to be heat-treated in any way. These compositions are
both patented.
S. A. E. STANDARD STEELS
The following steel specifications are considered standard by the
Society of Automotive Engineers and represents automobile practice in
this country. These tables give the S. A. E. number, the composition
of the steel and the heat treatment. These are referred to by
letter--the heat treatments being given in detail on pages 134
to 137 in Chap. 8. It should be noted that the percentage of the
different ingredients desired is the mean, or halfway between the
minimum and maximum.
TABLE 4.--CARBON STEELS
------------------------------------------------------------------------------
S. A. E. | Carbon | Manganese | | |
Specification|(minimum and |(minimum and |Phosphorus| Sulphur | Heat
no. | maximum) | maximum) |(maximum) |(maximum)| treatment
-------------|-------------|-------------|----------|---------|---------------
1,010 | 0.05 to 0.15| 0.30 to 0.60| 0.045 | 0.05 |Quench at 1,500
1,020 | 0.15 to 0.25| 0.30 to 0.60| 0.045 | 0.05 | A or B
1,025 | 0.20 to 0.30| 0.50 to 0.80| 0.045 | 0.05 | H
| | | | |
1,035 | 0.30 to 0.40| 0.50 to 0.80| 0.045 | 0.05 | H, D or E
1,045 | 0.40 to 0.50| 0.50 to 0.80| 0.045 | 0.05 | H, D or E
1,095 | 0.90 to 1.05| 0.25 to 0.50| 0.040 | 0.05 | F
------------------------------------------------------------------------------
TABLE 5.--SCREW STOCK
---------------------------------------------------------------------------
S. A. E. | Carbon | Manganese | Phosphorus | Sulphur
Specification no.| | | (maximum) |
-----------------|--------------|--------------|------------|--------------
1,114 | 0.08 to 0.20 | 0.30 to 0.80 | 0.12 | 0.06 to 0.12
---------------------------------------------------------------------------
TABLE 6.--NICKEL STEELS
-----------------------------------------------------------------------------
S. A. E. | | | Phosphorus| | |
Specification | | | (maximum) | | |
no. ---- | \ / | |
| Carbon | Manganese | | Sulphur | Nickel | Heat
|(minimum and|(minimum and| |(maximum)|(minimum and|treatment
| maximum) | maximum) | | | maximum) |
---------|------------|------------|-------|---------|------------|----------
2,315 |0.10 to 0.20|0.50 to 0.80| 0.04 | 0.045 |3.25 to 3.75|G, H or K
2,320 |0.15 to 0.25|0.50 to 0.80| 0.04 | 0.045 |3.25 to 3.75|G, H or K
2,330 |0.25 to 0.35|0.50 to 0.80| 0.04 | 0.045 |3.25 to 3.75| H or K
| | | | | |
2,335 |0.30 to 0.40|0.50 to 0.80| 0.04 | 0.045 |3.25 to 3.75| H or K
2,340 |0.35 to 0.45|0.50 to 0.80| 0.04 | 0.045 |3.25 to 3.75| H or K
2,345 |0.40 to 0.50|0.50 to 0.80| 0.04 | 0.045 |3.25 to 3.75| H or K
-----------------------------------------------------------------------------
TABLE 7.--NICKEL-CHROMIUM STEELS
-------------------------------------------------------------------------------
S. A. E. | | | Phosphorus| Sulphur | | |
Specification| | | (maximum)|(maximum) | | | Heat
no. ------ | ------ | ---- | |treatment
| Carbon | Manganese | | | Nickel | Chromium \
|(minimum and|(minimum and| | |(minimum and|(minimum and |
| maximum) | maximum) | | | maximum) | maximum) |
------|------------|------------|----|-----|------------|-------------|--------
3,120|0.15 to 0.25|0.50 to 0.80|0.04|0.045|1.00 to 1.50|0.45 to 0.75*|G,H or D
3,125|0.20 to 0.30|0.50 to 0.80|0.04|0.045|1.00 to 1.50|0.45 to 0.75*|H,D or E
3,130|0.25 to 0.35|0.50 to 0.80|0.04|0.045|1.00 to 1.50|0.45 to 0.75*|H,D or E
| | | | | | |
3,135|0.30 to 0.40|0.50 to 0 80|0.04|0.045|1.00 to 1.50|0.45 to 0 75*|H,D or E
3,140|0.35 to 0.45|0.50 to 0.80|0.04|0.045|1.00 to 1.50|0.45 to 0.75*|H,D or E
3,220|0.15 to 0.25|0.30 to 0.60|0.04|0.040|1.50 to 2.00|0.90 to 1.25 |G,H or D
| | | | | | |
3,230|0.25 to 0.35|0.30 to 0.60|0.04|0.040|1.50 to 2.00|0.90 to 1.25 | H or D
3,240|0.35 to 0.45|0.30 to 0.60|0.04|0.040|1.50 to 2.00|0.90 to 1.25 | H or D
3,250|0.45 to 0.55|0.30 to 0.60|0.04|0.040|1.50 to 2.00|0.90 to 1.25 | M or Q
| | | | | | |
X3,315|0.10 to 0.20|0.45 to 0.75|0.04|0.040|2.75 to 3.25|0.60 to 0.95 | G
X3,335|0.30 to 0.40|0.45 to 0.75|0.04|0.040|2.75 to 3.25|0.60 to 0.95 | P or R
X3,350|0.45 to 0.55|0.45 to 0.75|0.04|0.040|2.75 to 3.25|0.60 to 0.95 | P or R
| | | | | | |
3,320|0.15 to 0.25|0.30 to 0.60|0.04|0.040|3.25 to 3.75|1.25 to 1.75 | L
3,330|0.25 to 0.35|0.30 to 0.60|0.04|0.040|3.25 to 3.75|1.25 to 1.75 | P or R
3,340|0.35 to 0.45|0.30 to 0.60|0.04|0.040|3.25 to 3.75|1.25 to 1.75 | P or R
-------------------------------------------------------------------------------
* Another grade of this type of steel is available with chromium content
of 0.15 per cent to 45 per cent. It has somewhat lower physical properties.
TABLE 8.--CHROMIUM STEELS
-------------------------------------------------------------------------------
S. A. E. | | | | | |
Specification| | | | | |
no. --- Carbon | Manganese | | | Chromium |
|(minimum and|(minimum and|Phosphorus|Sulphur |(minimum and| Heat
| maximum) | maximum) |(maximum) |(maximum)| maximum) |treatment
---------|------------|------------|----------|---------|------------|---------
5,120 |0.15 to 0.25| * | 0.04 | 0.045 |0.65 to 0.85| B
5,140 |0.35 to 0.45| * | 0.04 | 0.045 |0.65 to 0.85| H or D
5,165 |0.60 to 0.70| * | 0.04 | 0.045 |0.65 to 0.85| H or D
| | | | | |
5,195 |0.90 to 1.05|0.20 to 0.45| 0.03 | 0.03 |0.90 to 1.10|M, P or R
51,120 |1.10 to 1.30|0.20 to 0.45| 0.03 | 0.03 |0.90 to 1.10|M, P or R
5,295 |0.90 to 1.05|0.20 to 0.45| 0.03 | 0.03 |1.10 to 1.30|M, P or R
52,120 |1.10 to 1.30|0.20 to 0.45| 0.03 | 0.03 |1.10 to 1.30|M, P or R
-------------------------------------------------------------------------------
--Two types of steel are available in this class, one with manganese 0.25
to 0.50 per cent (0.35 per cent desired), and silicon not over 0.20 per
cent; the other with manganese 0.60 to 0.80 per cent (0.70 per cent
desired), and silicon 0.15 to 0.50 per cent.
TABLE 9.--CHROMIUM-VANADIUM STEELS
-------------------------------------------------------------------------------
S. A. E. | | |Phosphorus| Sulphur | |Vanadium |
Specification| | | (maximum)|(maximum)| |(minimum)|
no. ------ | -- | / - |
| Carbon | Manganese | | | Chromium | | Heat
|(minimum and|(minimum and| | |(minimum and| |treatment
| maximum) | maximum) | | | maximum) | |
------|------------|------------|-------|-------|------------|-------|---------
6,120 |0.15 to 0.25|0.50 to 0.80| 0.04 | 0.04 |0.80 to 1.10| 0.15 | S
6,125 |0.20 to 0.30|0.50 to 0.80| 0.04 | 0.04 |0.80 to 1.10| 0.15 | S or T
6,130 |0.25 to 0.35|0.50 to 0.80| 0.04 | 0.04 |0.80 to 1.10| 0.15 | T or U
6,135 |0.30 to 0.40|0.50 to 0.80| 0.04 | 0.04 |0.80 to 1.10| 0.15 | T or U
6,140 |0.35 to 0.45|0.50 to 0.80| 0.04 | 0.04 |0.80 to 1.10| 0.15 | T or U
6,145 |0.40 to 0.50|0.50 to 0.80| 0.04 | 0.04 |0.80 to 1.10| 0.15 | U
6,150 |0.45 to 0.55|0.50 to 0.80| 0.04 | 0.04 |0.80 to 1.10| 0.15 | U
6,195 |0.90 to 1.05|0.20 to 0.45| 0.03 | 0.03 |0.80 to 1.10| 0.15 U
-------------------------------------------------------------------------------
TABLE 10.--SILICO-MANGANESE STEELS
-----------------------------------------------------------------------------
S. A. E. | | | | | |
Specification| | | | | |
no. ----- Carbon| Manganese | | | Silicon |
|(minimum and|(minimum and|Phosphorus|Sulphur |(minimum and| Heat
| maximum) | maximum) |(maximum) |(maximum)| maximum) |treatment
-------|------------|------------|----------|---------|------------|---------
9,250 |0.45 to 0.55|0.60 to 0.80| 0.045* | 0.045 |1.80 to 2.10| V
9,260 |0.55 to 0.65|0.50 to 0.70| 0.045* | 0.045 |1.50 to 1.80| V
-----------------------------------------------------------------------------
* Steel made by the acid process may contain maximum 0.05 phosphorus.
LIBERTY MOTOR CONNECTING RODS
The requirements for materials for the Liberty motor connecting rods
are so severe that the methods of securing the desired qualities
will be of value in other lines. The original specifications called
for chrome-nickel but the losses due to the difficulty of handling
caused the Lincoln Motor Company to suggest the substitution of
chrome-vanadium steel, and this was accepted by the Signal Corps. The
rods were accordingly made from chromium-vanadium steel, containing
carbon, 0.30 to 0.40 per cent; manganese, 0.50 to 0.80 per cent;
phosphorus, not over 0.04 per cent; sulphur, not over 0.04 per
cent; chromium, 0.80 to 1.10 per cent; vanadium, not less than 0.15
per cent. This steel is ordinarily known in the trade as 0.35 carbon
steel, S. A. E., specification 6,135, which provides a first-rate
quality steel for structural parts that are to be heat-treated.
The fatigue resisting or endurance qualities of this material are
excellent. It has a tensile strength of 150,000 lb. minimum per
square inch; elastic limit, 115,000 lb. minimum per square inch;
elongation, 5 per cent minimum in 2 in.; and minimum reduction
in area, 25 per cent.
The original production system as outlined for the manufacturers
had called for a heat treatment in the rough-forged state for the
connecting rods, and then semi-machining the rod forgings before
giving them the final treatment. The Lincoln Motor Company insisted
from the first that the proper method would be a complete heat
treatment of the forging in the rough state, and machining the
rod after the heat treatment. After a number of trial lots, the
Signal Corps acceded to the request and production was immediately
increased and quality benefited by the change. This method was
later included in a revised specification issued to all producers.
The original system was one that required a great deal of labor
per unit output. The Lincoln organization developed a method of
handling connecting rods whereby five workmen accomplished the
same result that would have required about 30 or 32 by the original
method. Even after revising the specification so as to allow complete
heat treatments in the rough-forged state, the ordinary methods
employed in heat-treating would have required 12 to 15 men. With
the fixtures employed, five men could handle 1,300 connecting rods,
half of which are plain and half, forked, in a working period of
little over 7 hr.
[Illustration: Fig. 14.--Rack for holding rods.]
[Illustration: Fig. 15.--Sliding rods into tank.]
The increase in production was gained by devising fixtures which
enabled fewer men to handle a greater quantity of parts with less
effort and in less time.
In heat-treating the forgings were laid on a rack or loop _A_,
Fig. 14, made of 1-1/4-in. double extra-heavy pipe, bent up with
parallel sides about 9 in. apart, one end being bent straight across
and the other end being bent upward so as to afford an easy grasp
for the hook. Fifteen rods were laid on each loop, there being
four loops of rods charged into a furnace with a hearth area of 36
by 66 in. The rods were charged at a temperature of approximately
900°F. They were heated for refining over a period of 3 hr. to
1,625°F., soaked 15 min, at this degree of heat and quenched in
soluble quenching oil.
In pulling the heat to quench the rods, the furnace door was raised
and the operator pulls one of the loops _A_, Fig. 15 forward to
the shelf of the furnace, supporting the straight end of the loop
by means of the porter bar _B_. They swung the loop of rods around
from the furnace shelf and set the straight end of the loop on
the edge of the quenching tank, then raise the curved end _C_,
by means of their hook _D_ so that all the rods on the loop slide
into the oil bath.
Before the rods cooled entirely, the baskets in the quenching tank
were raised and the oil allowed to partly drain off the forgings,
and they were stacked on curved-end loops or racks and charged into
the furnace for the second or hardening heat. The temperature of
the furnace was raised in 1-1/2 hr. to 1,550°F., the rods soaked
for 15 min. at this degree of heat and quenched in the same manner
as above.
They were again drained while yet warm, placed on loops and charged
into the furnace for the third or tempering heat. The temperature of
the furnace was brought to 1,100°F. in 1 hr., and the rods soaked at
this degree of heat for 1 hr. They were then removed from the furnace
the same as for quenching, but were dumped onto steel platforms
instead of into the quenching oil, and allowed to cool on these
steel platforms down to the room temperature.
PICKLING THE FORGINGS
The forgings were then pickled in a hot solution of either niter
cake or sulphuric acid and water at a temperature of 170°F., and
using a solution of about 25 per cent. The solution was maintained
at a constant point by taking hydrometer readings two or three
times a day, maintaining a reading of about 1.175. Sixty forked or
one hundred single rods were placed in wooden racks and immersed
in a lead-lined vat 30 by 30 by 5 ft. long. The rack was lowered
or lifted by means of an air hoist and the rods were allowed to
stay in solution from 1/2 to 1 hr., depending on the amount of
scale. The rods were then swung and lowered in the rack into running
hot water until all trace of the acid was removed.
The rod was finally subjected to Brinell test. This shows whether
or not the rod has been heat-treated to the proper hardness. If
the rods did not read between 241 and 277, they were re-treated
until the proper hardness is obtained.
CHAPTER IV
APPLICATION OF LIBERTY ENGINE MATERIALS TO THE AUTOMOTIVE INDUSTRY[1]
[Footnote 1: Paper presented at the summer meeting of the S. A.
E. at Ottawa Beach in June, 1919.]
The success of the Liberty engine program was an engineering achievement
in which the science of metallurgy played an important part. The
reasons for the use of certain materials and certain treatments
for each part are given with recommendations for their application
to the problems of automotive industry.
The most important items to be taken into consideration in the
selection of material for parts of this type are uniformity and
machineability. It has been demonstrated many times that the ordinary
grades of bessemer screw stock are unsatisfactory for aviation
purposes, due to the presence of excessive amounts of unevenly
distributed phosphorus and sulphide segregations. For this reason,
material finished by the basic open hearth process was selected,
in accordance with the following specifications: Carbon, 0.150 to
0.250 per cent; manganese, 0.500 to 0.800 per cent; phosphorus,
0.045 maximum per cent; sulphur, 0.060 to 0.090 per cent.
This material in the cold-drawn condition will show: Elastic limit,
50,000 lb. per square inch, elongation in 2 in., 10 per cent, reduction
of area, 35 per cent.
This material gave as uniform physical properties as S. A. E. No.
1020 steel and at the same time was sufficiently free cutting to
produce a smooth thread and enable the screw-machine manufacturers
to produce, to the same thread limits, approximately 75 per cent
as many parts as from bessemer screw stock.
There are but seven carbon-steel carbonized parts on the Liberty
engine. The most important are the camshaft, the camshaft rocker
lever roller and the tappet. The material used for parts of this
type was S. A. E. No. 1,020 steel, which is of the following chemical
analysis: Carbon 0.150 to 0.250 per cent; manganese, 0.300 to 0.600
per cent; phosphorus, 0.045 maximum per cent; sulphur, 0.050 maximum
per cent.
The heat treatment consisted in carbonizing at a temperature of
from 1,650 to 1,700°F. for a sufficient length of time to secure
the proper depth of case, cool slowly or quench; then reheat to a
temperature of 1,380 to 1,430°F. to refine the grain of the case,
and quench in water. The only thing that should limit the rate of
cooling from the carbonizing heat is distortion. Camshaft rocker
lever rollers and tappets, as well as gear pins, were quenched
directly from the carbonizing heat in water and then case-refined
and rehardened by quenching in water from a temperature of from
1,380 to 1,430°F.
The advantage of direct quenching from the carbonizing heat is
doubtless one of economy, and in many cases will save the cost
of a reheating. Specifications for case hardening, issued by the
Society of Automotive Engineers, have lately been revised; whereas
they formerly called for a slow cooling, they now permit a quenching
from the pot. Doubtless this is a step in advance. Warpage caused
by quenching can be reduced to a minimum by thoroughly annealing
the stock before any machine work is done on it.
Another advantage obtained from rapid cooling from the carbonizing
heat is the retaining of the majority of the excess cementite in
solution which produces a less brittle case and by so doing reduces
the liability of grinding checks and chipping of the case in actual
service.
In the case of the camshaft, it is not possible to quench directly
from the carbonizing heat because of distortion and therefore excessive
breakage during straightening operations. All Liberty camshafts
were cooled slowly from carbonizing heat and hardened by a single
reheating to a temperature of from 1,380 to 1,430°F. and quenching
in water.
Considerable trouble has always been experienced in obtaining uniform
hardness on finished camshafts. This is caused by insufficient
water circulation in the quenching tank, which allows the formation
of steam pockets to take place, or by decarbonization of the case
during heating by the use of an overoxidizing flame. Another cause,
which is very often overlooked, is due to the case being ground off
one side of cam more than the other and is caused by the roughing
master cam being slightly different from the finishing master cam.
Great care should be taken to see that this condition does not occur,
especially when the depth of case is between 1/32 and 3/64 in.
CARBON-STEEL FORGINGS
Low-stressed, carbon-steel forgings include such parts as carbureter
control levers, etc. The important criterion for parts of this type
is ease of fabrication and freedom from over-heated and burned
forgings. The material used for such parts was S. A. E. No. 1,030
steel, which is of the following chemical composition: Carbon, 0.250
to 0.350 per cent; manganese, 0.500 to 0.800 per cent; phosphorus,
0.045 maximum per cent; sulphur, 0.050 maximum per cent.
To obtain good machineability, all forgings produced from this
steel were heated to a temperature of from 1,575 to 1,625°F. to
refine the grain of the steel thoroughly and quenched in water
and then tempered to obtain proper machineability by heating to a
temperature of from 1,000 to 1,100°F. and cooled slowly or quenched.
Forgings subjected to this heat treatment are free from hard spots
and will show a Brinell hardness of 177 to 217, which is proper for
all ordinary machining operations. Great care should be taken not
to use steel for parts of this type containing less than 0.25 per
cent carbon, because the lower the carbon the greater the liability
of hard spots, and the more difficult it becomes to eliminate them.
The only satisfactory method so far in commercial use for the
elimination of hard spots is to give forgings a very severe quench
from a high temperature followed by a proper tempering heat to
secure good machine ability as outlined above.
The important carbon-steel forgings consisted of the cylinders,
the propeller-hubs, the propeller-hub flange, etc. The material
used for parts of this type was S. A. E. No. 1,045 steel, which
is of the following chemical composition: Carbon, 0.400 to 0.500
per cent; manganese, 0.500 to 0.800 per cent; phosphorus, 0.045
maximum per cent; sulphur, 0.050 maximum per cent.
All forgings made from this material must show, after heat treatment,
the following minimum physical properties: Elastic limit, 70,000;
lb. per square inch, elongation in 2 in., 18 per cent, reduction
of area, 45; per cent, Brinell hardness, 217 to 255.
To obtain these physical properties, the forgings were quenched in
water from a temperature of 1,500 to 1,550°F., followed by tempering
to meet proper Brinell requirements by heating to a temperature
of 1,150 to 1,200°F. and cooled slowly or quenched. No trouble
of any kind was ever experienced with parts of this type.
The principal carbon-steel pressed parts used on the Liberty engine
were the water jackets and the exhaust manifolds. The material
used for parts of this type was S. A. E. No. 1,010 steel, which
is of the following chemical composition: Carbon, 0.05 to 0.15 per
cent; manganese, 0.30 to 0.60 per cent; phosphorus, 0.045 maximum
per cent; sulphur, 0.045 maximum per cent.
No trouble was experienced in the production of any parts from
this material with the exception of the water jacket. Due to the
particular design of the Liberty cylinder assembly, many failures
occurred in the early days, due to the top of the jacket cracking
with a brittle fracture. It was found that these failures were
caused primarily from the use of jackets which showed small scratches
or die marks at this joint and secondarily by improper annealing of
the jackets themselves between the different forming operations.
By a careful inspection for die marks and by giving the jackets
1,400°F. annealing before the last forming operation, it was possible
to completely eliminate the trouble encountered.
HIGHLY STRESSED PARTS
The highly stressed parts on the Liberty engine consisted of the
connecting-rod bolt, the main-bearing bolt, the propeller-hub key,
etc. The material used for parts of this type was selected at the
option of the manufacturer from standard S. A. E. steels, the
composition of which are given in Table 11.
TABLE 11.--COMPOSITION OF S. A. E. STEELS Nos. 2,330, 3,135 AND 6,130
Steel No 2,330 3,135 6,130
Carbon, minimum 0.250 0.300 0.250
Carbon, maximum 0.350 0.400 0.450
Manganese, minimum 0.500 0.500 0.500
Manganese, maximum 0.800 0.800 0.800
Phosphorus, maximum 0.045 0.040 0.040
Sulphur, maximum 0.045 0.045 0.045
Nickel, minimum 3.250 1.000
Nickel, maximum 3.750 1.500
Chromium, minimum 0.450 0.800
Chromium, maximum 0.750 1.100
Vanadium, minimum 0.150
All highly stressed parts on the Liberty engine must show, after
heat treatment, the following minimum physical properties: Elastic
limit, 100,000 lb. per square inch; elongation in 2 in., 16 per
cent; reduction of area, 45 per cent; scleroscope hardness, 40
to 50.
The heat treatment employed to obtain these physical properties
consisted in quenching from a temperature of 1,525 to 1,575°F., in
oil, followed by tempering at a temperature of from 925 to 975°F.
Due to the extremely fine limits used on all threaded parts for
the Liberty engine, a large percentage of rejection was due to
warpage and scaling of parts. To eliminate this objection, many
of the Liberty engine builders adopted the use of heat-treated
and cold-drawn alloy steel for their highly stressed parts. On
all sizes up to and including 3/8 in. in diameter, the physical
properties were secured by merely normalizing the hot-rolled bars
by heating to a temperature of from 1,525 to 1,575°F., and cooling
in air, followed by the usual cold-drawing reductions. For parts
requiring stock over 3/8 in. in diameter, the physical properties
desired were obtained by quenching and tempering the hot-rolled bars
before cold-drawing. It is the opinion that the use of heat-treated
and cold-drawn bars is very good practice, provided proper inspection
is made to guarantee the uniformity of heat treatment and, therefore,
the uniformity of the physical properties of the finished parts.
The question has been asked many times by different manufacturers, as
to which alloy steel offers the best machineability when heat-treated
to a given Brinell hardness. The general consensus of opinion among
the screw-machine manufacturers is that S. A. E. No. 6,130 steel
gives the best machineability and that S. A. E. No. 2,330 steel
would receive second choice of the three specified.
In the finishing of highly stressed parts for aviation engines,
extreme care must be taken to see that all tool marks are eliminated,
unless they are parallel to the axis of strain, and that proper
radii are maintained at all changes of section. This is of the
utmost importance to give proper fatigue resistance to the part
in question.
GEARS
The material used for all gears on the Liberty engine was selected
at the option of the manufacturer from the following standard S.
A. E. steels, the composition of which are given in Table 12,
TABLE 12.--COMPOSITION OF STEELS NOS. X-3,340 AND 6,140
Steel No X-3,340 6,140
Carbon, minimum 0.350 0.350
Carbon, maximum 0.450 0.450
Manganese, minimum 0.450 0.500
Manganese, maximum 0.750 0.800
Phosphorus, maximum 0.040 0.040
Sulphur, maximum 0.045 0.045
Nickel, minimum 2.750
Nickel, maximum 3.250
Chromium, minimum 0.700 0.800
Chromium, maximum 0.950 1.100
Vanadium, minimum 0.150
All gears were heat-treated to a scleroscope hardness of from 55
to 55. The heat treatment used to secure this hardness consisted
in quenching the forgings from a temperature of 1,550 to 1,600°F.
in oil and annealing for good machineability at a temperature of
from 1,300 to 1,350°F. Forgings treated in this manner showed a
Brinell hardness of from 177 to 217.
RATE OF COOLING
At the option of the manufacturer, the above treatment of gear
forgings could be substituted by normalizing the forgings at a
temperature of from 1,550 to 1,600°F. The most important criterion
for proper normalizing, consisted in allowing the forgings to cool
through the critical temperature of the steel, at a rate not to exceed
50°F. per hour. For the two standard steels used, this consisted in
cooling from the normalizing temperature down to a temperature
of 1,100°F., at the rate indicated. Forgings normalized in this
manner will show a Brinell hardness of from 177 to 217. The question
has been repeatedly asked as to which treatment will produce the
higher quality finished part. In answer to this I will state that
on simple forgings of comparatively small section, the normalizing
treatment will produce a finished part which is of equal quality to
that of the quenched and annealed forgings. However, in the case of
complex forgings, or those of large section, more uniform physical
properties of the finished part will be obtained by quenching and
annealing the forgings in the place of normalizing.
The heat treatment of the finished gears consisted of quenching
in oil from a temperature of from 1,420 to 1,440°F. for the No.
X-3,340 steel, or from a temperature of from 1,500 to 1,540°F.
for No. 6,140 steel, followed by tempering in saltpeter or in an
electric furnace at a temperature of from 650 to 700°F.
The question has been asked by many engineers, why is the comparatively
low scleroscope hardness specified for gears? The reason for this is
that at best the life of an aviation engine is short, as compared with
that of an automobile, truck or tractor, and that shock resistance
is of vital importance. A sclerescope hardness of from 55 to 65
will give sufficient resistance to wear to prevent replacements
during the life of an aviation engine, while at the same time this
hardness produces approximately 50 per cent greater shock-resisting
properties to the gear. In the case of the automobile, truck or
tractor, resistance to wear is the main criterion and for that
reason the higher hardness is specified.
Great care should be taken in the design of an aviation engine
gear to eliminate sharp corners at the bottom of teeth as well
as in keyways. Any change of section in any stressed part of an
aviation engine must have a radius of at least 1/32 in. to give
proper shock and fatigue resistance. This fact has been demonstrated
many times during the Liberty engine program.
CONNECTING RODS
The material used for all connecting rods on the Liberty engine
was selected at the option of the manufacturer from one of two
standard S. A. E. steels, the composition of which are given in
Table 13.
TABLE 13.--COMPOSITION OF STEELS NOS. X-3,335 AND 6,135
Steel No. X-3,335 6,135
Carbon, minimum 0.300 0.300
Carbon, maximum 0.400 0.400
Manganese, minimum 0.450 0.500
Manganese, maximum 0.750 0.800
Phosphorus, maximum 0.040 0.040
Sulphur, maximum 0.045 0.045
Nickel, minimum 2.750
Nickel, maximum 3.250
Chromium, minimum 0.700 0.800
Chromium, maximum 0.950 1.100
Vanadium minimum 0.150
All connecting rods were heat-treated to show the following minimum
physical properties; Elastic limit, 105,000 lb. per square inch:
elongation in 2 in., 17.5; per cent, reduction of area 50.0; per
cent., Brinell hardness, 241 to 277.
The heat treatment used to secure these physical properties consisted
in normalizing the forgings at a temperature of from 1,550 to 1,600°F.,
followed by cooling in the furnace or in air. The forgings were then
quenched in oil from a temperature of from 1,420 to 1,440°F. for the
No. X-3,335 steel, or from a temperature of from 1,500 to 1,525°F.
for No. 6,135 steel, followed by tempering at a temperature of from
1,075 to 1,150°F. At the option of the manufacturer, the normalizing
treatment could be substituted by quenching the forgings from a
temperature of from 1,550 to 1,600°F., in oil, and annealing for
the best machineability at a temperature of from 1,300 to 1,350°F.
The double quench, however, did not prove satisfactory on No. X-3,335
steel, due to the fact that it was necessary to remove forgings
from the quenching bath while still at a temperature of from 300
to 500°F. to eliminate any possibility of cracking. In view of the
fact that this practice is difficult to carry out in the average
heat-treating plant, considerable trouble was experienced.
The most important criterion in the production of aviation engine
connecting rods is the elimination of burned or severely overheated
forgings. Due to the particular design of the forked rod, considerable
trouble was experienced in this respect because of the necessity
of reheating the forgings before they are completely forged. As
a means of elimination of burned forgings, test lugs were forged
on the channel section as well as on the top end of fork. After
the finish heat treatment, these test lugs were nicked and broken
and the fracture of the steel carefully examined. This precaution
made it possible to eliminate burned forgings as the test lugs were
placed on sections which would be most likely to become burned.
There is a great difference of opinion among engineers as to what
physical properties an aviation engine connecting rod should have.
Many of the most prominent engineers contend that a connecting rod
should be as stiff as possible. To produce rods in this manner in
any quantity, it is necessary for the final heat treatment to be made
on the semi-machined rod. This practice would make it necessary for a
larger percentage of the semi-machined rods to be cold-straightened
after the finish heat treatment. The cold-straightening operation
on a part having important functions to perform as a connecting
rod is extremely dangerous.
In view of the fact that a connecting rod functions as a strut,
it is considered that this part should be only stiff enough to
prevent any whipping action during the running of the engine. The
greater the fatigue-resisting property that one can put into the
rod after this stiffness is reached, the longer the life of the
rod will be. This is the reason for the Brinell limits mentioned
being specified.
In connection with the connecting rod, emphasis must be laid on the
importance of proper radii at all changes of section. The connecting
rods for the first few Liberty engines were machined with sharp
corners at the point where the connecting-rod bolt-head fits on
assembly. On the first long endurance test of a Liberty engine
equipped with rods of this type, failure resulted from fatigue
starting at this point. It is interesting to note that every rod on
the engine which did not completely fail at this point had started
to crack. The adoption of a 1/32-in. radius at this point completely
eliminated fatigue failures on Liberty rods.
CRANKSHAFT
The crankshaft was the most highly stressed part of the entire
Liberty engine, and, therefore, every metallurgical precaution
was taken to guarantee the quality of this part. The material used
for the greater portion of the Liberty crankshafts produced was
nickel-chromium steel of the following chemical composition: Carbon,
0.350 to 0.450 per cent; manganese, 0.300 to 0.600 per cent; phosphorus,
0.040 maximum per cent; sulphur, 0.045 maximum per cent; nickel,
1.750 to 2.250 per cent; chromium, 0.700 to 0.900 per cent.
Each crankshaft was heat-treated to show the following minimum
physical properties: Elastic limit, 116,000 lb. per square inch;
elongation in 2 in., 16 per cent, reduction of area, 50 per cent,
Izod impact, 34 ft.-lb.; Brinell hardness, 266 to 321.
For every increase of 4,000 lb. per square inch in the elastic
limit above 116,000 lb. per square inch, the minimum Izod impact
required was reduced 1 ft.-lb.
The heat treatment used to produce these physical properties consisted
in normalizing the forgings at a temperature of from 1,550 to 1,600°F.,
followed by quenching in water at a temperature of from 1,475 to
1,525°F. and tempering at a temperature of from 1,000 to 1,100°F.
It is absolutely necessary that the crankshafts be removed from the
quenching tank before being allowed to cool below a temperature of
500°F., and immediately placed in the tempering furnace to eliminate
the possibility of quenching cracks.
A prolongation of not less than the diameter of the forging bearing
was forged on one end of each crankshaft. This was removed from
the shaft after the finish heat treatment, and physical tests were
made on test specimens which were cut from it at a point half way
between the center and the surface. One tensile test and one impact
test were made on each crankshaft, and the results obtained were
recorded against the serial number of the shaft in question. This
serial number was carried through all machining operations and
stamped on the cheek of the finished shaft. In addition to the
above tensile and impact tests, at least two Brinell hardness
determinations were made on each shaft.
All straightening operations on the Liberty crankshaft which were
performed below a temperature of 500°F. were followed by retempering
at a temperature of approximately 200°F. below the original tempering
temperature.
Another illustration of the importance of proper radii at all changes
of section is given in the case of the Liberty crankshaft. The presence
of tool marks or under cuts must be completely eliminated from an
aviation engine crankshaft to secure proper service. During the
duration of the Liberty program, four crankshafts failed from fatigue,
failures starting from sharp corners at bottom of propeller-hub
keyway. Two of the shafts that failed showed torsional spirals
running more than completely around the shaft. As soon as this
difficulty was removed no further trouble was experienced.
One of the most important difficulties encountered in connection
with the production of Liberty crankshafts was hair-line seams. The
question of hair-line seams has been discussed to greater length
by engineers and metallurgists during the war than any other single
question. Hair-line seams are caused by small non-metallic inclusions
in the steel. There is every reason to believe that these inclusions
are in the greater majority of cases manganese sulphide. There is
a great difference of opinion as to the exact effect of hair-line
seams on the service of an aviation engine crankshaft. It is the
opinion of many that hair-line seams do not in any way affect the
endurance of a crankshaft in service, provided they are parallel to
the grain of the steel and do not occur on a fillet. Of the 20,000
Liberty engines produced, fully 50 per cent of the crankshafts
used contain hair-line seams but not at the locations mentioned.
There has never been a failure of a Liberty crankshaft which could
in any way be traced to hair-line seams.
It was found that hair-line seams occur generally on high
nickel-chromium steels. One of the main reasons why the comparatively
mild analysis nickel-chromium steel was used was due to the very
few hair-line seams present in it. It was also determined that
the hair lines will in general be found near the surface of the
forgings. For that reason, as much finish as possible was allowed
for machining. A number of tests have been made on forging bars
to determine the depths at which hair-line seams are found, and
many cases came up in which hair-line seams were found 3/8 in.
from the surface of the bar. This means that in case a crankshaft
does not show hair-line seams on the ground surface this is no
indication that it is free from such a defect.
One important peculiarity of nickel-chromium steel was brought
out from the results obtained on impact tests. This peculiarity
is known as "blue brittleness." Just what the effect of this is
on the service of a finished part depends entirely upon the design
of the particular part in question. There have been no failures of
any nickel-chromium steel parts in the automotive industry which
could in any way be traced to this phenomena.
Whether or not nickel-chromium-steel forgings will show "blue
brittleness" depends entirely upon the temperature at which they
are tempered and their rate of cooling from this temperature. The
danger range for tempering nickel-chromium steels is between a
temperature of from 400 to 1,100°F. From the data so far gathered
on this phenomena, it is necessary that the nickel-chromium steel
to show "blue brittleness" be made by the acid process. There has
never come to my attention a single instance in which basic open
hearth steel has shown this phenomena. Just why the acid open hearth
steel should be sensitive to "blue brittleness" is not known.
All that is necessary to eliminate the presence of "blue brittleness"
is to quench all nickel-chromium-steel forgings in water from their
tempering temperature. The last 20,000 Liberty crankshafts that
were made were quenched in this manner.
PISTON PIN
The piston pin on an aviation engine must possess maximum resistance
to wear and to fatigue. For this reason, the piston pin is considered,
from a metallurgical standpoint, the most important part on the
engine to produce in quantities and still possess the above
characteristics. The material used for the Liberty engine piston
pin was S. A. E. No. 2315 steel, which is of the following chemical
composition: Carbon, 0.100 to 0.200 per cent; manganese, 0.500
to 0.800 per cent; phosphorus, 0.040 maximum per cent; sulphur,
0.045 maximum per cent; nickel, 3.250 to 3.750 per cent.
Each finished piston pin, after heat treatment, must show a minimum
scleroscope hardness of the case of 70, a scleroscope hardness of
the core of from 35 to 55 and a minimum crushing strength when
supported as a beam and the load applied at the center of 35,000
lb. The heat treatment used to obtain the above physical properties
consisted in carburizing at a temperature not to exceed 1,675°F.,
for a sufficient length of time to secure a case of from 0.02 to
0.04 in. deep. The pins are then allowed to cool slowly from the
carbonizing heat, after which the hole is finish-machined and the
pin cut to length. The finish heat treatment of the piston pin
consisted in quenching in oil from a temperature of from 1,525 to
1,575°F. to refine the grain of core properly and then quenching in
oil at a temperature of from 1,340 to 1,380°F. to refine and harden
the grain of the case properly, as well as to secure proper hardness
of core. After this quenching, all piston pins are tempered in oil
at a temperature of from 375 to 400°F. A 100 per cent inspection
for scleroscope hardness of the case and the core was made, and
no failures were ever recorded when the above material and heat
treatment was used.
APPLICATION TO THE AUTOMOTIVE INDUSTRY
The information given on the various parts of the Liberty engine
applies with equal force to the corresponding parts in the construction
of an automobile, truck or tractor. We recommend as first choice for
carbon-steel screw-machine parts material produced by the basic
open hearth process and having the following chemical composition;
Carbon, 0.150 to 0.250 per cent; manganese, 0.500 to 0.800 per
cent; phosphorus, 0.045 maximum per cent; sulphur, 0.075 to 0.150
per cent.
This material is very uniform and is nearly as free cutting as
bessemer screw stock. It is sufficiently uniform to be used for
unimportant carburized parts, as well as for non-heat-treated
screw-machine parts. A number of the large automobile manufacturers
are now specifying this material in preference to the regular bessemer
grades.
As second choice for carbon-steel screw-machine parts we recommend
ordinary bessemer screw stock, purchased in accordance with S. A.
E. specification No. 1114. The advantage of using No. 1114 steel
lies in the fact that the majority of warehouses carry standard
sizes of this material in stock at all times. The disadvantage
of using this material is due to its lack of uniformity.
The important criterion for transmission gears is resistance to
wear. To secure proper resistance to wear a Brinell hardness of
from 512 to 560 must be obtained. The material selected to obtain
this hardness should be one which can be made most nearly uniform,
will undergo forging operations the easiest, will be the hardest
to overheat or burn, will machine best and will respond to a good
commercial range of heat treatment.
It is a well-known fact that the element chromium, when in the form
of chromium carbide in alloy steel, offers the greatest resistance to
wear of any combination yet developed. It is also a well-known fact that
the element nickel in steel gives excellent shock-resisting properties
as well as resistance to wear but not nearly as great a resistance
to wear as chromium. It has been standard practice for a number of
years for many manufacturers to use a high nickel-chromium steel
for transmission gears. A typical nickel-chromium gear specification
is as follows: Carbon, 0.470 to 0.520 per cent; manganese, 0.500
to 0.800 per cent; phosphorus, 0.040 maximum per cent; sulphur,
0.045 maximum per cent; chromium, 0.700 to 0.950 per cent.
There is no question but that a gear made from material of such an
analysis will give excellent service. However, it is possible to
obtain the same quality of service and at the same time appreciably
reduce the cost of the finished part. The gear steel specified is
of the air-hardening type. It is extremely sensitive to secondary
pipe, as well as seams, and is extremely difficult to forge and
very easy to overheat. The heat-treatment range is very wide, but
the danger from quenching cracks is very great. In regard to the
machineability, this material is the hardest to machine of any
alloy steel known.
COMPOSITION OF TRANSMISSION-GEAR STEEL
If the nickel content of this steel is eliminated, and the percentage
of chromium raised slightly, an ideal transmission-gear material is
obtained. This would, therefore, be of the following composition:
Carbon, 0.470 to 0.520 per cent; manganese, 0.500 to 0.800 per
cent; phosphorus, 0.040 maximum per cent; sulphur, 0.045 maximum
per cent; chromium, 0.800 to 1.100 per cent.
The important criterion in connection with the use of this material
is that the steel be properly deoxidized, either through the use
of ferrovanadium or its equivalent. Approximately 2,500 sets of
transmission gears are being made daily from material of this analysis
and are giving entirely satisfactory results in service. The heat
treatment of the above material for transmission gears is as follows:
"Normalize forgings at a temperature of from 1,5.50 to 1,600°F.
Cool from this temperature to a temperature of 1,100°F. at the
rate of 50° per hour. Cool from 1,100°F., either in air or quench
in water."
Forgings so treated will show a Brinell hardness of from 177 to
217, which is the proper range for the best machineability. The
heat treatment of the finished gears consists of quenching in oil
from a temperature of 1,500 to 1,540°F., followed by tempering
in oil at a temperature of from 375 to 425°F. Gears so treated
will show a Brinell hardness of from 512 to 560, or a scleroscope
hardness of from 72 to 80. One tractor builder has placed in service
20,000 sets of gears of this type of material and has never had to
replace a gear. Taking into consideration the fact that a tractor
transmission is subjected to the worst possible service conditions,
and that it is under high stress 90 per cent of the time, it seems
inconceivable that any appreciable transmission trouble would be
experienced when material of this type is used on an automobile,
where the full load is applied not over 1 per cent of the time,
or on trucks where the full load is applied not over 50 per cent
of the time.
The gear hardness specified is necessary to reduce to a minimum
the pitting or surface fatigue of the teeth. If gears having a
Brinell hardness of over 560 are used, danger is encountered, due
to low shock-resisting properties. If the Brinell hardness is under
512, trouble is experienced due to wear and surface fatigue of
the teeth.
For ring gears and pinions material of the following chemical
composition is recommended: Carbon, 0.100 to 0.200 per cent; manganese,
0.350 to 0.650 per cent; phosphorus, 0.040 maximum per cent; sulphur,
0.045 maximum per cent; chromium, 0.550 to 0.750 per cent; nickel,
0.400 to 0.600 per cent.
Care should be taken to see that this material is properly deoxidized
either by the use of ferrovanadium or its equivalent. The advantage
of using a material of the above type lies in the fact that it will
produce a satisfactory finished part with a very simple treatment.
The heat treatment of ring gears and pinions is as follows: "Carburize
at a temperature of from 1,650 to 1,700°F. for a sufficient length
of time to secure a depth of case of from 1/32 to 3/64 in., and
quench directly from carburizing heat in oil. Reheat to a temperature
of from 1,430 to 1,460°F. and quench in oil. Temper in oil at a
temperature of from 375 to 425°F. The final quenching operation
on a ring gear should be made on a fixture similar to the Gleason
press to reduce distortion to a minimum."
One of the largest producers of ring gears and pinions in the automotive
industry has been using this material and treatment for the last 2
years, and is of the opinion that he is now producing the highest
quality product ever turned out by that plant.
On some designs of automobiles a large amount of trouble is experienced
with the driving pinion. If the material and heat treatment specified
will not give satisfaction, rather than to change the design it is
possible to use the following analysis material, which will raise
the cost of the finished part but will give excellent service:
Carbon, 0.100 to 0.200 per cent; manganese, 0.350 to 0.650 per
cent; phosphorus, 0.040 maximum per cent; sulphur, 0.045 maximum
per cent; nickel, 4.750 to 5.250 per cent.
The heat treatment of pinions produced from this material consists
in carburizing at a temperature of from 1,600 to 1,650°F. for a
sufficient length of time to secure a depth of case from 1/32 to
3/64 in. The pinions are then quenched in oil from a temperature
of 1,500 to 1,525°F. to refine the grain of the core and quenched
in oil from a temperature of from 1,340 to 1,360°F. To refine and
harden the case. The use of this material however, is recommended
only in an emergency, as high-nickel steel is very susceptible
to seams, secondary pipe and laminations.
The main criterion on rear-axle and pinion shafts, steering knuckles
and arms and parts of this general type is resistance to fatigue and
torsion. The material recommended for parts of this character is
either S. A. E. No. 6135 or No. 3135 steel, which have the chemical
composition given in Tables 9 and 7.
HEAT TREATMENT OF AXLES
Parts of this general type should be heat-treated to show the following
minimum physical properties: Elastic limit, 115,000 lb. per square
inch; elongation in 2 in., 16 per cent; reduction of area, 50 per
cent; Brinell hardness, 277 to 321.
The heat treatment used to secure these physical properties consists
in quenching from a temperature of from 1,520 to 1,540°F. in water
and tempering at a temperature of from 975 to 1,025°F. Where the
axle shaft is a forging, and in the case of steering knuckles and
arms, this heat treatment should be preceded by normalizing the
forgings at a temperature of from 1,550 to 1,600°F. It will be
noted that these physical properties correspond to those worked
out for an ideal aviation engine crankshaft. If parts of this type
are designed with proper sections, so that this range of physical
properties can be used, the part in question will give maximum
service.
One of the most important developments during the Liberty engine
program was the fact that it is not necessary to use a high-analysis
alloy steel to secure a finished part which will give proper service.
This fact should save the automotive industry millions of dollars
on future production.
If the proper authority be given the metallurgical engineer to
govern the handling of the steel from the time it is purchased
until it is assembled into finished product, mild-analysis steels
can be used and the quality of the finished product guaranteed.
It was only through the careful adherence to these fundamental
principles that it was possible to produce 20,000 Liberty engines,
which are considered to be the most highly stressed mechanism ever
produced, without the failure of a single engine from defective
material or heat treatment.
MAKING STEEL BALLS
Steel balls are made from rods or coils according to size, stock
less than 9/16-in. comes in coils. Stock 5/8-in. and larger comes
in rods. Ball stock is designated in thousandths so that 5/8-in.
rods are known as 0.625-in. stock.
Steel for making balls of average size is made up of:
Carbon 0.95 to 1.05 per cent
Silicon 0.20 to 0.35 per cent
Manganese 0.30 to 0.45 per cent
Chromium 0.35 to 0.45 per cent
Sulphur and phosphorus not to exceed 0.025 per cent
For the larger sizes a typical analysis is:
Carbon 1.02 per cent
Silicon 0.21 per cent
Manganese 0.40 per cent
Chromium 0.65 per cent
Sulphur 0.026 per cent
Phosphorus 0.014 per cent
Balls 5/8 in. and below are formed cold on upsetting or heading
machines, the stock use is as follows:
TABLE 14.--SIZES OF STOCK FOR FORMING BALLS ON HEADER
-------------------------------------------------------
Diameter of | Diameter of | Diameter of | Diameter of
ball, inch | stock inch | ball, inch | stock, inch
-------------|-------------|-------------|-------------
1/8 | 0.100 | 5/16 | 0.235
5/32 | 0.120 | 3/8 | 0.275
3/16 | 0.145 | 7/16 | 0.320
7/32 | 0.170 | 1/2 | 0.365
1/4 | 0.190 | 9/16 | 0.395
9/32 | 0.220 | 5/8 | 0.440
-------------------------------------------------------
For larger balls the blanks are hot-forged from straight bars.
They are usually forged in multiples of four under a spring hammer
and then separated by a suitable punching or shearing die in a
press adjoining the hammer. The dimensions are:
-----------------------------------------------------------
Diameter of ball, | Diameter of die, | Diameter of stock,
inch | inch | inch
-------------------|------------------|--------------------
3/4 | 0.775 | 0.625
7/8 | 0.905 | 0.729
1 | 1.035 | 0.823
-----------------------------------------------------------
Before hardening, the balls are annealed to relieve the stresses
of forging and grinding, this being done by passing them through a
revolving retort made of nichrome or other heat-resisting substance.
The annealing temperature is 1,300°F.
The hardening temperature is from 1,425 to 1,475°F. according to
size and composition of steel. Small balls, 5/16 and under, are
quenched in oil, the larger sizes in water. In some special cases
brine is used. Quenching small balls in water is too great a shock
as the small volume is cooled clear through almost instantly. The
larger balls have metal enough to cool more slowly.
Balls which are cooled in either water or brine are boiled in water
for 2 hr. to relieve internal stresses, after which the balls are
finished by dry-grinding and oil-grinding.
The ball makers have an interesting method of testing stock for
seams which do not show in the rod or wire. The Hoover Steel Ball
Company cut off pieces of rod or wire 7/16 in. long and subject
them to an end pressure of from 20,000 to 50,000 lb. A pressure
of 20,000 lb. compresses the piece to 3/16 in. and the 50,000 lb.
pressure to 3/32 in. This opens any seam which may exist but a
solid bar shows no seam.
Another method which has proved very successful is to pass the
bar or rod to be tested through a solenoid electro-magnet. With
suitable instruments it is claimed that this is an almost infallible
test as the instruments show at once when a seam or flaw is present
in the bar.
CHAPTER V
THE FORGING OF STEEL
So much depends upon the forging of steel that this operation must
be carefully supervised. This is especially true because of the
tendency to place unskilled and ignorant men as furnace-tenders
and hammer men. The main points to be supervised are the slow and
careful heating to the proper temperature; forging must be continued
at a proper rate to the correct temperature. The bar of stock from
which a forging was made may have had a fairly good structure, but
if the details of the working are not carefully watched, a seamy,
split article of no value may easily result.
HEATING.--Although it is possible to work steels cold, to an extent
depending upon their ductility, and although such operations are
commonly performed, "forging" usually means working _heated_ steel.
_Heating_ is therefore a vital part of the process.
Heating should be done slowly in a soaking heat. A soft "lazy"
flame with excess carbon is necessary to avoid burning the corners
of the bar or billet, and heavily scaling the surface. If the
temperature is not raised slowly, the outer part of the metal may
be at welding heat while the inner part is several hundred degrees
colder and comparatively hard and brittle.
The above refers to muffle furnaces. If the heating is done in
a small blacksmith's forge, the fire should be kept clean, and
remade at intervals of about two hours. Ashes and cinders should
be cleaned from the center down to the tuyere and oily waste and
wood used to start a new fire. As this kindles a layer of coke
from the old fire is put on top, and another layer of green coal
(screened and dampened blacksmiths' coal) as a cover. When the
green coal on top has been coked the fire is ready for use. As
the fuel burns out in the center, the coke forming around the edge
is pushed inward, and its place taken by more green coal. Thus the
fire is made up of three parts; the center where coke is burning
and the iron heating; a zone where coke is forming, and the outside
bank of green coal.
STEEL WORKED IN AUSTENITIC STATE.--As a general rule steel should
be worked when it is in the austenitic state. (See page 108.) It
is then soft and ductile.
As the steel is heated above the critical temperature the size of
the austenite crystals tends to grow rapidly. When forging starts,
however, these grains are broken up. The growth is continually
destroyed by the hammering, which should consequently be continued
down to the upper critical temperature when the austenite crystals
break up into ferrite and cementite. The size of the final grains
will be much smaller and hence a more uniform structure will result
if the "mother" austenite was also fine grained. A final steel
will be composed of pearlite; ferrite and pearlite; or cementite
and pearlite, according to the carbon content.
The ultimate object is to secure a fine, uniform grain throughout
the piece and this can be secured by uniform heating and by thoroughly
rolling it or working it at a temperature just down to its critical
point. If this is correctly done the fracture will be fine and
silky. Steel which has been overheated slightly and the forging
stopped at too high a temperature will show a "granular" fracture.
A badly overheated or "burned" steel will have iridescent colors
on a fresh fracture, it will be brittle both hot and cold, and
absolutely ruined.
STEEL CAN BE WORKED COLD.--As noted above, steel can be worked cold,
as in the case of cold-rolled steel. Heat treatment of cold-worked
steel is a very delicate operation. Cold working hardens and strengthens
steel. It also introduces internal stresses. Heat-treatments are
designed to eliminate the stresses without losing the hardness
and strength. This is done by tempering at a low heat. Avoid the
"blue" range (350 to 750°C.). Tempering for a considerable time just
under the critical is liable to cause great brittleness. Annealing
(reheating through the critical) destroys the effect of cold work.
FORGING
HIGH-SPEED STEEL.--Heat very slowly and carefully to from 1,800
to 2,000°F. and forge thoroughly and uniformly. If the forging
operation is prolonged do not continue forging the tool when the
steel begins to stiffen under the hammer. Do not forge below 1,700°F.
(a dark lemon or orange color). Reheat frequently rather than prolong
the hammering at the low heats.
After finishing the forging allow the tool to cool as slowly as
possible in lime or dry ashes; avoid placing the tool on the damp
ground or in a draught of air. Use a good clean fire for heating.
Do not allow the tool to soak at the forging heat. Do not heat any
more of the tool than is necessary in order to forge it to the
desired shape.
CARBON TOOL STEEL.--Heat to a bright red, about 1,500 to 1,550°F.
Do not hammer steel when it cools down to a dark cherry red, or
just below its hardening point, as this creates surface cracks.
OIL-HARDENING STEEL.--Heat slowly and uniformly to 1,450°F. and
forge thoroughly. Do not under any circumstances attempt to harden
at the forging heat. After cooling from forging reheat to about
1,450°F. and cool slowly so as to remove forging strains.
CHROME-NICKEL STEEL.--Forging heat of chrome-nickel steel depends
very largely on the percentage of each element contained in the
steel. Steel containing from 1/2 to 1 per cent chromium and from
1-1/2 to 3-1/2 per cent nickel, with a carbon content equal to
the chromium, should be heated very slowly and uniformly to
approximately 1,600° F., or salmon color. After forging, reheat
the steel to about 1,450° and cool slowly so as to remove forging
strains. Do not attempt to harden the steel before such annealing.
A great deal of steel is constantly being spoiled by carelessness
in the forging operation. The billets may be perfectly sound, but
even if the steel is heated to a good forging heat, and is hammered
too lightly, a poor forging results. A proper blow will cause the
edges and ends to bulge slightly outwards--the inner-most parts
of the steel seem to flow faster than the surface. Light blows
will work the surface out faster; the edges and ends will curve
inwards. This condition in extreme cases leaves a seam in the axis
of the forging.
Steel which is heated quickly and forging begun before uniform
heat has penetrated to its center will open up seams because the
cooler central portion is not able to flow with the hot metal
surrounding it. Uniform heating is absolutely necessary for the
best results.
Figure 16 shows a sound forging. The bars in Fig. 17 were burst
by improper forging, while the die, Fig. 18, burst from a piped
center.
Figure 19 shows a piece forged with a hammer too light for the size
of the work. This gives an appearance similar to case-hardening,
the refining effect of the blows reaching but a short distance
from the surface.
While it is impossible to accurately rate the capacity of steam
hammers with respect to the size of work they should handle, on
account of the greatly varying conditions, a few notes from the
experience of the Bement works of the Niles-Bement-Pond Company
will be of service.
[Illustration: FIG. 16.--A sound forging.]
[Illustration: FIG. 17.--Burst from improper forging.]
For making an occasional forging of a given size, a smaller hammer
may be used than if we are manufacturing this same piece in large
quantities. If we have a 6-in. piece to forge, such as a pinion or
a short shaft, a hammer of about 1,100-lb. capacity would answer
very nicely. But should the general work be as large as this, it
would be very much better to use a 1,500-lb. hammer. If, on the
other hand, we wish to forge 6-in. axles economically, it would
be necessary to use a 7,000- or 8,000-lb. hammer. The following
table will be found convenient for reference for the proper size
of hammer to be used on different classes of general blacksmith
work, although it will be understood that it is necessary to modify
these to suit conditions, as has already been indicated.
[Illustration: FIG. 18.--Burst from a piped center.]
[Illustration: FIG. 19.--Result of using too light a hammer.]
Diameter of stock Size of hammer
3-1/2 in. 250 to 350 lb.
4 in. 350 to 600 lb.
4-1/2 in. 600 to 800 lb.
5 in. 800 to 1,000 lb.
6 in. 1,100 to 1,500 lb.
Steam hammers are always rated by the weight of the ram, and the
attached parts, which include the piston and rod, nothing being
added on account of the steam pressure behind the piston. This makes
it a little difficult to compare them with plain drop or tilting
hammers, which are also rated in the same way.
[Illustration: FIG. 20.--Good and bad ingots.]
Steam hammers are usually operated at pressures varying from 75
to 100 lb. of steam per square inch, and may also be operated by
compressed air at about the same pressures. It is cheaper, however,
in the case of compressed air to use pressures from 60 to 80 lb.
instead of going higher.
Forgings must, however, be made from sound billets if satisfactory
results are to be secured. Figure 20 shows three cross-sections
of which _A_ is sound, _B_ is badly piped and _C_ is worthless.
PLANT FOR FORGING RIFLE BARRELS
The forging of rifle barrels in large quantities and heat-treating
them to meet the specifications demanded by some of the foreign
governments led Wheelock, Lovejoy & Company to establish a complete
plant for this purpose in connection with their warehouse in Cambridge,
Mass. This plant, designed and constructed by their chief engineer,
K. A. Juthe, had many interesting features. Many features of this
plant can be modified for other classes of work.
[Illustration: FIG 21.--Cutting up barrels.]
[Illustration: FIG. 22.--Upsetting the ends.]
The stock, which came in bars of mill length, was cut off so as to
make a barrel with the proper allowances for trimming (Fig. 21).
They then pass to the forging or upsetting press in the adjoining
room. This press, which is shown in more detail in Fig. 22, handled
the barrels from all the heating furnaces shown. The men changed
work at frequent intervals, to avoid excessive fatigue.
[Illustration: FIG. 23.--Continuous heating furnace.]
Then the barrels were reheated in the continuous furnace, shown
in Fig. 23, and straightened before being tested.
The barrels were next tested for straightness. After the heat-treating,
the ends are ground, a spot ground on the enlarged end and each
barrel tested on a Brinell machine. The pressure used is 3,000 kg.,
or 6,614 lb., on a 10-millimeter ball, which is standard. Hardness
of 240 was desired.
The heat-treating of the rifle blanks covered four separate operations:
(1) Heating and soaking the steel above the critical temperature
and quenching in oil to harden the steel through to the center;
(2) reheating for drawing of temper for the purpose of meeting the
physical specifications; (3) reheating to meet the machine ability
test for production purposes; and (4) reheating to straighten the
blanks while hot.
A short explanation of the necessity for the many heats may be
interesting. For the first heat, the blanks were slowly brought
to the required heat, which is about 150°F. above the critical
temperature. They are then soaked at a high heat for about 1 hr.
before quenching. The purpose of this treatment is to eliminate
any rolling or heat stresses that might be in the bars from mill
operations; also to insure a thorough even heat through a cross-section
of the steel. This heat also causes blanks with seams or slight
flaws to open up in quenching, making detection of defective blanks
very easy.
The quenching oil was kept at a constant temperature of 100°F.,
to avoid subjecting the steel to shocks, thereby causing surface
cracks. The drawing of temper was the most critical operation and
was kept within a 10° fluctuation. The degree of heat necessary
depends entirely on the analysis of the steel, there being a certain
variation in the different heats of steel as received from the mill.
MACHINEABILITY
Reheating for machine ability was done at 100° less than the drawing
temperature, but the time of soaking is more than double. After
both drawing and reheating, the blanks were buried in lime where
they remain, out of contact with the air, until their temperature
had dropped to that of the workroom.
For straightening, the barrels were heated to from 900 to 1,000°F.
in an automatic furnace 25 ft. long, this operation taking about 2
hr. The purpose of hot straightening was to prevent any stresses
being put into the blanks, so that after rough-turning, drilling
or rifling operations they would not have a tendency to spring
back to shape as left by the quenching bath.
A method that produces an even better machining rifle blank, which
practically stays straight through the different machining operations,
was to rough-turn the blanks, then subject them to a heat of practically
1,0000 for 4 hr. Production throughout the different operations is
materially increased, with practically no straightening required
after drilling, reaming, finish-turning or rifling operations.
[Illustration: FIG. 24. FIG. 25.
FIGS. 24 and 25.--Roof system of cooling quenching oil.]
This method was tested out by one of the largest manufacturers and
proved to be the best way to eliminate a very expensive finished
gun-barrel straightening process.
[Illustration: FIG. 26.--Details of the cooler.]
The heat-treating required a large amount of cooling oil, and the
problem of keeping this at the proper temperature required considerable
study. The result was the cooling plant on the roof, as shown in
Figs. 24, 25 and 26. The first two illustrations show the plant as
it appeared complete. Figure 26 shows how the oil was handled in
what is sometimes called the ebulator system. The oil was pumped
up from the cooling tanks through the pipe _A_ to the tank _B_.
From here it ran down onto the breakers or separators _C_, which
break the oil up into fine particles that are caught by the fans
_D_. The spray is blown up into the cooling tower _E_, which contains
banks of cooling pipes, as can be seen, as well as baffies _F_. The
spray collects on the cool pipes and forms drops, which fall on
the curved plates _G_ and run back to the oil-storage tank below
ground.
The water for this cooling was pumped from 10 artesian wells at the
rate of 60 gal. per minute and cooled 90 gal. of oil per minute,
lowering the temperature from 130 or 140 to 100°F. The water as
it came from the wells averaged around 52°F. The motor was of a
7-1/2-hp. variable-speed type with a range of from 700 to 1,200
r.p.m., which could be varied to suit the amount of oil to be cooled.
The plant handled 300 gal. of oil per minute.
CHAPTER VI
ANNEALING
There is no mystery or secret about the proper annealing of different
steels, but in order to secure the best results it is absolutely
necessary for the operator to know the kind of steel which is to
be annealed. The annealing of steel is primarily done for one of
three specific purposes: To soften for machining purposes; to change
the physical properties, largely to increase ductility; or to release
strains caused by rolling or forging.
Proper annealing means the heating of the steel slowly and uniformly
to the right temperature, the holding of the temperature for a given
period and the gradual cooling to normal temperature. The proper
temperature depends on the kind of steel, and the suggestions of the
maker of the special steel being used should be carefully followed.
For carbon steel the temperatures recommended for annealing vary
from 1,450 to 1,600°F. This temperature need not be long continued.
The steel should be cooled in hot sand, lime or ashes. If heated in
the open forge the steel should be buried in the cooling material
as quickly as possible, not allowing it to remain in the open air
any longer than absolutely necessary. Best results, however, are
secured when the fire does not come in direct contact with the
steel.
Good results are obtained by packing the steel in iron boxes or
tubes, much as for case-hardening or carbonizing, using the same
materials. Pieces do not require to be entirely surrounded by carbon
for annealing, however. Do not remove from boxes until cold.
Steel to be annealed may be classified into four different groups,
each of which must be treated according to the elements contained in
its particular analysis. Different methods are therefore necessary
to bring about the desired result. The classifications are as follows:
High-speed steel, alloy steel, tool or crucible steel, and high-carbon
machinery steel.
ANNEALING OF HIGH-SPEED STEEL
For annealing high-speed steel, some makers recommend using ground
mica, charcoal, lime, fine dry ashes or lake sand as a packing
in the annealing boxes. Mixtures of one part charcoal, one part
lime and three parts of sand are also suggested, or two parts of
ashes may be substituted for the one part of lime.
To bring about the softest structure or machine ability of high-speed
steel, it should be packed in charcoal in boxes or pipes, carefully
sealed at all points, so that no gases will escape or air be admitted.
It should be heated slowly to not less than 1,450°F. and the steel
must not be removed from its packing until it is cool. Slow heating
means that the high heat must have penetrated to the very core of
the steel.
When the steel is heated clear through it has been in the furnace
long enough. If the steel can remain in the furnace and cool down
with it, there will be no danger of air blasts or sudden or uneven
cooling. If not, remove the box and cover quickly with dry ashes,
sand or lime until it becomes cold.
Too high a heat or maintaining the heat for too long a period,
produces a harsh, coarse grain and greatly increases the liability
to crack in hardening. It also reduces the strength and toughness
of the steel.
Steel which is to be used for making tools with teeth, such as
taps, reamers and milling cutters, should not be annealed too much.
When the steel is too soft it is more apt to tear in cutting and
makes it more difficult to cut a smooth thread or other surface.
Moderate annealing is found best for tools of this kind.
TOOL OR CRUCIBLE STEEL
Crucible steel can be annealed either in muffled furnace or by
being packed. Packing is by far the most satisfactory method as it
prevents scaling, local hard spots, uneven annealing, or violent
changes in shape. It should be brought up slowly to just above
its calescent or hardening temperature. The operator must know
before setting his heats the temperature at which the different
carbon content steels are hardened. The higher the carbon contents
the lower is the hardening heat, but this should in no case be
less than 1,450°F.
ANNEALING ALLOY STEEL
The term alloy steel, from the steel maker's point of view, refers
largely to nickel and chromium steel or a combination of both. These
steels are manufactured very largely by the open-hearth process,
although chromium steels are also a crucible product. It is next
to impossible to give proper directions for the proper annealing
of alloy steel unless the composition is known to the operator.
Nickel steels may be annealed at lower temperatures than carbon
steels, depending upon their alloy content. For instance, if a
pearlitic carbon steel may be annealed at 1,450°C., the same analysis
containing 2-1/2 per cent nickel may be annealed at 1,360°C. and
a 5 per cent nickel steel at 1,270°.
In order that high chromium steels may be readily machined, they
must be heated at or slightly above the critical for a very long
time, and cooled through the critical at an extremely slow rate.
For a steel containing 0.9 to 1.1 per cent carbon, under 0.50 per
cent manganese, and about 1.0 per cent chromium, Bullens recommends
the following anneal:
1. Heat to 1,700 or 1,750°F.
2. Air cool to about 800°F.
3. Soak at 1,425 to 1,450°F.
4. Cool slowly in furnace.
HIGH-CARBON MACHINERY STEEL
The carbon content of this steel is above 30 points and is hardly
ever above 60 points or 0.60 per cent. Annealing such steel is
generally in quantity production and does not require the care that
the other steels need because it is very largely a much cheaper
product and a great deal of material is generally removed from
the outside surface.
The purpose for which this steel is annealed is a deciding factor
as to what heat to give it. If it is for machineability only, the
steel requires to be brought up slowly to just below the critical and
then slowly cooled in the furnace or ash pit. It must be thoroughly
covered so that there will be no access of cool air. If the annealing
is to increase ductility to the maximum extent it should be slowly
heated to slightly over the upper critical temperature and kept at
this heat for a length of time necessary for a thorough penetration
to the core, after which it can be cooled to about 1,200°F., then
reheated to about 1,360°F., when it can be removed and put in an
ash pit or covered with lime. If the annealing is just to relieve
strains, slow heating is not necessary, but the steel must be brought
up to a temperature not much less than a forging or rolling heat
and gradually cooled. Covering in this case is only necessary in
steel of a carbon content of more than 40 points.
ANNEALING IN BONE
Steel and cast iron may both be annealed in granulated bone. Pack the
work the same as for case-hardening except that it is not necessary
to keep the pieces away from each other. Pack with bone that has
been used until it is nearly white. Heat as hot as necessary for
the steel and let the furnace cool down. If the boxes are removed
from furnace while still warm, cover boxes and all in warm ashes
or sand, air slaked lime or old, burned bone to retain heat as
long as possible. Do not remove work from boxes until cold.
ANNEALING OF RIFLE COMPONENTS AT SPRINGFIELD ARMORY
In general, all forgings of the components of the arms manufactured
at the Armory and all forgings for other ordnance establishments
are packed in charcoal, lime or suitable material and annealed
before being transferred from the forge shop.
Except in special cases, all annealing will be done in annealing
pots of appropriate size. One fire end of a thermo-couple is inserted
in the center of the annealing pot nearest the middle of the furnace
and another in the furnace outside of but near the annealing pots.
The temperatures used in annealing carbon steel components of the
various classes used at the Armory vary from 800°C. To 880°C. or
1,475 to 1,615°F.
The fuel is shut off from the annealing furnace gradually as the
temperature of the pot approaches the prescribed annealing temperature
so as to prevent heating beyond that temperature.
The forgings of the rifle barrel and the pistol barrel are exceptions
to the above general rule. These forgings will be packed in lime
and allowed to cool slowly from the residual heat after forging.
CHAPTER VII
CASE-HARDENING OR SURFACE-CARBURIZING
Carburizing, commonly called case-hardening, is the art of producing
a high-carbon surface, or case, upon a low carbon steel article.
Wrenches, locomotive link motions, gun mechanisms, balls and ball
races, automobile gears and many other devices are thereby given
a high-carbon _case_ capable of assuming extreme hardness, while
the interior body of metal, the _core_, remains soft and tough.
The simplest method is to heat the piece to be hardened to a bright
red, dip it in cyanide of potassium (or cover it by sprinkling
the cyanide over it), keep it hot until the melted cyanide covers
it thoroughly, and quench in water. Carbon and nitrogen enter the
outer skin of the steel and harden this skin but leave the center
soft. The hard surface or "case" varies in thickness according to
the size of the piece, the materials used and the length of time
which the piece remains at the carburizing temperature. Cyanide
case-hardening is used only where a light or thin skin is sufficient.
It gives a thickness of about 0.002 in.
In some cases of cyanide carburizing, the piece is heated in cyanide
to the desired temperature and then quenched. For a thicker case
the steel is packed in carbon materials of various kinds such as
burnt leather scraps, charcoal, granulated bone or some of the
many carbonizing compounds.
Machined or forged steel parts are packed with case-hardening material
in metal boxes and subjected to a red heat. Under such conditions,
carbon is absorbed by the steel surfaces, and a carburized case is
produced capable of responding to ordinary hardening and tempering
operations, the core meanwhile retaining its original softness and
toughness.
Such case-hardened parts are stronger, cheaper, and more serviceable
than similar parts made of tool steel. The tough core resists breakage
by shock. The hardened case resists wear from friction. The low cost
of material, the ease of manufacture, and the lessened breakage
in quenching all serve to promote cheap production.
For successful carburizing, the following points should be carefully
observed:
The utmost care should be used in the selection of pots for carburizing;
they should be as free as possible from both scaling and warping.
These two requirements eliminate the cast iron pot, although many
are used, thus leaving us to select from malleable castings, wrought
iron, cast steel, and special alloys, such as nichrome or silchrome.
If first cost is not important, it will prove cheaper in the end
to use pots of some special alloy.
[Illustrations: FIGS. 27 to 30.--Case-hardening or carburizing boxes.]
[Illustration: FIG. 31.--A lid that is easily luted.]
The pots should be standardized to suit the product. Pots should be
made as small as possible in width, and space gained by increasing
the height; for it takes about 1-1/2 hr. to heat the average small
pot of 4 in. in width, between 3 and 4 hr. to heat to the center
of an 8-in. box, and 5 to 6 hr. to heat to the center of a 12-in.
box; and the longer the time required to heat to the center, the
more uneven the carburizing.
The work is packed in the box surrounded by materials which will
give up carbon when heated. It must be packed so that each piece
is separate from the others and does not touch the box, with a
sufficient amount of carburizing material surrounding each. Figures
27 to 31 show the kind of boxes used and the way the work should be
packed. Figure 31 shows a later type of box in which the edges can
be easily luted. Figure 30 shows test wires broken periodically to
determine the depth of case. Figure 28 shows the minimum clearance
which should be used in packing and Fig. 29 the way in which the
outer pieces receive the heat first and likewise take up the carbon
before those in the center. This is why a slow, soaking heat is
necessary in handling large quantities of work, so as to allow
the heat and carbon to soak in equally.
While it has been claimed that iron below its critical temperature
will absorb some carbon, Giolitti has shown that this absorption
is very slow. In order to produce quick and intense carburization
the iron should preferably be above its upper critical temperature
or 1,600°F.,--therefore the carbon absorbed immediately goes into
austenite, or solid solution. It is also certain that the higher
the temperature the quicker will carbon be absorbed, and the deeper
it will penetrate into the steel, that is, the deeper the "case."
At Sheffield, England, where wrought iron is packed in charcoal and
heated for days to convert it into "blister steel," the temperatures
are from 1,750 to 1,830°F. Charcoal by itself carburizes slowly,
consequently commercial compounds also contain certain "energizers"
which give rapid penetration at lower temperatures.
The most important thing in carburizing is the human element. Most
careful vigilance should be kept when packing and unpacking, and the
operator should be instructed in the necessity for clean compound
free from scale, moisture, fire clay, sand, floor sweepings, etc.
From just such causes, many a good carburizer has been unjustly
condemned. It is essential with most carburizers to use about 25 to
50 per cent of used material, in order to prevent undue shrinking
during heating; therefore the necessity of properly screening used
material and carefully inspecting it for foreign substances before
it is used again. It is right here that the greatest carelessness
is generally encountered.
Don't pack the work to be carburized too closely; leave at least
1 in. from the bottom, 3/4 in. from the sides, and 1 in. from the
top of pots, and for a 6-hr. run, have the pieces at least 1/2
in. apart. This gives the heat a chance to thoroughly permeate
the pot, and the carburizing material a chance to shrink without
allowing carburized pieces to touch and cause soft spots.
Good case-hardening pots and annealing tubes can be made from the
desired size of wrought iron pipe. The ends are capped or welded,
and a slot is cut in the side of the pot, equal to one quarter of
its circumference, and about 7/8 of its length. Another piece of
the same diameter pipe cut lengthwise into thirds forms a cover
for this pot. We then have a cheap, substantial pot, non-warping,
with a minimum tendency to scale, but the pot is difficult to seal
tightly. This idea is especially adaptable when long, narrow pots
are desired.
When pots are packed and the carburizer thoroughly tamped down,
the covers of the pot are put on and sealed with fire clay which
has a little salt mixed into it. The more perfect the seal the
more we can get out of the carburizer. The rates of penetration
depend on temperature and the presence of proper gas in the required
volume. Any pressure we can cause will, of course, have a tendency
to increase the rate of penetration.
If you have a wide furnace, do not load it full at one time. Put
one-half your load in first, in the center of the furnace, and
heat until pots show a low red, about 1,325 to 1,350°F. Then fill
the furnace by putting the cold pots on the outside or, the section
nearest the source of heat. This will give the work in the slowest
portion of the furnace a chance to come to heat at the same time
as the pots that are nearest the sources of heat.
To obtain an even heating of the pots and lessen their tendency
to warp and scale, and to cause the contents of the furnace to
heat up evenly, we should use a reducing fire and fill the heating
chamber with flame. This can be accomplished by partially closing
the waste gas vents and reducing slightly the amount of air used
by the burners. A short flame will then be noticed issuing from
the partially closed vents. Thus, while maintaining the temperature
of the heating chamber, we will have a lower temperature in the
combustion chamber, which will naturally increase its longevity.
Sometimes it is advisable to cool the work in the pots. This saves
compound, and causes a more gradual diffusion of the carbon between
the case and the core, and is very desirable condition, inasmuch
as abrupt cases are inclined to chip out.
The most satisfactory steel to carburize contains between 0.10
and 0.20 per cent carbon, less than 0.35 per cent manganese, less
than 0.04 per cent phosphorus and sulphur, and low silicon. But
steel of this composition does not seem to satisfy our progressive
engineers, and many alloy steels are now on the market, these,
although more or less difficult to machine, give when carburized
the various qualities demanded, such as a very hard case, very tough
core, or very hard case and tough core. However, the additional
elements also have a great effect both on the rate of penetration
during the carburizing operation, and on the final treatment,
consequently such alloy steels require very careful supervision
during the entire heat treating operations.
RATE OF ABSORPTION
According to Guillet, the absorption of carbon is favored by those
special elements which exist as double carbides in steel. For example,
manganese exists as manganese carbide in combination with the iron
carbide. The elements that favor the absorption of carbon are:
manganese, tungsten, chromium and molybdenum those opposing it,
nickel, silicon, and aluminum. Guillet has worked out the effect
of the different elements on the rate of penetration in comparison
with steel that absorbed carbon at a given temperature, at an average
rate of 0.035 in. per hour.
His tables show that the following elements require an increased
time of exposure to the carburizing material in order to obtain
the same depth of penetration as with simple steel:
When steel contains Increased time of exposure
2.0 per cent nickel 28 per cent
7.0 per cent nickel 30 per cent
1.0 per cent titanium 12 per cent
2.0 per cent titanium 28 per cent
0.5 per cent silicon 50 per cent
1.0 per cent silicon 80 per cent
2.0 per cent silicon 122 per cent
5.0 per cent silicon No penetration
1.0 per cent aluminum 122 per cent
2.0 per cent aluminum 350 per cent
The following elements seem to assist the rate of penetration of
carbon, and the carburizing time may therefore be reduced as follows:
When steel contains Decreased time of exposure
0.5 per cent manganese 18 per cent
1.0 per cent manganese 25 per cent
1.0 per cent chromium 10 per cent
2.0 per cent chromium 18 per cent
0.5 per cent tungsten 0
1.0 per cent tungsten 0
2.0 per cent tungsten 25 per cent
1.0 per cent molybdenum 0
2.0 per cent molybdenum 18 per cent
The temperature at which carburization is accomplished is a very
important factor. Hence the necessity for a reliable pyrometer,
located so as to give the temperature just below the tops of the
pots. It must be remembered, however, that the pyrometer gives
the temperature of only one spot, and is therefore only an aid to
the operator, who must use his eyes for successful results.
The carbon content of the case generally is governed by the temperature
of the carburization. It generally proves advisable to have the
case contain between 0.90 per cent and 1.10 carbon; more carbon
than this gives rise to excess free cementite or carbide of iron,
which is detrimental, causing the case to be brittle and apt to chip.
T. G. Selleck gives a very useful table of temperatures and the
relative carbon contents of the case of steels carburized between
4 and 6 hrs. using a good charcoal carburizer. This data is as
follows:
TABLE 15.--CARBON CONTENT OBTAINED AT VARIOUS TEMPERATURES
At 1,500°F., the surface carbon content will be 0.90 per cent
At 1,600°F., the surface carbon content will be 1.00 per cent
At 1,650°F., the surface carbon content will be 1.10 per cent
At 1,700°F., the surface carbon content will be 1.25 per cent
At 1,750°F., the surface carbon content will be 1.40 per cent
At 1,800°F., the surface carbon content will be 1.75 per cent
To this very valuable table, it seems best to add the following
data, which we have used for a number of years. We do not know
the name of its author, but it has proved very valuable, and seems
to complete the above information. The table is self-explanatory,
giving depth of penetration of the carbon of the case at different
temperatures for different lengths of time:
---------------------------------------------------------
| Temperature
Penetration |-----------------------------
| 1,550 | 1,650 | 1,800
---------------------------|---------|---------|---------
Penetration after 1/2 hr. | 0.008 | 0.012 | 0.030
Penetration after 1 hr. | 0.018 | 0.026 | 0.045
Penetration after 2 hr. | 0.035 | 0.048 | 0.060
Penetration after 3 hr. | 0.045 | 0.055 | 0.075
Penetration after 4 hr. | 0.052 | 0.061 | 0.092
Penetration after 6 hr. | 0.056 | 0.075 | 0.110
Penetration after 8 hr. | 0.062 | 0.083 | 0.130
---------------------------------------------------------
From the tables given, we may calculate with a fair degree of certainty
the amount of carbon in the case, and its penetration. These figures
vary widely with different carburizers, and as pointed out immediately
above, with different alloy steels.
CARBURIZING MATERIAL
The simplest carburizing substance is charcoal. It is also the
slowest, but is often used mixed with something that will evolve
large volumes of carbon monoxide or hydrocarbon gas on being heated.
A great variety of materials is used, a few of them being charcoal
(both wood and bone), charred leather, crushed bone, horn, mixtures
of charcoal and barium carbonate, coke and heavy oils, coke treated
with alkaline carbonates, peat, charcoal mixed with common salt,
saltpeter, resin, flour, potassium bichromate, vegetable fibre,
limestone, various seed husks, etc. In general, it is well to avoid
complex mixtures.
H. L. Heathcote, on analyzing seventeen different carburizers, found
that they contained the following ingredients:
Per cent
Moisture 2.68 to 26.17
Oil 0.17 to 20.76
Carbon (organic) 6.70 to 54.19
Calcium phosphate 0.32 to 74.75
Calcium carbonate 1.20 to 11.57
Barium carbonate nil to 42.00
Zinc oxide nil to 14.50
Silica nil to 8.14
Sulphates (SO3) trace to 3.45
Sodium chloride nil to 7.88
Sodium carbonate nil to 40.00
Sulphides (S) nil to 2.80
Carburizing mixtures, though bought by weight, are used by volume,
and the weight per cubic foot is a big factor in making a selection.
A good mixture should be porous, so that the evolved gases, which
should be generated at the proper temperature, may move freely
around the steel objects being carburized; should be a good conductor
of heat; should possess minimum shrinkage when used; and should
be capable of being tamped down.
Many "secret mixtures" are sold, falsely claimed to be able to
convert inferior metal into crucible tool steel grade. They are
generally nothing more than mixtures of carbonaceous and cyanogen
compounds possessing the well-known carburizing properties of those
substances.
QUENCHING
It is considered good practice to quench alloy steels from the pot,
especially if the case is of any appreciable depth. The texture
of carbon steel will be weakened by the prolonged high heat of
carburizing, so that if we need a tough core, we must reheat it
above its critical range, which is about 1,600°F. for soft steel,
but lower for manganese and nickel steels. Quenching is done in
either water, oil, or air, depending upon the results desired.
The steel is then very carefully reheated to refine the case, the
temperature varying from 1,350 to 1,450°F., depending on whether
the material is an alloy or a simple steel, and quenched in either
water or oil.
[Illustration: FIG. 32.--Case-hardening depths.]
There are many possibilities yet to be developed with the carburizing
of alloy steels, which can produce a very tough, tenacious austenitic
case which becomes hard on cooling in air, and still retains a
soft, pearlitic core. An austenitic case is not necessarily file
hard, but has a very great resistance to abrasive wear.
The more carbon a steel has to begin with the more slowly will it
absorb carbon and the lower the temperature required. Low-carbon
steel of from 15 to 20 points is generally used and the carbon
brought up to 80 or 85 points. Tool steels may be carbonized as
high as 250 points.
In addition to the carburizing materials given, a mixture of 40
per cent of barium carbonate and 60 per cent charcoal gives much
faster penetration than charcoal, bone or leather. The penetration
of this mixture on ordinary low-carbon steel is shown in Fig. 32,
over a range of from 2 to 12 hr.
EFFECT OF DIFFERENT CARBURIZING MATERIAL
[Illustrations: FIGS. 33 to 37.]
Each of these different packing materials has a different effect
upon the work in which it is heated. Charcoal by itself will give
a rather light case. Mixed with raw bone it will carburize more
rapidly, and still more so if mixed with burnt bone. Raw bone and
burnt bone, as may be inferred, are both quicker carbonizers than
charcoal, but raw bone must never be used where the breakage of
hardened edges is to be avoided, as it contains phosphorus and
tends to make the piece brittle. Charred leather mixed with charcoal
is a still faster material, and horns and hoofs exceed even this
in speed; but these two compounds are restricted by their cost
to use with high-grade articles, usually of tool or high-carbon
steel, that are to be hardened locally--that is, "pack-hardened."
Cyanide of potassium or prussiate of potash are also included in
the list of carbonizing materials; but outside of carburizing by
dipping into melted baths of this material, their use is largely
confined to local hardening of small surfaces, such as holes in
dies and the like.
Dr. Federico Giolitti has proven that when carbonizing with charcoal,
or charcoal plus barium carbonate, the active agent which introduces
carbon into the steel is a gas, carbon monoxide (CO), derived by
combustion of the charcoal in the air trapped in the box, or by
decomposition of the carbonate. This gas diffuses in and out of
the hot steel, transporting carbon from the charcoal to the outer
portions of the metal:
If energizers like tar, peat, and vegetable fiber are used, they
produce hydrocarbon gases on being heated--gases principally composed
of hydrogen and carbon. These gases are unstable in the presence of
hot iron: it seems to decompose them and sooty carbon is deposited
on the surface of the metal. This diffuses into the metal a little,
but it acts principally by being a ready source of carbon, highly
active and waiting to be carried into the metal by the carbon
monoxide--which as before, is the principal transfer agent.
Animal refuse when used to speed up the action of clean charcoal
acts somewhat in the same manner, but in addition the gases given
off by the hot substance contain nitrogen compounds. Nitrogen and
cyanides (compounds of carbon and nitrogen) have long been known
to give a very hard thin case very rapidly. It has been discovered
only recently that this is due to the steel absorbing nitrogen
as well as carbon, and that nitrogen hardens steel and makes it
brittle just like carbon does. In fact it is very difficult to
distinguish between these two hardening agents when examining a
carburized steel under the microscope.
One of the advantages of hardening by carburizing is the fact that
you can arrange to leave part of the work soft and thus retain
the toughness and strength of the original material. Figures 33
to 37 show ways of doing this. The inside of the cup in Fig. 34
is locally hardened, as illustrated in Fig. 34, "spent" or used
bone being packed around the surfaces that are to be left soft,
while cyanide of potassium is put around those which are desired
hard. The threads of the nut in Fig. 35 are kept soft by carburizing
the nut while upon a stud. The profile gage, Fig. 36, is made of
high-carbon steel and is hardened on the inside by packing with
charred leather, but kept soft on the outside by surrounding it
with fireclay. The rivet stud shown in Fig. 37 is carburized while
of its full diameter and then turned down to the size of the rivet
end, thus cutting away the carburized surface.
After packing the work carefully in the boxes the lids are sealed
or luted with fireclay to keep out any gases from the fire. The
size of box should be proportioned to the work. The box should
not be too large especially for light work that is run on a short
heat. If it can be just large enough to allow the proper amount
of material around it, the work is apt to be more satisfactory
in every way.
Pieces of this kind are of course not quenched and hardened in
the carburizing heat, but are left in the box to cool, just as in
box annealing, being reheated and quenched as a second operation.
In fact, this is a good scheme to use for the majority of carburizing
work of small and moderate size. Material is on the market with which
one side of the steel can be treated; or copper-plating one side
of it will answer the same purpose and prevent that side becoming
carburized.
QUENCHING THE WORK
In some operations case-hardened work is quenched from the box by
dumping the whole contents into the quenching tank. It is common
practice to leave a sieve or wire basket to catch the work, allowing
the carburizing material to fall to the bottom of the tank where it
can be recovered later and used again as a part of a new mixture.
For best results, however, the steel is allowed to cool down slowly
in the box after which it is removed and hardened by heating and
quenching the same as carbon steel of the same grade. It has absorbed
sufficient carbon so that, in the outer portions at least, it is
a high-carbon steel.
THE QUENCHING TANK
The quenching tank is an important feature of apparatus in
case-hardening--possibly more so than in ordinary tempering. One
reason for this is because of the large quantities of pieces usually
dumped into the tank at a time. One cannot take time to separate
the articles themselves from the case-hardening mixture, and the
whole content of the box is droped into the bath in short order,
as exposure to air of the heated work is fatal to results. Unless
it is split up, it is likely to go to the bottom as a solid mass,
in which case very few of the pieces are properly hardened.
[Illustration: FIG. 38.--Combination cooling tank for case-hardening.]
A combination cooling tank is shown in Fig. 38. Water inlet and
outlet pipes are shown and also a drain plug that enables the tank
to be emptied when it is desired to clean out the spent carburizing
material from the bottom. A wire-bottomed tray, framed with angle
iron, is arranged to slide into this tank from the top and rests
upon angle irons screwed to the tank sides. Its function is to
catch the pieces and prevent them from settling to the tank bottom,
and it also makes it easy to remove a batch of work. A bottomless
box of sheet steel is shown at _C_. This fits into the wire-bottomed
tray and has a number of rods or wires running across it, their
purpose being to break up the mass of material as it comes from
the carbonizing box.
Below the wire-bottomed tray is a perforated cross-pipe that is
connected with a compressed-air line. This is used when case-hardening
for colors. The shop that has no air compressor may rig up a
satisfactory equivalent in the shape of a low-pressure hand-operated
air pump and a receiver tank, for it is not necessary to use
high-pressure air for this purpose. When colors are desired on
case-hardened work, the treatment in quenching is exactly the same
as that previously described except that air is pumped through
this pipe and keeps the water agitated. The addition of a slight
amount of powdered cyanide of potassium to the packing material
used for carburizing will produce stronger colors, and where this is
the sole object, it is best to maintain the box at a dull-red heat.
[Illustration: FIG. 39.--Why heat treatment of case-hardened work
is necessary.]
The old way of case-hardening was to dump the contents of the box
at the end of the carburizing heat. Later study in the structure
of steel thus treated has caused a change in this procedure, the
use of automobiles and alloy steels probably hastening this result.
The diagrams reproduced in Fig. 39 show why the heat treatment of
case-hardened work is necessary. Starting at _A_ with a close-grained
and tough stock, such as ordinary machinery steel containing from 15
to 20 points of carbon, if such work is quenched on a carbonizing
heat the result will be as shown at _B_. This gives a core that is
coarse-grained and brittle and an outer case that is fine-grained
and hard, but is likely to flake off, owing to the great difference
in structure between it and the core. Reheating this work beyond
the critical temperature of the core refines this core, closes
the grain and makes it tough, but leaves the case very brittle;
in fact, more so than it was before.
REFINING THE GRAIN
This is remedied by reheating the piece to a temperature slightly
above the critical temperature of the case, this temperature
corresponding ordinarily to that of steel having a carbon content
of 85 points, When this is again quenched, the temperature, which
has not been high enough to disturb the refined core, will have
closed the grain of the case and toughened it. So, instead of but
one heat and one quenching for this class of work, we have three
of each, although it is quite possible and often profitable to
omit the quenching after carburizing and allow the piece or pieces
and the case-carburizing box to cool together, as in annealing.
Sometimes another heat treatment is added to the foregoing, for
the purpose of letting down the hardness of the case and giving
it additional toughness by heating to a temperature between 300°
and 500°. Usually this is done in an oil bath. After this the piece
is allowed to cool.
It is possible to harden the surface of tool steel extremely hard
and yet leave its inner core soft and tough for strength, by a
process similar to case-hardening and known as "pack-hardening."
It consists in using tool steel of carbon contents ranging from
60 to 80 points, packing this in a box with charred leather mixed
with wood charcoal and heating at a low-red heat for 2 or 3 hr.,
thus raising the carbon content of the exterior of the piece. The
article when quenched in an oil bath will have an extremely hard
exterior and tough core. It is a good scheme for tools that must
be hard and yet strong enough to stand abuse. Raw bone is never
used as a packing for this class of work, as it makes the cutting
edges brittle.
CASE-HARDENING TREATMENTS FOR VARIOUS STEELS
Plain water, salt water and linseed oil are the three most common
quenching materials for case-hardening. Water is used for ordinary
work, salt water for work which must be extremely hard on the surface,
and oil for work in which toughness is the main consideration. The
higher the carbon of the case, the less sudden need the quenching
action take hold of the piece; in fact, experience in case-hardening
work gives a great many combinations of quenching baths of these
three materials, depending on their temperatures. Thin work, highly
carbonized, which would fly to pieces under the slightest blow if
quenched in water or brine, is made strong and tough by properly
quenching in slightly heated oil. It is impossible to give any
rules for the temperature of this work, so much depending on the
size and design of the piece; but it is not a difficult matter to
try three or four pieces by different methods and determine what
is needed for best results.
The alloy steels are all susceptible of case-hardening treatment;
in fact, this is one of the most important heat treatments for such
steels in the automobile industry. Nickel steel carburizes more
slowly than common steel, the nickel seeming to have the effect
of slowing down the rate of penetration. There is no cloud without
its silver lining, however, and to offset this retardation, a single
treatment is often sufficient for nickel steel; for the core is not
coarsened as much as low-carbon machinery steel and thus ordinary
work may be quenched on the carburizing heat. Steel containing
from 3 to 3.5 per cent of nickel is carburized between 1,650 and
1,750°F. Nickel steel containing less than 25 points of carbon,
with this same percentage of nickel, may be slightly hardened by
cooling in air instead of quenching.
Chrome-nickel steel may be case-hardened similarly to the method just
described for nickel steel, but double treatment gives better results
and is used for high-grade work. The carburizing temperature is the
same, between 1,650 and 1,750°F., the second treatment consisting
of reheating to 1,400° and then quenching in boiling salt water,
which gives a hard surface and at the same time prevents distortion
of the piece. The core of chrome-nickel case-hardened steel, like
that of nickel steel, is not coarsened excessively by the first
heat treatment, and therefore a single heating and quenching will
suffice.
CARBURIZING BY GAS
The process of carburizing by gas, briefly mentioned on page 88,
consists of having a slowly revolving, properly heated, cylindrical
retort into which illuminating gas (a mixture of various hydrocarbons)
is continuously injected under pressure. The spent gases are vented
to insure the greatest speed in carbonizing. The work is constantly
and uniformly exposed to a clean carbonizing atmosphere instead
of partially spent carbonaceous solids which may give off very
complex compounds of phosphorus, sulphur, carbon and nitrogen.
Originally this process was thought to require a gas generator but
it has been discovered that city gas works all right. The gas consists
of vapors derived from petroleum or bituminous coal. Sometimes the
gas supply is diluted by air, to reduce the speed of carburization
and increase the depth.
PREVENTING CARBURIZING BY COPPER-PLATING
Copper-plating has been found effective and must have a thickness
of 0.0005 in. Less than this does not give a continuous coating.
The plating bath used has a temperature of 170°F. A voltage of
4.1 is to be maintained across the terminals. Regions which are
to be hardened can be kept free from copper by coating them with
paraffin before they enter the plating tank. The operation is as
follows:
Operation
No. Contents of bath Purpose
1 Gasoline To remove grease
2 Sawdust To dry
3 Warm potassium hydroxide solution To remove grease and dirt
4 Warm water To wash
5 Warm sulphuric acid solution To acid clean
6 Warm water To wash
7 Cold water Additional wash
8 Cold potassium cyanide solution Cleanser
9 Cold water To wash
10 Electric cleaner, warm sodium Cleanser to give good
hydroxide case-iron anode plating surface
11 Copper plating bath of copper Plating bath
sulphate and potassium cyanide
solution warm
There are also other methods of preventing case-hardening, one
being to paint the surface with a special compound prepared for
this purpose. In some cases a coating of plastic asbestos is used
while in others thin sheet asbestos is wired around the part to
be kept soft.
PREPARING PARTS FOR LOCAL CASE-HARDENING
At the works of the Dayton Engineering Laboratories Company, Dayton,
Ohio, they have a large quantity of small shafts, Fig. 40, that
are to be case-hardened at _A_ while the ends _B_ and _C_ are to
be left soft. Formerly, the part _A_ was brush-coated with melted
paraffin but, as there were many shafts, this was tedious and great
care was necessary to avoid getting paraffin where it was not wanted.
[Illustration: FIG. 40.--Shaft to be coated with paraffin.]
To insure uniform coating the device shown in Fig. 41 was made.
Melted paraffin is poured in the well _A_ and kept liquid by setting
the device on a hot plate, the paraffin being kept high enough
to touch the bottoms of the rollers. The shaft to be coated is
laid between the rollers with one end against the gage _B_, when
a turn or two of the crank _C_ will cause it to be evenly coated.
[Illustration: FIG. 41.--Device for coating the shaft.]
THE PENETRATION OF CARBON
Carburized mild steel is used to a great extent in the manufacture
of automobile and other parts which are likely to be subjected to
rough usage. The strength and ability to withstand hard knocks
depend to a very considerable degree on the thoroughness with which
the carburizing process is conducted.
Many automobile manufacturers have at one time or another passed
through a period of unfortunate breakages, or have found that for
a certain period the parts turned out of their hardening shops
were not sufficiently hard to enable the rubbing surfaces to stand
up against the pressure to which they were subjected.
So many factors govern the success of hardening that often this
succession of bad work has been actually overcome without those
interested realizing what was the weak point in their system of
treatment. As the question is one that can create a bad reputation
for the product of any firm it is well to study the influential
factors minutely.
INTRODUCTION OF CARBON
The matter to which these notes are primarily directed is the
introduction of carbon into the case of the article to be hardened.
In the first place the chances of success are increased by selecting
as few brands of steel as practicable to cover the requirements of
each component of the mechanism. The hardener is then able to become
accustomed to the characteristics of that particular material, and
after determining the most suitable treatment for it no further
experimenting beyond the usual check-test pieces is necessary.
Although a certain make of material may vary in composition from
time to time the products of a manufacturer of good steel can be
generally relied upon, and it is better to deal directly with him
than with others.
In most cases the case-hardening steels can be chosen from the
following: (1) Case-hardening mild steel of 0.20 per cent carbon;
(2) case-hardening 3-1/2 per cent nickel steel; (3) case-hardening
nickel-chromium steel; (4) case-hardening chromium vanadium. After
having chosen a suitable steel it is best to have the sample analyzed
by reliable chemists and also to have test pieces machined and pulled.
To prepare samples for analysis place a sheet of paper on the table
of a drilling machine, and with a 3/8-in. diameter drill, machine
a few holes about 3/8 in. deep in various parts of the sample bar,
collecting about 3 oz. of fine drillings free from dust. This can be
placed in a bottle and dispatched to the laboratory with instructions
to search for carbon, silicon, manganese, sulphur, phosphorus and
alloys. The results of the different tests should be carefully
tabulated, and as there would most probably be some variation an
average should be made as a fair basis of each element present,
and the following tables may be used with confidence when deciding
if the material is reliable enough to be used.
TABLE 16.--CASE-HARDENING MILD STEEL OF 0.20 PER CENT CARBON
Carbon 0.15 to 0.25 per cent
Silicon Not over 0.20 per cent
Manganese 0.30 to 0.60 per cent
Sulphur Not over 0.04 per cent
Phosphorus Not over 0.04 per cent
A tension test should register at least 60,000 lb. per square inch.
TABLE 17.--CASE-HARDENING 3-1/2 PER CENT NICKEL STEEL
Carbon 0.12 to 0.20 per cent
Manganese 0.65 per cent
Sulphur Not over 0.045 per cent
Phosphorus Not over 0.04 per cent
Nickel 3.25 to 3.75 per cent
TABLE 18.--CASE-HARDENING NICKEL CHROMIUM STEEL
Carbon 0.15 to 0.25 per cent
Manganese 0.50 to 0.80 per cent
Sulphur Not over 0.045 per cent
Phosphorus Not over 0.04 per cent
Nickel 1 to 1.5 per cent
Chromium 0.45 to 0.75 per cent
TABLE 19.--CASE-HARDENING CHROMIUM VANADIUM STEEL
Carbon Not over 0.25 per cent
Manganese 0.50 to 0.85 per cent
Sulphur Not over 0.04 per cent
Phosphorus Not over 0.04 per cent
Chromium 0.80 to 1.10 per cent
Vanadium Not less than 0.15 per cent
Having determined what is required we now proceed to inquire into
the question of carburizing, which is of vital importance.
USING ILLUMINATING GAS
The choice of a carburizing furnace depends greatly on the facilities
available in the locality where the shop is situated and the nature
and quantity of the work to be done. The furnaces can be heated with
producer gas in most cases, but when space is of value illuminating
gas from a separate source of supply has some compensations. When
the latter is used it is well to install a governor if the pressure
is likely to fluctuate, particularly where the shop is at a high
altitude or at a long distance from the gas supply.
Many furnaces are coal-fired, and although greater care is required
in maintaining a uniform temperature good results have been obtained.
The use of electricity as a means of reaching the requisite temperature
is receiving some attention, and no doubt it would make the control
of temperature comparatively simple. However, the cost when applied
to large quantities of work will, for the present at least, prevent
this method from becoming popular. It is believed that the results
obtainable \with the electric furnace would surpass any others; but
the apparatus is expensive, and unless handled with intelligence
would not last long.
The most elementary medium of carburization is pure carbon, but
the rate of carburization induced by this material is very low,
and other components are necessary to accelerate the process. Many
mixtures have been marketed, each possessing its individual merits,
and as the prices vary considerably it is difficult to decide which
is the most advantageous.
Absorption from actual contact with solid carbon is decidedly slow,
and it is necessary to employ a compound from which gases are liberated,
and the steel will absorb the carbon from the gases much more readily.
Both bone and leather charcoal give off more carburizing gases
than wood charcoal, and although the high sulphur content of the
leather is objectionable as being injurious to the steel, as also
is the high phosphorus content of the bone charcoal, they are both
preferable to the wood charcoal.
By mixing bone charcoal with barium carbonate in the proportions
of 60 per cent of the former to 40 per cent of the latter a very
reliable compound is obtained.
The temperature to which this compound is subjected causes the
liberation of carbon monoxide when in contact with hot charcoal.
Many more elaborate explanations may be given of the actions and
reactions taking place, but the above is a satisfactory guide to
indicate that it is not the actual compound which causes carburization,
but the gases released from the compound.
Until the temperature of the muffle reaches about 1,300°F. carburization
does not take place to any useful extent, and consequently it is
advisable to avoid the use of any compound from which the carburizing
gases are liberated much before that temperature is reached. In
the case of steel containing nickel slightly higher temperatures
may be used and are really necessary if the same rate of carbon
penetration is to be obtained, as the presence of nickel resists
the penetration.
At higher temperatures the rate of penetration is higher, but not
exactly in proportion to the temperature, and the rate is also
influenced by the nature of the material and the efficiency of the
compound employed.
The so-called saturation point of mild steel is reached when the
case contains 0.90 per cent of carbon, but this amount is frequently
exceeded. Should it be required to ascertain the amount of carbon
in a sample at varying depths below the skin this can be done by
turning off a small amount after carburizing and analyzing the
turnings. This can be repeated several times, and it will probably
be found that the proportion of carbon decreases as the test piece
is reduced in diameter unless decarburization has taken place.
[Illustration: FIG. 42.--Chart showing penetration of carbon.]
The chart, Fig. 42, is also a good guide.
In order to use the chart it is necessary to harden the sample
we desire to test as we would harden a piece of tool steel, and
then test by scleroscope. By locating on the chart the point on
the horizontal axis which represents the hardness of the sample
the curve enables one to determine the approximate amount of carbon
present in the case.
Should the hardness lack uniformity the soft places can be identified
by etching. To accomplish this the sample should be polished after
quenching and then washed with a weak solution of nitric acid in
alcohol, whereupon the harder points will show up darker than the
softer areas.
The selection of suitable boxes for carburizing is worthy of a
little consideration, and there can be no doubt that in certain
cases results are spoiled and considerable expense caused by using
unsuitable containers.
As far as initial expense goes cast-iron boxes are probably the
most expedient, but although they will withstand the necessary
temperatures they are liable to split and crack, and when they
get out of shape there is much difficulty in straightening them.
The most suitable material in most cases is steel boiler plate 3/8
or 1/2 in. thick, which can be made with welded joints and will
last well.
The sizes of the boxes employed depend to a great extent on the
nature of the work being done, but care should be exercised to
avoid putting too much in one box, as smaller ones permit the heat
to penetrate more quickly, and one test piece is sufficient to
give a good indication of what has taken place. If it should be
necessary to use larger boxes it is advisable to put in three or four
test pieces in different positions to ascertain if the penetration
of carbon has been satisfactory in all parts of the box, as it
is quite possible that the temperature of the muffle is not the
same at all points, and a record shown by one test piece would
not then be applicable to all the parts contained in the box. It
has been found that the rate of carbon penetration increases with
the gas pressure around the articles being carburized, and it is
therefore necessary to be careful in sealing up the boxes after
packing. When the articles are placed within and each entirely
surrounded by compound so that the compound reaches to within 1
in. of the top of the box a layer of clay should be run around the
inside of the box on top of the compound. The lid, which should
be a good fit in the box, is then to be pressed on top of this,
and another layer of clay run just below the rim of the box on
top of the cover.
A SATISFACTORY LUTING MIXTURE
A mixture of fireclay and sand will be found very satisfactory
for closing up the boxes, and by observing the appearance of the
work when taken out we can gage the suitability of the methods
employed, for unless the boxes are carefully sealed the work is
generally covered with dark scales, while if properly done the
articles will be of a light gray.
By observing the above recommendations reliable results can be obtained,
and we can expect uniform results after quenching.
GAS CONSUMPTION FOR CARBURIZING
Although the advantages offered by the gas-fired furnace for carburizing
have been generally recognized in the past from points of view as
close temperature regulation, decreased attendance, and greater
convenience, very little information has been published regarding
the consumption of gas for this process. It has therefore been a
matter of great difficulty to obtain authentic information upon
this point, either from makers or users of such furnaces.
In view of this, the details of actual consumption of gas on a
regular customer's order job will be of interest. The "Revergen"
furnace, manufactured by the Davis Furnace Company, Luton, Bedford,
England, was used on this job, and is provided with regenerators
and fired with illuminating gas at ordinary pressure, the air being
introduced to the furnace at a slight pressure of 3 to 4 in. water
gage. The material was charged into a cold furnace, raised to 1,652°F.,
and maintained at that temperature for 8 hr. to give the necessary
depth of case. The work consisted of automobile gears packed in
six boxes, the total weight being 713 lb. The required temperature
of 1,652°F. was obtained in 70 min. from lighting up, and a summary
of the data is shown in the following table:
Cubic Foot Total
Per Pound Number of
of Load Cubic Foot
Gas to raise furnace and charge from
cold to 1,652°F., 70 min. 1.29 925
Gas to maintain 1,652°F. for 1st hour 0.38 275
Gas to maintain 1,652°F. for 2nd hour 0.42 300
Gas to maintain 1,652°F. for 3rd hour 0.38 275
Gas to maintain 1,652°F. for 4th hour 0.42 300
Gas to maintain 1,652°F. for 5th hour 0.49 350
Gas to maintain 1,652°F. for 6th hour 0.49 350
Gas to maintain 1,652°F. for 7th hour 0.45 325
Gas to maintain 1,652°F. for 8th hour 0.45 325
The overall gas consumption for this run of 9 hr. 10 min. was only
4.8 cu. ft. per pound of load.
THE CARE OF CARBURIZING COMPOUNDS
Of all the opportunities for practicing economy in the heat-treatment
department, there is none that offers greater possibilities for
profitable returns than the systematic cleaning, blending and reworking
of artificial carburizers, or compounds.
The question of whether or not it is practical to take up the work
depends upon the nature of the output. If the sole product of the
hardening department consists of a 1.10 carbon case or harder,
requiring a strong highly energized material of deep penetrative
power such as that used in the carburizing of ball races, hub-bearings
and the like, it would be best to dispose of the used material to
some concern whose product requires a case with from 0.70 to 0.90
carbon, but where there is a large variety of work the compound
may be so handled that there will be practically no waste.
This is accomplished with one of the most widely known artificial
carburizers by giving all the compound in the plant three distinct
classifications: "New," being direct from the maker; "half and
half," being one part of new and one part first run; and "2 to 1,"
which consists of two parts of old and one part new.
SEPARATING THE WORK FROM THE COMPOUND
During the pulling of the heat, the pots are dumped upon a cast-iron
screen which forms a table or apron for the furnace. Directly beneath
this table is located one of the steel conveyor carts, shown in Fig.
43, which is provided with two wheels at the rear and a dolly clevis
at the front, which allows it to be hauled away from beneath the
furnace apron while filled with red-hot compound. A steel cover is
provided for each box, and the material is allowed to cool without
losing much of the evolved gases which are still being thrown off
by the compound.
[Illustration: FIG. 43.--The cooling carts.]
[Illustration: FIG. 44.--Machine for blending the mixture.]
As this compound comes from the carburizing pots it contains bits
of fireclay which represent a part of the luting used for sealing,
and there may be small parts of work or bits of fused material
in it as well. After cooling, the compound is very dusty and
disagreeable to handle, and, before it can be used again, must be
sifted, cleaned and blended.
Some time ago the writer was confronted with this proposition for
one of the largest consumers of carburizing compound in the world,
and the problem was handled in the following manner: The cooled
compound was dumped from the cooling cars and sprinkled with a
low-grade oil which served the dual purposes of settling the dust
and adding a certain percentage of valuable hydrocarbon to the
compound. In Fig. 44 is shown the machine that was designed to do
the cleaning and blending.
BLENDING THE COMPOUND
Essentially, this consists of the sturdy, power-driven separator
and fanning mill which separates the foreign matter from the compound
and elevates it into a large settling basin which is formed by
the top of the steel housing that incloses the apparatus. After
reaching the settling basin, the compound falls by gravity into
a power-driven rotary mixing tub which is directly beneath the
settling basin. Here the blending is done by mixing the proper
amount of various grades of material together. After blending the
compound, it is ready to be stored in labeled containers and delivered
to the packing room.
It will be seen that by this simple system there is the least possible
loss of energy from the compound. The saving commences the moment
the cooling cart is covered and preserves the valuable dust which is
saved by the oiling and the settling basin of the blending machine.
Then, too, there is the added convenience of the packers who have
a thoroughly cleaned, dustless, and standardized product to work
with. Of course, this also tends to insure uniformity in the
case-hardening operation.
With this outfit, one man cleans and blends as much compound in
one hour as he formerly did in ten.
CHAPTER VII
HEAT TREATMENT OF STEEL
Heat treatment consists in heating and cooling metal at definite
rates in order to change its physical condition. Many objects may
be attained by correct heat treatment, but nothing much can be
expected unless the man who directs the operations knows what is
the essential difference in a piece of steel at room temperature
and at a red heat, other than the obvious fact that it is hot. The
science of metallography has been developed in the past 25 years,
and aided by precise methods of measuring temperature, has done
much to systematize the information which we possess on metallic
alloys, and steel in particular.
CRITICAL POINTS
One of the most important means of investigating the properties of
pure metals and their alloys is by an examination of their heating
and cooling curves. Such curves are constructed by taking a small
piece and observing and recording the temperature of the mass at
uniform intervals of time during a _uniform_ heating or cooling.
These observations, when plotted in the form of a curve will show
whether the temperature of the mass rises or falls uniformly.
The heat which a body absorbs serves either to raise the temperature
of the mass or change its physical condition. That portion of the
heat which results in an increase in temperature of the body is
called "sensible heat," inasmuch as such a gain in heat is apparent
to the physical senses of the observer. If heat were supplied to the
body at a uniform rate, the temperature would rise continuously,
and if the temperature were plotted against time, a smooth rising
curve would result. Or, if sensible heat were abstracted from the
body at a uniform rate, a time-temperature curve would again be a
smooth falling curve. Such a curve is called a "cooling curve."
However, we find that when a body is melting, vaporizing, or otherwise
suffering an abrupt change in physical properties, a quantity of
heat is absorbed which disappears without changing the temperature
of the body. This heat absorbed during a change of state is called
"latent heat," because it is transformed into the work necessary to
change the configuration and disposition of the molecules in the
body; but it is again liberated in equal amount when the reverse
change takes place.
From these considerations it would seem that should the cooling
curve be continuous and smooth, following closely a regular course,
all the heat abstracted during cooling is furnished at the expense
of a fall in temperature of the body; that is to say, it disappears
as "sensible heat." These curves, however, frequently show horizontal
portions or "arrests" which denote that at that temperature all
of the heat constantly radiating is being supplied by internal
changes in the alloy itself; that is, it is being supplied by the
evolution of a certain amount of "latent heat."
In addition to the large amount of heat liberated when a metal
solidifies, there are other changes indicated by the thermal analysis
of many alloys which occur _after_ the body has become entirely
solidified. These so-called transformation points or ranges may
be caused by chemical reactions taking place within the solid,
substances being precipitated from a "solid solution," or a sudden
change in some physical property of the components, such as in
magnetism, hardness, or specific gravity.
It may be difficult to comprehend that such changes can occur in
a body after it has become entirely solidified, owing to the usual
conception that the particles are then rigidly fixed. However, this
rigidity is only comparative. The molecules in the solid state
have not the large mobility they possess as a liquid, but even so,
they are still moving in circumscribed orbits, and have the power,
under proper conditions, to rearrange their position or internal
configuration. In general, such rearrangement is accompanied by a
sudden change in some physical property and in the total energy
of the molecule, which is evidenced by a spontaneous evolution or
absorption of latent heat.
Cooling curves of the purest iron show at least two well-defined
discontinuities at temperatures more than 1,000°F., below its
freezing-point. It seems that the soft, magnetic metal so familiar
as wrought iron, and called "alpha iron" or "ferrite" by the
metallurgist, becomes unstable at about 1,400°F. and changes into
the so-called "beta" modification, becoming suddenly harder, and
losing its magnetism. This state in turn persists no higher than
1,706°C., when a softer, non-magnetic "gamma" iron is the stable
modification up to the actual melting-point of the metal. These
various changes occur in electrolytic iron, and therefore cannot be
attributed to any chemical reaction or solution; they are entirely
due to the existence of "allotropic modifications" of the iron in
its solid state.
[Illustration: FIG. 45.--Inverse Rate Cooling Curve of 0.38 C Steel.]
Steels, or iron containing a certain amount of carbon, develop
somewhat different cooling curves from those produced by pure iron.
Figure 45 shows, for instance, some data observed on a cooling
piece of 0.38 per cent carbon steel, and the curve constructed
therefrom. It will be noted that the time was noted when the needle
on the pyrometer passed each dial marking. If the metal were not
changing in its physical condition, the time between each reading
would be nearly constant; in fact for a time it required about 50
sec. to cool each unit. When the dial read about 32.5 (corresponding
in this instrument to a temperature of 775°C. or 1,427°F.) the
cooling rate shortened materially, 55 sec. then 65, then 100, then
100; showing that some change inside the metal was furnishing some
of the steadily radiating heat. This temperature is the so-called
"upper critical" for this steel. Further down, the "lower critical"
is shown by a large heat evolution at 695°C. or 1,283°F.
Just the reverse effects take place upon heating, except that the
temperatures shown are somewhat higher--there seems to be a lag
in the reactions taking place in the steel. This is an important
point to remember, because if it was desired to anneal a piece of
0.38 carbon steel, it is necessary to heat it up to and beyond
1,476° F. (1,427°F. _plus_ this lag, which may be as much as 50°).
It may be said immediately that above the upper critical the carbon
exists in the iron as a "solid solution," called "austenite" by
metallographers. That is to say, it is uniformly distributed as atoms
throughout the iron; the atoms of carbon are not present in any fixed
combination, in fact any amount of carbon from zero to 1.7 per cent
can enter into solid solution above the upper critical. However,
upon cooling this steel, the carbon again enters into combination
with a definite proportion of iron (the carbide "cementite," Fe3C),
and accumulates into small crystals which can be seen under a good
microscope. Formation of all the cementite has been completed by
the time the temperature has fallen to the lower critical, and
below that temperature the steel exists as a complex substance
of pure iron and the iron carbide.
It is important to note that the critical points or critical range
of a plain steel varies with its carbon content. The following
table gives some average figures:
Carbon Content. Upper Critical. Lower Critical.
0.00 1,706°F. 1,330°F.
0.20 1,600°F. 1,330°F.
0.40 1,480°F. 1,330°F.
0.60 1,400°F. 1,330°F.
0.80 1,350°F. 1,330°F.
0.90 1,330°F. 1,330°F.
1.00 1,470°F. 1,330°F.
1.20 1,650°F. 1,330°F.
1.40 1,830°F. 1,330°F.
1.60 2,000°F. 1,330°F.
It is immediately noted that the critical range narrows with increasing
carbon content until all the heat seems to be liberated at one
temperature in a steel of 0.90 per cent carbon. Beyond that composition
the critical range widens rapidly. Note also that the lower critical
is constant in plain carbon steels containing no alloying elements.
[Illustration: FIG. 46.--Microphotograph of steel used in S. K.
F. bearings, polished and etched with nitric acid and magnified
1,000 times. Made by H. O. Walp.]
This steel of 0.90 carbon content is an important one. It is called
"eutectoid" steel. Under the microscope a properly polished and
etched sample shows the structure to consist of thin sheets of
two different substances (Fig. 46). One of these is pure iron,
and the other is pure cementite. This structure of thin sheets
has received the name "pearlite," because of its pearly appearance
under sunlight. Pearlite is a constituent found in all annealed
carbon steels. Pure iron, having no carbon, naturally would show no
pearlite when examined under a microscope; only abutting granules
of iron are delicately traced. The metallographist calls this pure
iron "ferrite." As soon as a little carbon enters the alloy and a
soft steel is formed, small angular areas of pearlite appear at the
boundaries of the ferrite crystals (Fig. 47). With increasing carbon
in the steel the volume of iron crystals becomes less and less, and
the relative amount of pearlite increases, until arriving at 0.90
per cent carbon, the large ferrite crystals have been suppressed and
the structure is all pearlite. Higher carbon steels show films of
cementite outlining grains of pearlite (Fig. 48).
This represents the structure of annealed, slowly cooled steels.
It is possible to change the relative sizes of the ferrite and
cementite crystals by heat treatment. Large grains are associated
with brittleness. Consequently one must avoid heat treatments which
produce coarse grains.
[Illustration: FIG. 47.--Structure of low carbon steel, polished,
etched and viewed under 100 magnifications. Tiny white granules
of pure iron (ferrite) have small accumulations of dark-etching
pearlite interspersed between them. Photograph by H. S. Rawdon.]
[Illustration: FIG. 48.--Slowly cooled high-carbon steel, polished,
etched and viewed at 100 magnifications. The dark grains are pearlite,
separated by white films of iron carbide (cementite). Photograph
by H. S. Rawdon.]
In general it may be said that the previous crystalline structure
of a steel is entirely obliterated when it passes just through the
critical range. At that moment, in fact, the ferrite, cementite or
pearlite which previously existed has lost its identity by everything
going into the solid solution called austenite. If sufficient time
is given, the chemical elements comprising a good steel distribute
themselves uniformly through the mass. If the steel be then cooled,
the austenite breaks up into new crystals of ferrite, cementite
and pearlite; and in general if the temperature has not gone far
above the critical, and cooling is not excessively slow, a very
fine texture will result. This is called "refining" the grain;
or in shop parlance "closing" the grain. However, if the heating
has gone above the critical very far, the austenite crystals start
to grow; a very short time at an extreme temperature will cause
a large grain growth. Subsequent cooling gives a coarse texture,
or an arrangement of ferrite, cementite and pearlite grains which
is greatly coarsened, reflecting the condition of the austenite
crystals from which they were born.
It maybe noted in passing that the coarse crystals of cast metal
cannot generally be refined by heat treatment unless some forging
or rolling has been done in the meantime. Heat treatment alone does
not seem to be able to break up the crystals of an ingot structure.
HARDENING
Steel is hardened by quenching from above the upper critical. Apparently
the quick cooling prevents the normal change back to definite and
sizeable crystals of ferrite and cementite. Hardness is associated
with this suppressed change. If the change is allowed to continue
by a moderate reheating, like a tempering, the hardness decreases.
If a piece of steel could be cooled instantly, doubtless austenite
could be preserved and examined. In the ordinary practice of hardening
steels, the quenching is not so drastic, and the transformation of
austenite back to ferrite and cementite is more or less completely
effected, giving rise to certain transitory forms which are known
as "martensite," "troostite," "sorbite," and finally, pearlite.
Austenite has been defined as a solid solution of cementite (Fe3C)
in gamma iron. It is stable at various temperatures dependent upon
its carbon content, which may be any amount up to the saturated
solution containing 1.7 per cent. Austenite is not nearly as hard
as martensite, owing to its content of the soft gamma iron. Fig.
49 shows austenite to possess the typical appearance of any pure,
crystallized substance.
In the most quickly quenched high carbon steels, austenite commonly
forms the ground mass which is interspersed with martensite, a large
field of which is illustrated in Fig. 50. Martensite is usually
considered to be a solid solution of cementite in beta iron. It
represents an unstable condition in which the metal is caught during
rapid cooling. It is very hard, and is the chief constituent of
hardened high-carbon steels, and of medium-carbon nickel-steel
and manganese-steel.
Troostite is of doubtful composition, but possibly is an unstable
mixture of untransformed martensite with sorbite. It contains more
or less untransformed material, as it is too hard to be composed
entirely of the soft alpha modification, and it can also be tempered
more or less without changing in appearance. Its normal appearance as
rounded grains is given in Fig. 51; larger patches show practically
no relief in their structure, and a photograph merely shows a dark,
structureless area.
[Illustration: FIG. 49.--Coarse-grained martensite, polished and
etched with nitric acid and magnified 50 times. Made by Prof. Chas.
Y. Clayton.]
Sorbite is believed to be an early stage in the formation of pearlite,
when the iron and iron carbide originally constituting the solid
solution (austenite) have had an opportunity to separate from each
other, and the iron has entirely passed into the alpha modification,
but the particles are yet too small to be distinguishable under
the microscope. It also, possibly, contains some incompletely
transformed matter. Sorbite is softer and tougher than troostite,
and is habitually associated with pearlite. Its components are
tending to coagulate into pearlite, and will do so in a fairly
short time at temperatures near the lower critical, which heat will
furnish the necessary molecular freedom. The normal appearance,
however, is the cloudy mass shown in Fig. 52.
Pearlite is a definite conglomerate of ferrite and cementite containing
about six parts of the former to one of the latter. When pure, it
has a carbon content of about 0.95 per cent. It represents the
complete transformation of the eutectoid austenite accomplished by
slow-cooling of an iron-carbon alloy through the transformation
range. (See Fig. 46.)
[Illustration: FIG. 50.--Quenched high-carbon steel, polished,
etched and viewed at 100 magnifications. This structure is called
martensite and is desired when maximum hardness is essential. Photograph
by H. S. Rawdon.]
[Illustration: FIG. 51.--Martensite (light needles) passing into
troosite (dark patches). 130 X. From a piece of eutectoid steel
electrically welded.]
[Illustration: FIG. 52.--Sorbite (dark patches) passing into pearlite
(wavy striations). Light Areas are Patches of Ferrite. 220 X. From
a piece of hypo-eutectoid steel electrically welded.]
These observations are competent to explain annealing and toughening
practice. A quickly quenched carbon steel is mostly martensitic
which, as noted, is a solid solution of beta iron and cementite,
hard and brittle. Moderate reheating or annealing changes this
structure largely into troostite, which is a partly transformed
martensite, possessing much of the hardness of martensite, but with
a largely increased toughness and shock resistance. This toughness is
the chief characteristic of the next material in the transformation
series, sorbite, which is merely martensite wholly transformed into
a mixture of ultramicroscopic crystals of ferrite (alpha iron)
and cementite (Fe3C).
"Tempering" or "drawing" should be restricted to mean moderate
reheating, up to about 350° C., forming troostitic steel. "Toughening"
represents the practice of reheating hardened carbon steels from
350° C. up to just below the lower critical, and forms sorbitic
steel; while "annealing" refers to a heating for grain size at
or above the transformation ranges, followed by a slow cooling.
Any of these operations not only allows the transformations from
austenite to pearlite to proceed, but also relieves internal stresses
in the steel.
Normalizing is a heating like annealing, followed by a moderately
rapid quench.
JUDGING THE HEAT OF STEEL
While the use of a pyrometer is of course the only way to have
accurate knowledge as to the heat being used in either forging or
hardening steels, a color chart will be of considerable assistance
if carefully studied. These have been prepared by several of the
steel companies as a guide, but it must be remembered that the colors
and temperatures given are only approximate, and can be nothing
else.
[Illustration: FIG. 53.--Finding hardening heats with a magnet.]
_The Magnet Test_.--The critical point can also be determined by
an ordinary horse-shoe magnet. Touch the steel with a magnet during
the heating and when it reaches the temperature at which steel fails
to attract the magnet, or in other words, loses its magnetism,
the critical point has been reached.
Figures 53 and 54 show how these are used in practice.
The first (Fig. 53) shows the use of a permanent horse-shoe magnet
and the second (Fig. 54) an electro-magnet consisting of an iron
rod with a coil or spool magnet at the outer end. In either case
the magnet should not be allowed to become heated but should be
applied quickly.
[Illustration: FIG. 54.--Using electro-magnet to determine heat.]
The work is heated up slowly in the furnace and the magnet applied
from time to time. The steel being heated will attract the magnet
until the heat reaches the critical point. The magnet is applied
frequently and when the magnet is no longer attracted, the piece
is at the lowest temperature at which it can be hardened properly.
Quenching slightly above this point will give a tool of satisfactory
hardness. The method applies only to carbon steels and will not
work for modern high-speed steels.
HEAT TREATMENT OF GEAR BLANKS
This section is based on a paper read before the American Gear
Manufacturers' Association at White Sulphur Springs, W. Va., Apr.
18, 1918.
Great advancement has been made in the heat treating and hardening of
gears. In this advancement the chemical and metallurgical laboratory
have played no small part. During this time, however, the condition
of the blanks as they come to the machine shop to be machined has
not received its share of attention.
There are two distinct types of gears, both types having their
champions, namely, carburized and heat-treated. The difference
between the two in the matter of steel composition is entirely in
the carbon content, the carbon never running higher than 25-point
in the carburizing type, while in the heat-treated gears the carbon
is seldom lower than 35-point. The difference in the final gear
is the hardness. The carburized gear is file hard on the surface,
with a soft, tough and ductile core to withstand shock, while the
heat-treated gear has a surface that can be touched by a file with
a core of the same hardness as the outer surface.
ANNEALING WORK.--With the exception of several of the higher types
of alloy steels, where the percentages of special elements run quite
high, which causes a slight air-hardening action, the carburizing
steels are soft enough for machining when air cooled from any
temperature, including the finishing temperature at the hammer.
This condition has led many drop-forge and manufacturing concerns
to consider annealing as an unnecessary operation and expense.
In many cases the drop forging has only been heated to a low
temperature, often just until the piece showed color, to relieve
the so-called hammer strains. While this has been only a compromise
it has been better than no reheating at all, although it has not
properly refined the grain, which is necessary for good machining
conditions.
Annealing is heating to a temperature slightly above the highest
critical point and cooling slowly either in the air or in the furnace.
Annealing is done to accomplish two purposes: (1) to relieve mechanical
strains and (2) to soften and produce a maximum refinement of grain.
PROCESS OF CARBURIZING.--Carburizing imparts a shell of high-carbon
content to a low-carbon steel. This produces what might be termed
a "dual" steel, allowing for an outer shell which when hardened
would withstand wear, and a soft ductile core to produce ductility
and withstand shock. The operation is carried out by packing the
work to be carburized in boxes with a material rich in carbon and
maintaining the box so charged at a temperature in excess of the
highest critical point for a length of time to produce the desired
depth of carburized zone. Generally maintaining the temperature
at 1,650 to 1,700° F. for 7 hr. will produce a carburized zone
1/32 in. deep.
Heating to a temperature slightly above the highest critical point
and cooling suddenly in some quenching medium, such as water or oil
hardens the steel. This treatment produces a maximum refinement
with the maximum strength.
Drawing to a temperature below the highest critical point (the
temperature being governed by the results required) relieves the
hardening strains set up by quenching, as well as the reducing
of the hardness and brittleness of hardened steel.
EFFECTS OF PROPER ANNEALING.--Proper annealing of low-carbon steels
causes a complete solution or combination to take place between
the ferrite and pearlite, producing a homogeneous mass of small
grains of each, the grains of the pearlite being surrounded by
grains of ferrite. A steel of this refinement will machine to good
advantage, due to the fact that the cutting tool will at all times
be in contact with metal of uniform composition.
While the alternate bands of ferrite and pearlite are microscopically
sized, it has been found that with a Gleason or Fellows gear-cutting
machine that rough cutting can be traced to poorly annealed steels,
having either a pronounced banded structure or a coarse granular
structure.
TEMPERATURE FOR ANNEALING.--Theoretically, annealing should be
accomplished at a temperature at just slightly above the critical
point. However, in practice the temperature is raised to a higher
point in order to allow for the solution of the carbon and iron to
be produced more rapidly, as the time required to produce complete
solution is reduced as the temperature increases past the critical
point.
For annealing the simpler types of low-carbon steels the following
temperatures have been found to produce uniform machining conditions
on account of producing uniform fine-grain pearlite structure:
0.15 to 0.25 per cent carbon, straight carbon steel.--Heat to 1,650°F.
Hold at this temperature until the work is uniformly heated; pull
from the furnace and cool in air.
0.15 to 0.25 per cent carbon, 1-1/2 per cent nickel, 1/2 per cent
chromium steel.--Heat to 1,600°F. Hold at this temperature until
the work is uniformly heated; pull from the furnace and cool in air.
0.15 to 0.25 per cent carbon, 3-1/2 per cent nickel steel.--Heat
to 1,575°F. Hold at this temperature until the work is uniformly
heated; pull from the furnace and cool in air.
CARE IN ANNEALING.--Not only will benefits in machining be found
by careful annealing of forgings but the subsequent troubles in
the hardening plant will be greatly reduced. The advantages in
the hardening start with the carburizing operation, as a steel of
uniform and fine grain size will carburize more uniformly, producing
a more even hardness and less chances for soft spots. The holes in
the gears will also "close in more uniformly," not causing some
gears to require excessive grinding and others with just enough
stock. Also all strains will have been removed from the forging,
eliminating to a great extent distortion and the noisy gears which
are the result.
With the steels used, for the heat-treated gears, always of a higher
carbon content, treatment after forging is necessary for machining, as
it would be impossible to get the required production from untreated
forgings, especially in the alloy steels. The treatment is more
delicate, due to the higher percentage of carbon and the natural
increase in cementite together with complex carbides which are
present in some of the higher types of alloys.
Where poor machining conditions in heat-treated steels are present
they are generally due to incomplete solution of cementite rather
than bands of free ferrite, as in the case of case-hardening steels.
This segregation of carbon, as it is sometimes referred to, causes
hard spots which, in the forming of the tooth, cause the cutter
to ride over the hard metal, producing high spots on the face of
the tooth, which are as detrimental to satisfactory gear cutting
as the drops or low spots produced on the face of the teeth when
the pearlite is coarse-grained or in a banded condition.
In the simpler carburized steels it is not necessary to test the
forgings for hardness after annealing, but with the high percentages
of alloys in the carburizing steels and the heat-treated steels
a hardness test is essential.
To obtain the best results in machining, the microstructure of the
metal should be determined and a hardness range set that covers
the variations in structure that produce good machining results.
By careful control of the heat-treating operation and with the aid
of the Brinell hardness tester and the microscope it is possible
to continually give forgings that will machine uniformly and be
soft enough to give desired production. The following gives a few
of the hardness numerals on steel used in gear manufacture that
produce good machining qualities:
0.20 per cent carbon, 3 per cent nickel, 1-1/4; per cent
chromium--Brinell 156 to 170.
0.50 per cent carbon, 3 per cent nickel, 1 per cent chromium--Brinell
179 to 187.
0.50 per cent carbon chrome-vanadium--Brinell 170 to 179.
THE INFLUENCE OF SIZE
The size of the piece influences the physical properties obtained in
steel by heat treatment. This has been worked out by E. J. Janitzky,
metallurgical engineer of the Illinois Steel Company, as follows:
[Illustration: FIG. 55.--Effect of size on heating.]
"With an increase in the mass of steel there is a corresponding
decrease in both the minimum surface hardness and depth hardness,
when quenched from the same temperature, under identical conditions
of the quenching medium. In other words, the physical properties
obtained are a function of the surface of the metal quenched for
a given mass of steel. Keeping this primary assumption in mind, it
is possible to predict what physical properties may be developed in
heat treating by calculating the surface per unit mass for different
shapes and sizes. It may be pointed out that the figures and chart
that follow are not results of actual tests, but are derived by
calculation. They indicate the mathematical relation, which, based
on the fact that the physical properties of steel are determined
not alone by the rate which heat is lost per unit of surface, but
by the rate which heat is lost per unit of weight in relation to
the surface exposed for that unit. The unit of weight has for the
different shaped bodies and their sizes a certain surface which
determines their physical properties.
"For example, the surface corresponding to 1 lb. of steel has been
computed for spheres, rounds and flats. For the sphere with a unit
weight of 1 lb. the portion is a cone with the apex at the center
of the sphere and the base the curved surface of the sphere (surface
exposed to quenching). For rounds, a unit weight of 1 lb. may be
taken as a disk or cylinder, the base and top surfaces naturally do
not enter into calculation. For a flat, a prismatic or cylindrical
volume may be taken to represent the unit weight. The surfaces
that are considered in this instance are the top and base of the
section, as these surfaces are the ones exposed to cooling."
The results of the calculations are as follows:
TABLE 20.--SPHERE
Diameter Surface per
of sphere pound of steel
_X_ _Y_
8 in. 2.648 sq. in.
6 in. 3.531 sq. in.
4 in. 5.294 sq. in.
3 in. 7.062 sq. in.
2 in. 10.61 sq. in.
_XY_ = 21.185.
TABLE 21.--ROUND
Diameter Surface per
of round pound of steel
_X_ _Y_
8.0 in. 1.765 sq. in.
6.0 in. 2.354 sq. in.
5.0 in. 2.829 sq. in.
4.0 in. 3.531 sq. in.
3.0 in. 4.708 sq. in.
2.0 in. 7.062 sq. in.
1.0 in. 14.125 sq. in.
0.5 in. 28.25 sq. in.
0.25 in. 56.5 sq. in.
_XY_ = 14.124.
TABLE 22.--FLAT
Thickness Surface per
of flat pound of steel
_X_ _Y_
8.0 in. 0.8828 sq. in.
6.0 in. 1.177 sq. in.
5.0 in. 1.412 sq. in.
4.0 in. 1.765 sq. in.
3.0 in. 2.345 sq. in.
2.0 in. 3.531 sq. in.
1.0 in. 7.062 sq. in.
0.5 in. 14.124 sq. in.
0.25 in. 28.248 sq. in.
_XY_ = 7.062.
Having once determined the physical qualities of a certain specimen,
and found its position on the curve we have the means to predict the
decrease of physical qualities on larger specimens which receive
the same heat treatment.
When the surfaces of the unit weight as outlined in the foregoing
tables are plotted as ordinates and the corresponding diameters as
abscissæ, the resulting curve is a hyperbola and follows the law
_XY = C_. In making these calculations the radii or one-half of
the thickness need only to be taken into consideration as the heat
is conducted from the center of the body to the surface, following
the shortest path.
The equations for the different shapes are as follows:
For flats _XY_ = 7.062
For rounds _XY_ = 14.124
For spheres _XY_ = 21.185
It will be noted that the constants increase in a ratio of 1, 2,
and 3, and the three bodies in question will increase in hardness
on being quenched in the same ratio, it being understood that the
diameter of the sphere and round and thickness of the flat are
equal.
Relative to shape, it is interesting to note that rounds, squares,
octagons and other three axial bodies, with two of their axes equal,
have the same surface for the unit weight.
For example:
Size Length Surface Weight Surface for 1 lb.
2 in. Sq. 12 in. 96.0 sq. in. 13.60 lb. 7.06 sq. in.
2 in. Round 12 in. 75.4 sq. in. 10.68 lb. 7.06 sq. in.
Although this discussion is at present based upon mathematical
analysis, it is hoped that it will open up a new field of investigation
in which but little work has been done, and may assist in settling
the as yet unsolved question of the effect of size and shape in
the heat treatment of steel.
HEAT-TREATING EQUIPMENT AND METHODS FOR MASS PRODUCTION
The heat-treating department of the Brown-Lipe-Chapin Company,
Syracuse, N. Y., runs day and night, and besides handling all the
hardening of tools, parts of jigs, fixtures, special machines and
appliances, carburizes and heat-treats every month between 150,000
and 200,000 gears, pinions, crosses and other components entering
into the construction of differentials for automobiles.
The treatment of the steel really begins in the mill, where the
steel is made to conform to a specific formula. On the arrival
of the rough forgings at the Brown-Lipe-Chapin factory, the first
of a long series of inspections begins.
ANNEALING METHOD.--Forgings which are too hard to machine are put
in pots with a little charcoal to cause a reducing atmosphere and
to prevent scale. The covers are then luted on and the pots placed
in the furnace. Carbon steel from 15 to 25 points is annealed at
1,600°F. Nickel steel of the same carbon and containing in addition
3-1/2 per cent nickel is annealed at 1,450°F. When the pots are
heated through, they are rolled to the yard and allowed to cool.
This method of annealing gives the best hardness for quick machining.
The requirements in the machine operations are very rigid and, in
spite of great care and probably the finest equipment of special
machines in the world, a small percentage of the product fails
to pass inspection during or at the completion of the machine
operations. These pieces, however, are not a loss, for they play
an important part in the hardening process, indicating as they do
the exact depth of penetration of the carburizing material and
the condition of both case and core.
HEAT-TREATING DEPARTMENT.--The heat-treating department occupies an
L-shaped building. The design is very practical, with the furnace
and the floor on the same level so that there is no lifting of
heavy pots. Fuel oil is used in all the furnaces and gives highly
satisfactory results. The consumption of fuel oil is about 2 gal.
per hour per furnace.
The work is packed in the pots in a room at the entrance to the
heat-treatment building. Before packing, each gear is stamped with
a number which is a key to the records of the analysis and complete
heat treatment of that particular gear. Should a question at any time
arise regarding the treatment of a certain gear, all the necessary
information is available if the number on the gear is legible. For
instance, date of treatment, furnace, carburizing material, position
of the pot in the furnace, position of gear in pot, temperature of
furnace and duration of treatment are all tabulated and filed for
reference.
After marking, all holes and parts which are to remain uncarburized
are plugged or luted with a mixture of kaolin and Mellville gravel
clay, and the gear is packed in the carburizing material. Bohnite,
a commercial carburizing compound is used exclusively at this plant.
This does excellent work and is economical. Broadly speaking, the
economy of a carburizing compound depends on its lightness. The
space not occupied by work must be filled with compound; therefore)
other things being equal, a compound weighing 25 lb. would be worth
more than twice as much as one weighing 60 lb. per cubic foot. It
has been claimed that certain compounds can be used over and over
again, but this is only true in a limited way, if good work is
required. There is, of course, some carbon in the compound after
the first use, but for first-class work, new compound must be used
each time.
THE PACKING DEPARTMENT.--In Fig. 56 is shown the packing pots where
the work is packed. These are of malleable cast iron, with an internal
vertical flange around the hole _A_. This fits in a bell on the
end of the cast-iron pipe _B_, which is luted in position with
fireclay before the packing begins. At _C_ is shown a pot ready
for packing. The crown gears average 10 to 12 in. in diameter and
weigh about 11 lb. each. When placed in the pots, they surround
the central tube, which allows the heat to circulate. Each pot
contains five gears. Two complete scrap gears are in each furnace
(_i.e._, gears which fail to pass machining inspection), and at
the top of front pot are two or more short segments of scrap gear,
used as test pieces to gage depth of case.
[Illustration: FIG. 56.--Packing department and special pots.]
After filling to the top with compound, the lid _D_ is luted on.
Ten pots are then placed in a furnace. It will be noted that the
pots to the right are numbered 1, 2, 3, 4, indicating the position
they are to occupy in the furnace.
The cast-iron ball shown at _E_ is small enough to drop through
the pipe _B_, but will not pass through the hole _A_ in the bottom
of the pot. It is used as a valve to plug the bottom of the pot
to prevent the carburizing compound from dropping through when
removing the carburized gears to the quenching bath.
Without detracting from the high quality of the work, the metallurgist
in this plant has succeeded in cutting out one entire operation
and reducing the time in the hardening room by about 24 hr.
Formerly, the work was carburized at about 1,700°F. for 9 hr. The
pots were then run out into the yard and allowed to cool slowly.
When cool, the work was taken out of the pots, reheated and quenched
at 1,600°F. to refine the core. It was again reheated to 1,425°F.
and quenched to refine the case. Finally, it was drawn to the proper
temper.
SHORT METHOD OF TREATMENT.--In the new method, the packed pots are
run into the case-hardening furnaces, which are heated to 1,600°F.
On the insertion of the cold pots, the temperature naturally falls.
The amount of this fall is dependent upon a number of variables,
but it averages nearly 500°F. as shown in the pyrometer chart,
Fig. 61. The work and furnace must be brought to 1,600°F. Within
2-1/2 hr.; otherwise, a longer time will be necessary to obtain
the desired depth of case. On this work, the depth of case required
is designated in thousandths, and on crown gears, the depth in
0.028 in. Having brought the work to a temperature of 1,600°F.
the depth of case mentioned can be obtained in about 5-1/2 hr. by
maintaining this temperature.
As stated before, at the top of each pot are several test pieces
consisting of a whole scrap gear and several sections. After the
pots have been heated at 1,600°F. for about 5-1/4 hr., they are
removed, and a scrap-section test-piece is quenched direct from
the pot in mineral oil at _not more than_ 100°F. The end of a tooth
of this is then ground and etched to ascertain the depth of case.
As these test pieces are of exactly the same cross-section as the
gears themselves, the carburizing action is similar. When the depth
of case has been found from the etched test pieces to be satisfactory,
the pots are removed. The iron ball then is dropped into the tube
to seal the hole in the bottom of the pot; the cover and the tube
are removed, and the gears quenched direct from the pot in mineral
oil, which is kept at a temperature not higher than 100°F.
THE EFFECT.--The heating at 1,600°F. gives the first heat treatment
which refines the core, which under the former high heat (1,700°F.)
was rendered coarsely crystalline. All the gears, including the
scrap gears, are quenched direct from the pot in this manner.
The gears then go to the reheating furnaces, situated in front of
a battery of Gleason quenching machines. These furnaces accommodate
from 12 to 16 crown gears. The carbon-steel gears are heated in a
reducing atmosphere to about 1,425°F. (depending on the carbon
content) placed in the dies in the Gleason quenching machine, and
quenched between dies in mineral oil at less than 100°F. The test
gear receives exactly the same treatment as the others and is then
broken, giving a record of the condition of both case and core.
AFFINITY OF NICKEL STEEL FOR CARBON.--The carbon- and nickel-steel
gears are carburized separately owing to the difference in time
necessary for their carburization. Practically all printed information
on the subject is to the effect that nickel steel takes longer to
carburize than plain carbon steel. This is directly opposed to
the conditions found at this plant. For the same depth of case,
other conditions being equal, a nickel-steel gear would require
from 20 to 30 min. less than a low carbon-steel gear.
From the quenching machines, the gears go to the sand-blasting
machines, situated in the wing of the heat-treating building, where
they are cleaned. From here they are taken to the testing department.
The tests are simple and at the same time most thorough.
TESTING AND INSPECTION OF HEAT TREATMENT.--The hard parts of the
gear must be so hard that a new mill file does not bite in the
least. Having passed this file test at several points, the gears go
to the center-punch test. The inspector is equipped with a wooden
trough secured to the top of the bench to support the gear, a number
of center punches (made of 3/4-in. hex-steel having points sharpened
to an angle of 120 deg.) and a hammer weighing about 4 oz. With
these simple tools, supplemented by his skill, the inspector can
_feel_ the depth and quality of the case and the condition of the
core. The gears are each tested in this way at several points on
the teeth and elsewhere, the scrap gear being also subjected to
the test. Finally, the scrap gear is securely clamped in the
straightening press shown in Fig. 57. With a 3-1/2-lb. hammer and
a suitable hollow-ended drift manipulated by one of Sandow's
understudies, teeth are broken out of the scrap gear at various
points. These give a record confirming the center-punch tests,
which, if the angle of the center punch is kept at 120 deg. and
the weight of the hammer and blow are uniform, is very accurate.
After passing the center-punch test the ends of the teeth are peened
lightly with a hammer. If they are too hard, small particles fly
off. Such gears are drawn in oil at a temperature of from 300 to
350°F., depending on their hardness. Some builders prefer to have
the extreme outer ends of the teeth drawn somewhat lower than the
rest. This drawing is done on gas-heated red-hot plates, as shown
at _A_ in Fig. 58.
[Illustration: FIG. 57.--Press for holding test gears for breaking.]
Nickel steel, in addition to all the tests given to carbon steel,
is subjected to a Brinell test. For each steel, the temperature
and the period of treatment are specific. For some unknown reason,
apparently like material with like treatment will, in isolated
cases, not produce like results. It then remains for the treatment
to be repeated or modified, but the results obtained during inspection
form a valuable aid to the metallurgist in determining further
treatment.
TEMPERATURE RECORDING AND REGULATION.--Each furnace is equipped
with pyrometers, but the reading and recording of all temperatures
are in the hands of one man, who occupies a room with an opening
into the end of the hardening department. The opening is about 15
ft. above the floor level. On each side of it, easily legible from
all of the furnaces, is a board with the numbers of the various
furnaces, as shown in Figs. 59 and 60. Opposite each furnace number
is a series of hooks whereon are hung metal numbers representing the
pyrometer readings of the temperature in that particular furnace.
Within the room, as shown in Fig. 60, the indicating instrument
is to the right, and to the left is a switchboard to connect it
with the thermo-couples in the various furnaces. The boards shown
to the right and the left swing into the room, which enables the
attendant easily to change the numbers to conform to the pyrometer
readings. Readings of the temperatures of the carburizing furnaces
are taken and tabulated every ten minutes. These, numbered 1 to
10, are shown on the board to the right in Fig. 59. The card shown
in Fig. 61 gives such a record. These records are filed away for
possible future reference.
[Illustration: FIG. 58.--Gas heated drawing plate for tooth ends.]
The temperatures of the reheating furnaces, numbered from 1 to
26 and shown on the board to the left in Fig. 59, are taken every
5 min.
Each furnace has a large metal sign on which is marked the temperature
at which the furnace regulator is required to keep his heat. As
soon as any variation from this is posted on the board outside
the pyrometer room, the attendant sees it and adjusts the burners
to compensate.
[Illustration: FIG. 59.--Pyrometer recording room.]
[Illustration: FIG. 60.--Inside of Pyrometer switch room.]
DIES FOR GLEASON TEMPERING MACHINES.--In Fig. 62 is shown a set
of dies for the Gleason tempering machine. These accurately made
dies fit and hold the gear true during quenching, thus preventing
distortion.
[Illustration: FIG. 61.--Carburizing furnace record.]
Referring to Fig. 62, the die _A_ has a surface _B_ which fits the
face of the teeth of the gear _C_. This surface is perforated by
a large number of holes which permit the quenching oil to circulate
freely. The die _A_ is set in the upper end of the plunger _A_
of the tempering machine, shown in Fig. 63, a few inches above
the surface of the quenching oil in the tank _N_. Inside the die
_A_ are the centering jaws _D_, Fig. 62, which are an easy fit
for the bore of the gear _C_. The inner surface of the centering
jaws is in the shape of a female cone. The upper die is shown at
_E_. In the center (separate from it, but a snug sliding fit in
it) is the expander _G_, which, during quenching, enters the taper
in the centering jaws _D_, expanding them against the bore of the
gear _C_. The faces _F_ of the upper die _E_ fit two angles at the
back of the gear and are grooved for the passage of the quenching
oil. The upper die _E_ is secured to the die carrier _B_, shown in
Fig. 9, and inside the die is the expander _G_, which is backed
up by compression springs.
[Illustration: FIG. 62.--Dies for Gleason gear-hardening machine.]
HARDENING OPERATION.--Hardening a gear is accomplished as follows:
The gear is taken from the furnace by the furnaceman and placed in
the lower die, surrounding the centering jaws, as shown at _H_ in
Fig. 62 and _C_ in Fig. 63. Air is then turned into the cylinder
_D_, and the piston rod _E_, the die carrier _B_, the top die _F_
and the expander _G_ descend. The pilot _H_ enters a hole in the
center of the lower die, and the expander _G_ enters the centering
jaws _I_, causing them to expand and center the gear _C_ in the
lower die. On further advance of the piston rod _E_, the expander
_G_ is forced upward against the pressure of the springs _J_ and
the upper die _F_ comes in contact with the upper surface of the
gear. Further downward movement of the dies, which now clamp the
work securely, overcomes the resistance of the pressure weight
_K_ (which normally keeps up the plunger _A_), and the gear is
submerged in the oil. The quenching oil is circulated through a
cooling system outside the building and enters the tempering machine
through the inlet pipe _L_. When the machine is in the position
shown, the oil passes out through the ports _M_ in the lower plunger
to the outer reservoir _N_, passing to the cooling system by way of
the overflow _O_. When the lower plunger _A_ is forced downward,
the ports _M_ are automatically closed and the cool quenching oil
from the inlet pipe _L_, having no other means of escape, passes
through the holes in the lower die and the grooves in the upper,
circulating in contact with the surfaces of the gear and passes to
the overflow. When the air pressure is released, the counterweights
return the parts to the positions shown in Fig. 63, and the operator
removes the gear.
[Illustration: FIG. 63.--Gleason tempering machine.]
The gear comes out uniformly hard all over and of the same degree of
hardness as when tempered in an open tank. The output of the machine
depends on the amount of metal to be cooled, but will average from
8 to 16 per hour. Each machine is served by one man, two furnaces
being required to heat the work. A slight excess of oil is used
in the firing of the furnaces to give a reducing atmosphere and
to avoid scale.
[Illustration: FIG. 64.--Hardening and shrinking sleeves.]
CARBURIZING LOW-CARBON SLEEVES.--Low-carbon sleeves are carburized
and pushed on malleable-iron differential-case hubs. Formerly,
these sleeves were given two treatments after carburization in
order to refine the case and the core, and then sent to the grinding
department, where they were ground to a push fit for the hubs. After
this they were pushed on the hubs. By the method now employed,
the first treatment refines the core, and on the second treatment,
the sleeves are pushed on the hub and at the same time hardened.
This method cuts out the internal grinding time, pressing on hubs,
and haulage from one department to another. Also, less work is
lost through splitting of the sleeves.
The machine for pushing the sleeves on is shown in Fig. 64. At
_A_ is the stem on which the hot sleeve _B_ is to be pushed. The
carburized sleeves are heated in an automatic furnace, which takes
them cold at the back and feeds them through to the front, by which
time they are at the correct temperature. The loose mandrel _C_
is provided with a spigot on the lower end, which fits the hole
in the differential-case hub. The upper end is tapered as shown
and acts as a pilot for the ram _D_. The action of pushing on and
quenching is similar to the action of the Gleason tempering machine,
with the exception that water instead of oil is used as a quenching
medium. The speed of operation depends on a number of variables,
but from 350 to 500 can be heated and pressed on in 11 hr.
CYANIDE BATH FOR TOOL STEELS.--All high-carbon tool steels are
heated in a cyanide bath. With this bath, the heat can be controlled
within 3 deg. The steel is evenly heated without exposure to the
air, resulting in work which is not warped and on which there is no
scale. The cyanide bath is, of course, not available for high-speed
steel because of the very high temperatures necessary.
DROP FORGING DIES
The kind of steel used in the die of course influences the heat
treatment it is to receive, but this also depends on the kind of
work the die is to perform. If the die is for a forging which is
machined all over and does not have to be especially close to size,
where a variation of 1/16 in. is not considered excessive, a low
grade steel will be perfectly satisfactory.
In cases of fine work, however, where the variation cannot be over
0.005 to 0.01 in. we must use a fine steel and prevent its going
out of shape in the heating and quenching. A high quality crucible
steel is suggested with about the following analysis: Carbon 0.75
per cent, manganese 0.25 per cent, silicon 0.15 per cent, sulphur
0.015 per cent, and phosphorus 0.015 per cent. Such a steel will
have a decalescent point in the neighborhood of 1,355°F. and for
the size used, probably in a die of approximately 8 in., it will
harden around 1,450°F.
To secure best results care must be taken at every step. The block
should be heated slowly to about 1,400°F., the furnace closed tight
and allowed to cool slowly in the furnace itself. It should not
soak at the high temperature.
After machining, and before it is put in the furnace for hardening,
it should be slowly preheated to 800 or 900°F. This can be done in
several ways, some putting the die block in front of the open door
of a hardening furnace and keeping the furnace at about 1,000°F.
The main thing is to heat the die block very slowly and evenly.
The hardening heat should be very slow, 7 hr. being none too long
for such a block, bringing the die up gradually to the quenching
temperature of 1,450°. This should be held for 1/2 hr. or even a
little more, when the die can be taken out and quenched. There
should be no guess work about the heating, a good pyrometer being
the only safe way of knowing the correct temperature.
The quenching tank should be of good size and have a spray or stream
of water coming up near the surface. Dip the die block about 3 in.
deep and let the stream of water get at the face so as to play
on the forms. By leaving the rest of the die out of the water,
moving the die up and down a trifle to prevent a crack at the line
of immersion, the back of the block is left tough while the face
is very hard. To overcome the tendency to warp the face it is a
good plan to pour a little water on the back of the die as this
tends to even up the cooling. The depth to which the die is dipped
can be easily regulated by placing bars across the tank at the
proper depth.
After the scleroscope shows the die to be properly hardened, which
means from 98 to 101, the temper should be drawn as soon as convenient.
A lead pot in which the back of the die can be suspended so as
to heat the back side, makes a good method. Or the die block can
be placed back to the open door of a furnace. On a die of this
size it may take several hours to draw it to the desired temper.
This can be tested while warm by the scleroscope method, bearing
in mind that the reading will not be the same as when cold. If
the test shows from 76 to 78 while warm, the hardness when cold
will be about 83, which is about right for this work.
S. A. E. HEAT TREATMENTS
The Society of Automotive Engineers have adopted certain heat treatments
to suit different steels and varying conditions. These have already
been referred to on pages 39 to 41 in connection with the different
steels used in automobile practice. These treatments are designated
by letter and correspond with the designations in the table.
HEAT TREATMENTS
_Heat Treatment A_
After forging or machining:
1. Carbonize at a temperature between 1,600°F. and 1,750°F.
(1,650-1,700°F. desired.)
2. Cool slowly or quench.
3. Reheat to 1,450-1,500°F. and quench.
_Heat Treatment B_
After forging or machining:
1. Carbonize between 1,600°F. and 1,750°F. (1,650-1,700°F.
Desired.)
2. Cool slowly in the carbonizing mixture.
3. Reheat to 1,550-1,625°F.
4. Quench.
5. Reheat to 1,400-1,450°F.
6. Quench.
7. Draw in hot oil at 300 to 450°F., depending upon the degree of
hardness desired.
_Heat Treatment D_
After forging or machining:
1. Heat to 1,500-1,600°F.
2. Quench.
3. Reheat to 1,450-1,500°F.
4. Quench.
5. Reheat to 600-1,200°F. and cool slowly.
_Heat Treatment E_
After forging or machining:
1. Heat to 1,500-1,550°F.
2. Cool slowly.
3. Reheat to 1,450-1,500°F.
4. Quench.
5. Reheat to 600-1,200°F. and cool slowly.
_Heat Treatment F_
After shaping or coiling:
1. Heat to 1,425-1,475°F.
2. Quench in oil.
3. Reheat to 400-900°F., in accordance with temper desired and cool
slowly.
_Heat Treatment G_
After forging or machining:
1. Carbonize at a temperature between 1,600°F. and 1,750°F.
(1,650-1,700°F. desired).
2. Cool slowly in the carbonizing mixture.
3. Reheat to 1,500-1,550°F.
4. Quench.
5. Reheat to 1,300-1,400°F.
6. Quench.
7. Reheat to 250-500°F. (in accordance with the necessities of the case)
and cool slowly.
_Heat Treatment H_
After forging or machining:
1. Heat to 1,500-1,600°F.
2. Quench.
3. Reheat to 600-1,200°F. and cool slowly.
_Heat Treatment K_
After forging or machining:
1. Heat to 1,500-1,550°F.
2. Quench.
3. Reheat to 1,300-1,400°F.
4. Quench.
5. Reheat to 600-1,200°F. and cool slowly.
_Heat Treatment L_
After forging or machining:
1. Carbonize between 1,600°F. and 1,750°F. (1,650-1,700°F. desired).
2. Cool slowly in the carbonizing mixture.
3. Reheat to 1,400-1,500°F.
4. Quench.
5. Reheat to 1,300-1,400°F.
6. Quench.
7. Reheat to 250-500°F. and cool slowly.
_Heat Treatment M_
After forging or machining:
1. Heat to 1,450-1,500°F.
2. Quench.
3. Reheat to 500-1.250°F. and cool slowly.
_Heat Treatment P_
After forging or machining:
1. Heat to 1,450-1,500°F.
2. Quench.
3. Reheat to 1,375-1,450°F. slowly.
4. Quench.
5. Reheat to 500-1,250°F. and cool slowly.
_Heat Treatment Q_
After forging:
1. Heat to 1,475-1,525°F. (Hold at this temperature one-half hour,
to insure thorough heating.)
2. Cool slowly.
3. Machine.
4. Reheat to 1,375-1,425°F.
5. Quench.
6. Reheat to 250-550°F. and cool slowly.
_Heat Treatment R_
After forging:
1. Heat to 1,500-1,550°F.
2. Quench in oil.
3. Reheat to 1,200-1,300°F. (Hold at this temperature three hours.)
4. Cool slowly.
5. Machine.
6. Reheat to 1,350-1,450°F.
7. Quench in oil.
8. Reheat to 250-500°F. and cool slowly.
_Heat Treatment S_
After forging or machining:
1. Carbonize at a temperature between 1,600 and 1,750°F.
(1,650-1,700°F. Desired.)
2. Cool slowly in the carbonizing mixture.
3. Reheat to 1,650-1,750°F.
4. Quench.
5. Reheat to 1,475-1,550°F.
6. Quench.
7. Reheat to 250-550°F. and cool slowly.
_Heat Treatment T_
After forging or machining:
1. Heat to 1,650-1,750°F.
2. Quench.
3. Reheat to 500-1,300°F. and cool slowly.
_Heat Treatment U_
After forging:
1. Heat to 1,525-1,600°F. (Hold for about one-half hour.)
2. Cool slowly.
3. Machine.
4. Reheat to 1,650-1,700°F.
5. Quench.
6. Reheat to 350-550°F. and cool slowly.
_Heat Treatment V_
After forging or machining:
1. Heat to 1,650-1,750°F.
2. Quench.
3. Reheat to 400-1,200°F. and cool slowly.
RESTORING OVERHEATED STEEL
The effect of heat treatment on overheated steel is shown graphically
in Fig. 65 to the series of illustrations on pages 137 to 144. This
was prepared by Thos. Firth & Sons, Ltd., Sheffield, England.
[Illustration: FIG. 65.--Chart of changes due to heating and cooling.]
The center piece Fig. 65 represents a block of steel weighing about
25 lb. The central hole accommodated a thermo-couple which was attached
to an autographic recorder. The curve is a copy of the temperature
record during heating and cooling. Into the holes in the side of
the block small pegs of overheated mild steel were inserted. One
peg was withdrawn and quenched at each of the temperatures indicated
by the numbered arrows, and after suitable preparation these pegs
were photographed in order to show the changes in structure taking
place during heating and cooling operations. The illustrations here
reproduced are selected from those photographs with the object
of presenting pictorially the changes involved in the refining of
overheated steel or steel castings. Figures 66 to 79 with their
captions show much that is of value to steel users.
[Illustration: FIG. 66.--The structure of overheated mild steel
from which all the pegs were made (magnified 25 diameters). The
pegs withdrawn at 720°C., or earlier, had this structure and were
quite soft.]
[Illustration: FIG. 67.--Peg withdrawn at 750°C. (magnified 25
diameters). The structure is apparently unaltered, but the peg was
hard and, unlike the earlier ones, would not bend double.]
[Illustration: FIG. 68.--A portion of 66 magnified 200 diameters
to show that the dark (pearlite) areas are laminated.]
[Illustration: FIG. 69.--A portion of 67 magnified 200 diameters,
showing that pearlite areas are no longer laminated and providing
reason for observed hardness.]
[Illustration FIG. 70.--Peg withdrawn at 780°C. (magnified 25
diameters), showing inter-diffusion of transformed pearlite and
ferrite areas.]
[Illustration: FIG. 71.--Peg withdrawn at 800°C. (magnified 25
diameters), showing inter-diffusion so far advanced that the original
outline of the crystals is now only faintly suggested.]
[Illustration: FIG. 72.--Peg withdrawn at 850°C. (magnified 100
diameters) after inter-diffusion was completed. Note the regular
outlines and the small size of the crystals as compared with 67.]
[Illustration: FIG. 73.--To facilitate comparison 67 was enlarged
to the same magnification as 62, and the one superimposed on the
other. The single large crystal occupied as much space as 8,000
of the smaller ones.]
[Illustration: FIG. 74.--The peg withdrawn on cooling at 800°C.
(magnified 100 diameters) shows the first reappearance of free
ferrite. All pegs withdrawn at higher temperatures were like Fig.
72.]
[Illustration: FIG. 75.--Peg withdrawn after cooling to 760°C. The
increased amount of free ferrite arranges itself about the crystals
as envelopes.]
[Illustration: FIG. 76.-Peg withdrawn after cooling to 740°C.]
[Illustration: FIG. 77.--Peg withdrawn after cooling to 670°C.
(magnified 800 diameters). Just at this moment the lamination of
pearlite, which now occupied its original area, was taking place.
In some parts the lamination was perfect, in other parts the iron
and iron-carbide were still dissolved in each other.]
[Illustration: FIG. 78.--Any peg withdrawn after 670°C. on cooling
(magnified 100 diameters).]
[Illustration: FIG. 79.--Structure of overheated steel before (left)
and after refining (right).]
CHAPTER IX
HARDENING CARBON STEEL FOR TOOLS
For years the toolmaker had full sway in regard to make of steel
wanted for shop tools, he generally made his own designs, hardened,
tempered, ground and usually set up the machine where it was to
be used and tested it.
Most of us remember the toolmaker during the sewing machine period
when interchangeable tools were beginning to find their way; rather
cautiously at first. The bicycle era was the real beginning of
tool making from a manufacturing standpoint, when interchangeable
tools for rapid production were called for and toolmakers were in
great demand. Even then, jigs, and fixtures were of the toolmaker's
own design, who practically built every part of it from start to
finish.
The old way, however, had to be changed. Instead of the toolmaker
starting his work from cutting off the stock in the old hack saw,
a place for cutting off stock was provided. If, for instance, a
forming tool was wanted, the toolmaker was given the master tool
to make while an apprentice roughed out the cutter. The toolmaker,
however, reserved the hardening process for himself. That was one
of the particular operations that the old toolmaker refused to
give up. It seemed preposterous to think for a minute that any
one else could possibly do that particular job without spoiling
the tools, or at least warp it out of shape (most of us did not
grind holes in cutters 15 to 20 years ago); or a hundred or more
things might happen unless the toolmaker did his own hardening
and tempering.
That so many remarkably good tools were made at that time is still
a wonder to many, when we consider that the large shop had from 30
to 40 different men, all using their own secret compounds, heating
to suit eyesight, no matter if the day was bright or dark, and then
tempering to color. But the day of the old toolmaker has changed.
Now a tool is designed by a tool designer, O.K.'d, and then a print
goes to the foreman of the tool department, who specifies the size
and gets the steel from the cutting-off department. After finishing
the machine work it goes to the hardening room, and this is the
problem we shall now take up in detail.
THE MODERN HARDENING ROOM.--A hardening room of today means a very
different place from the dirty, dark smithshop in the corner with
the open coal forge. There, when we wanted to be somewhat particular,
we sometimes shoveled the coal cinders to one side and piled a great
pile of charcoal on the forge. We now have a complete equipment;
a gas- or oil-heating furnace, good running water, several sizes
of lead pots, and an oil tank large enough to hold a barrel of
oil. By running water, we mean a large tank with overflow pipes
giving a constant supply. The ordinary hardening room equipment
should consist of:
Gas or oil muffle furnace for hardening.
Gas or oil forge furnace.
A good size gas or oil furnace for annealing and case-hardening.
A gas or oil furnace to hold lead pots.
Oil tempering tank, gas- or oil-heated.
Pressure blower.
Large oil tank to hold at least a barrel of oil.
Big water tank with screen trays connected with large pipe from bottom
with overflow.
Straightening press.
The furnace should be connected with pyrometers and tempering tank with
a thermometer.
Beside all this you need a good man. It does not make much difference
how completely the hardening department is fitted up, if you expect
good work, a small percentage of loss and to be able to tackle anything
that comes along, you must have a good man, one who understands
the difference between low- and high-carbon steel, who knows when
particular care must be exercised on particular work. In other
words, a man who knows how his work should be done, and has the
intelligence to follow directions on treatments of steel on which
he has had no experience.
Jewelers' tools, especially for silversmith's work, probably have
to stand the greatest punishment of any all-steel tools and to
make a spoon die so hard that it will not sink under a blow from
an 1,800-lb. hammer with a 4-ft. drop, and still not crack, demands
careful treatment.
To harden such dies, first cover the impression on the die with
paste made from bone dust or lampblack and oil. Place face down
in an iron box partly filled with crushed charcoal, leaving back
of die uncovered so that the heat can be seen at all times. Heat
slowly in furnace to a good cherry red. The heat depends on the
quality and the analysis of steel and the recommended actions of
the steel maker should be carefully followed. When withdrawn from
the fire the die should be quenched as shown in Fig. 80 with the
face of die down and the back a short distance out of the water.
When the back is black, immerse all over.
[Illustration: FIG. 80.--Quenching a die, face down.]
If such a tank is not at hand, it would pay to rig one up at once,
although a barrel of brine may be used, or the back of the die
may be first immersed to a depth of about 1/2 in. When the piece
is immersed, hold die on an angle as in Fig. 81.
[Illustration: FIG. 81.--Hold die at angle to quench.]
This is for the purpose of expelling all steam bubbles as they
form in contact with hot steel. We are aware of the fact that a
great many toolmakers in jewelry shops still cling to the overhead
bath, as in Fig. 82, but more broken pieces and more dies with
soft spots are due to this method than to all the others combined,
as the water strikes one spot in force, contracting the surface
so much faster than the rest of the die that the results are the
same as if an uneven heating had been given the steel.
TAKE TIME FOR HARDENING.--Uneven heating and poor quenching has
caused loss of many very valuable dies, and it certainly seems
that when a firm spends from $75 to $450 in cutting a die that
a few hours could be spared for proper hardening. But the usual
feeling is that a tool must be hurried as soon as the hardener
gets it, and if a burst die is the result from either uneven or
overheated steel and quenching same without judgment, the steel
gets the blame.
[Illustration: FIG. 82.--An obsolete method.]
Give the steel a chance to heat properly, mix a little common sense
with "your 30 years experience on the other fellows steel." Remember
that high-carbon steel hardens at a lower heat than low-carbon
steel, and quench when at the right heat in the two above ways,
and 99 per cent of the trouble will vanish.
When a die flies to pieces in quenching, don't rush to the
superintendent with a "poor-steel" story, but find out first why it
broke so that the salesman who sold it will not be able to harden
piece after piece from the same bar satisfactorily. If you find
a "cold short," commonly called "a pipe," you can lay the blame
on the steelmaker. If it is a case of overheating and quenching
when too hot, you will find a coarse grain with many bright spots
like crystals to the hardening depth. If uneven heating is the
cause, you will find a wider margin of hardening depth on one side
than on the other, or find the coarse grain from over-heating on
one side while on the other you will find a close grain, which
may be just right. If you find any other faults than a "pipe,"
or are not able to harden deep enough, then take the blame like
a man and send for information. The different steel salesmen are
good fellows and most of them know a thing or two about their own
business.
For much work a cooling bath at from 50 to 75°F. is very good both
for small hobs, dies, cutter plates or plungers. Some work will
harden best in a barrel of brine, but in running cold water, splendid
results will be obtained. Cutter plates should always be dipped
corner first and if any have stripper holes, they should first
be plugged with asbestos or fire clay cement.
In general it may be said that the best hardening temperature for
carbon steel is the lowest temperature at which it will harden
properly.
CARBON IN TOOL STEEL
Carbon tool steel, or "tool steel" as it is commonly called, usually
contains from 80 to 125 points (or from 0.80 to 1.25 per cent)
of carbon, and none of the alloys which go to make up the high
speed steels. This was formerly known also as crucible or "cast"
steel, or crucible cast steel, from the way in which it was made.
This was before the days of steel castings. The advent of these
caused so much confusion that the term was soon dropped. When we
say "tool steel," we nearly always refer to carbon-tool steel,
high-speed steel being usually designated by that name.
For many purposes carbon-steel cutters are still found best, although
where a large amount of material is to be removed at a rapid rate,
it has given way to high-speed steels.
CARBON STEELS FOR DIFFERENT TOOLS
All users of tool steels should carefully study the different qualities
of the steels they handle. Different uses requires different kinds of
steel for best results, and for the purpose of designating different
steels some makers have adopted the two terms "temper," and "quality,"
to distinguish between them.
In this case temper refers to the amount of carbon which is combined
with the iron to make the metal into a steel. The quality means
the absence of phosphorous, sulphur and other impurities, these
depending on the ores and the methods of treatment.
Steel makers have various ways of designating carbon steels for
different purposes. Some of these systems involve the use of numbers,
that of the Latrobe Steel Company being given herewith. It will
be noted that the numbers are based on 20 points of carbon per
unit. The names given the different tempers are also of interest.
Other makers use different numbers.
The temper list follows:
LATROBE TEMPER LIST OF CARBON TOOL STEELS
No. 3 temper 0.60 to 0.69 per cent carbon
No. 3-1/2 temper 0.70 to 0.79 per cent carbon
No. 4 temper 0.80 to 0.89 per cent carbon
No. 4-1/2 temper 0.90 to 0.99 pet cent carbon
No. 5 temper 1.00 to 1.09 per cent carbon
No. 5-1/2 temper 1.10 to 1.19 per cent carbon
No. 6 temper 1.20 to 1.29 per cent carbon
No. 6-1/2 temper 1.30 to 1.39 per cent carbon
No. 7 temper 1.40 to 1.49 per cent carbon
USES OF THE VARIOUS TEMPERS OF CARBON TOOL STEEL
DIE TEMPER.--No. 3: All kinds of dies for deep stamping, pressing
and drop forgings. Mining drills to harden only. Easily weldable.
SMITHS' TOOL TEMPER.--No. 3-1/2: Large punches, minting and rivet
dies, nailmakers' tools, hammers, hot and cold sets, snaps and
boilermakers' tools, various smiths' tools, large shear blades,
double-handed chisels, caulking tools, heading dies, masons' tools
and tools for general welding purposes.
SHEAR BLADE TEMPER.--No. 4: Punches, large taps, screwing dies,
shear blades, table cutlery, circular and long saws, heading dies.
Weldable.
GENERAL PURPOSE TEMPER.--No. 4-1/2: Taps, small punches, screwing
dies, sawwebs, needles, etc., and for all general purposes. Weldable.
AXE TEMPER.--No. 5: Axes, chisels, small taps, miners' drills and
jumpers to harden and temper, plane irons. Weldable with care.
CUTLERY TEMPER.--No. 5-1/2: Large milling cutters, reamers, pocket
cutlery, wood tools, short saws, granite drills, paper and tobacco
knives. Weldable with very great care.
TOOL TEMPER.--No. 6: Turning, planing, slotting, and shaping tools,
twist drills, mill picks, scythes, circular cutters, engravers'
tools, surgical cutlery, circular saws for cutting metals, bevel
and other sections for turret lathes. Not weldable.
HARD TOOL TEMPER.--No. 6-1/2: Small twist drills, razors, small
and intricate engravers' tools, surgical instruments, knives. Not
weldable.
RAZOR TEMPER.--No. 7: Razors, barrel boring bits, special lathe
tools for turning chilled rolls. Not weldable.
STEEL FOR CHISELS AND PUNCHES
The highest grades of carbon or tempering steels are to be recommended
for tools which have to withstand shocks, such as for cold chisels
or punches. These steels are, however, particularly useful where
it is necessary to cut tempered or heat-treated steel which is
more than ordinarily hard, for cutting chilled iron, etc. They are
useful for boring, for rifle-barrel drilling, for fine finishing
cuts, for drawing dies for brass and copper, for blanking dies for
hard materials, for formed cutters on automatic screw machines
and for roll-turning tools.
Steel of this kind, being very dense in structure, should be given
more time in heating for forging and for hardening, than carbon
steels of a lower grade. For forging it should be heated slowly
and uniformly to a bright red and only light blows used as the
heat dies out. Do not hammer at all at a black heat. Reheat slowly
to a dark red for hardening and quench in warm water. Grind on a
wet grindstone.
Where tools have to withstand shocks and vibration, as in pneumatic
hammer work, in severe punching duty, hot or cold upsetting or
similar work, tool steels containing vanadium or chrome-vanadium
give excellent results. These are made particularly for work of
this kind.
CHISELS-SHAPES AND HEAT TREATMENT[1]
[Footnote 1: Abstract of paper by HENRY FOWLER, chief mechanical
engineer of the Midland Ry., England, before the Institution of
Mechanical Engineers.]
In the chief mechanical engineer's department of the Midland Ry.,
after considerable experimenting, it was decided to order chisel
steel to the following specifications: carbon, 0.75 to 0.85 per
cent, the other constituents being normal. This gives a complete
analysis as follows: carbon, 0.75 to 0.85; manganese, 0.30; silicon,
0.10; sulphur, 0.025; phosphorus, 0.025.
The analysis of a chisel which had given excellent service was as
follows: carbon, 0.75; manganese, 0.38; silicon, 0.16; sulphur,
0.028; phosphorus, 0.026. The heat treatment is unknown.
[Illustration: FIG. 83.--Forms of chisels standardized for the
locomotive shops of the Midland Ry., England.]
At the same time that chisel steel was standardized, the form of
the chisels themselves was revised, and a standard chart of these
as used in the locomotive shops was drawn up. Figure 83 shows the
most important forms, which are made to stock orders in the smithy
and forwarded to the heat-treatment room where the hardening and
tempering is carried out on batches of fifty. A standard system
of treatment is employed, which to a very large extent does away
with the personal element. Since the chemical composition is more
or less constant, the chief variant is the section which causes
the temperatures to be varied slightly. The chisels are carefully
heated in a gas-fired furnace to a temperature of from 730 to 740°C.
(1,340 to 1,364°F.) according to section. In practice, the first
chisel, is heated to 730°C.; and the second to 735°C. (1,355°F.);
and a 1 in. half round chisel to 740°C., because of their varying
increasing thickness of section at the points. Upon attaining this
steady temperature, the chisels are quenched to a depth of 3/8
to 1/2 in. from the point in water, and then the whole chisel is
immersed and cooled off in a tank containing linseed oil.
The oil-tank is cooled by being immersed in a cold-water tank through
which water is constantly circulated. After this treatment, the
chisels have a dead hard point and a tough or sorbitic shaft. They
are then tempered or the point "let down." This is done by immersing
them in another oil-bath which has been raised to about 215°C.
(419°F). The first result is, of course, to drop the temperature
of the oil, which is gradually raised to its initial point. On
approaching this temperature the chisels are taken out about every
2°C. rise and tested with a file, and at a point between 215 and
220°C. (428°F.), when it is found that the desired temper has been
reached, the chisels are removed, cleaned in sawdust, and allowed
to cool in an iron tray.
No comparative tests of these chisels with those bought and treated
by the old rule-of-thumb methods have been made, as no exact method of
carrying out such tests mechanically, other than trying the hardness
by the Brinell or scleroscope method, are known; any ordinary test
depends so largely upon the dexterity of the operator. The universal
opinion of foremen and those using the chisels as to the advantages
of the ones receiving the standard treatment described is that
a substantial improvement has been made. The chisels were not
"normalized." Tests of chisels normalized at about 900°C. (1,652°F.)
showed that they possessed no advantage.
Tools or pieces which have holes or deep depressions should be
filled before heating unless it is necessary to have the holes
hard on the inside. In that case the filling would keep the water
away from the surface and no hardening would take place. Where
filling is to be done, various materials are used by different
hardeners. Fireclay and common putty seem to be favored by many.
Every mechanic who has had anything to do with the hardening of
tools knows how necessary it is to take a cut from the surface of
the bar that is to be hardened. The reason is that in the process
of making the steel its outer surface has become decarbonized.
This change makes it low-carbon steel, which will of course not
harden. It is necessary to remove from 1/16 to 1/4 in. of diameter
on bars ranging from 1/2 to 4 in.
This same decarbonization occurs if the steel is placed in the
forge in such a way that unburned oxygen from the blast can get at
it. The carbon is oxidized, or burned out, converting the outside
of the steel into low-carbon steel. The way to avoid this is to use
a deep fire. Lack of this precaution is the cause of much spoiled
work, not only because of decarbonization of the outer surface
of the metal, but because the cold blast striking the hot steel
acts like boiling hot water poured into an ice-cold glass tumbler.
The contraction sets up stresses that result in cracks when the
piece is quenched.
PREVENTING DECARBONIZATION OF TOOL STEEL
It is especially important to prevent decarbonization in such tools
as taps and form cutters, which must keep their shape after hardening
and which cannot be ground away on the profile. For this reason
it is well to put taps, reamers and the like into pieces of pipe
in heating them. The pipe need be closed on one end only, as the
air will not circulate readily unless there is an opening at both
ends.
Even if used in connection with a blacksmith's forge the lead bath
has an advantage for heating tools of complicated shapes, since
it is easier to heat them uniformly and they are submerged and
away from the air. The lead must be stirred frequently or the heat
is not uniform in all parts of the lead bath. Covering the lead
with powdered charcoal will largely prevent oxidization and waste
of lead.
Such a bath is good for temperatures between 620 and 1,150°F. At
higher temperatures there is much waste of lead.
ANNEALING TO RELIEVE INTERNAL STRESSES
Work quenched from a high temperature and not afterward tempered
will, if complex in shape, contain many internal stresses which may
later cause it to break. They may be eased off by slight heating
without materially lessening the hardness of the piece. One way
to do this is to hold the piece over a fire and test it with a
moistened finger. Another way is to dip the piece in boiling water
after it has first been quenched in a cold bath. Such steps are
not necessary with articles which will afterward be tempered and
in which the strains are thus reduced.
In annealing steels the operation is similar to hardening, as far
as heating is concerned. The critical temperatures are the proper
ones for annealing as well as hardening. From this point on there
is a difference, for annealing consists in cooling as slowly as
possible. The slower the cooling the softer will be the steel.
Annealing may be done in the open air, in furnaces, in hot ashes
or lime, in powdered charcoal, in burnt bone, in charred leather
and in water. Open-air annealing will do as a crude measure in
cases where it is desired to take the internal stresses out of
a piece. Care must be taken in using this method that the piece
is not exposed to drafts or placed on some cold substance that
will chill it. Furnace annealing is much better and consists in
heating the piece in a furnace to the critical temperature and
then allowing the work and the furnace to cool together.
When lime or ashes are used as materials to keep air away from
the steel and retain the heat, they should be first heated to make
sure that they are dry. Powdered charcoal is used for high-grade
annealing, the piece being packed in this substance in an iron box
and both the work and the box raised to the critical temperature
and then allowed to cool slowly. Machinery steel may be annealed in
spent ground-bone that has been used in casehardening; _but tool
steel must never be annealed in this way_, as it will be injured
by the phosphorus contained in the bone. Charred leather is the
best annealing material for high-carbon steel, because it prevents
decarbonizing taking place.
DOUBLE ANNEALING
Water annealing consists in heating the piece, allowing it to cool
in air until it loses its red heat and becomes black and then
immediately quenching it in water. This plan works well for very
low-carbon steel; but for high-carbon steel what is known as the
"double annealing treatment" must be given, provided results are
wanted quickly. The process consists in heating the steel quickly to
200° or more above the upper critical, cooling in air down through
the recalescence point, then reheating it to just above the critical
point and again cooling slowly through the recalescence, then quenching
in oil. This process retains in the steel a fine-grained structure
combined with softness.
QUENCHING TOOL STEEL
To secure proper hardness, the cooling of quenching of steel is
as important as its heating. Quenching baths vary in nature, there
being a large number of ways to cool a piece of steel in contrast
to the comparatively few ways of heating it.
Plain water, brine and oil are the three most common quenching
materials. Of these three the brine will give the most hardness,
and plain water and oil come next. The colder that any of these
baths is when the piece is put into it the harder will be the steel;
but this does not mean that it is a good plan to dip the heated
steel into a tank of ice water, for the shock would be so great
that the bar would probably fly to pieces. In fact, the quenching
bath must be sometimes heated a bit to take off the edge of the
shock.
Brine solutions will work uniformly, or give the same degree of
hardness, until they reach a temperature of 150°F. above which
their grip relaxes and the metals quenched in them become softer.
Plain water holds its grip up to a temperature of approximately
100°F.; but oil baths, which are used to secure a slower rate of
cooling, may be used up to 500° or more. A compromise is sometimes
effected by using a bath consisting of an inch or two of oil floating
on the surface of water. As the hot steel passes through the oil,
the shock is not as severe as if it were to be thrust directly
into the water; and in addition, oil adheres to the tool and keeps
the water from direct contact with the metal.
The old idea that mercury will harden steel more than any other
quenching material has been exploded. A bath consisting of melted
cyanide of potassium is useful for heating fine engraved dies and
other articles that are required to come out free from scale. One
must always be careful to provide a hood or exhaust system to get
rid of the deadly fumes coming from the cyanide pot.
The one main thing to remember in hardening tool steel is to quench
on a rising heat. This does not mean a rapid heating as a slow
increase in temperature is much better in every way.
THE THEORY OF TEMPERING.--Steel that has been hardened is generally
harder and more brittle than is necessary, and in order to bring
it to the condition that meets our requirements a treatment called
tempering is used. This increases the toughness of the steel, _i.e._,
decrease the brittleness at the expense of a slight decrease in
hardness.
There are several theories to explain this reaction, but generally
it is only necessary to remember that in hardening we quench steel
from the austenite phase, and, due to this rapid cooling, the normal
change from austenite to the eutectoid composition does not have
time to take place, and as a consequence the steel exists in a
partially transformed, unstable and very hard condition at atmospheric
temperatures. But owing to the internal rigidity which exists in
cold metal the steel is unable to change into its more stable phase
until atoms can rearrange themselves by the application of heat.
The higher the heat, the greater the transformation into the softer
phases. As the transformation takes place, a certain amount of heat
of reaction, which under slow cooling would have been released in
the critical range, is now released and helps to cause a further
slight reaction.
If a piece of steel is heated to a certain temperature and held
there, the tempering color, instead of remaining unchanged at this
temperature, will advance in the tempering-color scale as it would
with increasing temperature. This means that the tempering colors
do not absolutely correspond to the temperatures of steels, but the
variations are so slight that we can use them in actual practice.
(See Table 23, page 158.)
TEMPERATURES TO USE.--As soon as the temperature of the steel reaches
100°C. (212°F.) the transformation begins, increasing in intensity
as the temperature is raised, until finally when the lower critical
range is reached, the steel has been all changed into the ordinary
constituents of unhardened steels.
If a piece of polished steel is heated in an ordinary furnace, a
thin film of oxides will form on its surface. The colors of this
film change with temperature, and so, in tempering, they are generally
used as an indication of the temperature of the steel. The steel
should have at least one polished face so that this film of oxides
may be seen.
An alternative method to the determination of temper by color is
to temper by heating in an oil or salt bath. Oil baths can be used
up to temperatures of 500°F.; above this, fused-salt baths are
required. The article to be tempered is put into the bath, brought
up to and held at the required temperature for a certain length
of time, and then cooled, either rapidly or slowly. This takes
longer than the color method, but with low temperatures the results
are more satisfactory, because the temperature of the bath can
be controlled with a pyrometer. The tempering temperatures given
in the following table are taken from a handbook issued by the
Midvale Steel Company.
TABLE 23.--TEMPERING TEMPERATURES FOR STEELS
----------------------------------------------------------------------------
Temperature | | Temperature |
for 1 hr. | | for 8 min. |
---------------| Color |---------------| Uses
Deg. F.|Deg. C.| |Deg. F.|Deg. C.|
-------|-------|------------|-------|-------|-------------------------------
370 | 188 |Faint yellow| 460 | 238 |Scrapers, brass-turning tools,
| | | | |reamers, taps, milling cutters,
| | | | |saw teeth.
390 | 199 |Light straw | 510 | 265 |Twist drills, lathe tools,
| | | | |planer tools, finishing tools
410 | 210 |Dark straw | 560 | 293 |Stone tools, hammer faces,
| | | | |chisels for hard work, boring
| | | | |cutters.
430 | 221 |Brown | 610 | 321 |Trephining tools, stamps.
450 | 232 |Purple | 640 | 337 |Cold chisels for ordinary work,
| | | | |carpenters' tools, picks, cold
| | | | |punches, shear blades, slicing
| | | | |tools, slotter tools.
490 | 254 |Dark blue | 660 | 343 |Hot chisels, tools for hot
| | | | |work, springs.
510 | 265 |Light blue | 710 | 376 |Springs, screw drivers.
----------------------------------------------------------------------------
It will be noted that two sets of temperatures are shown, one being
specified for a time interval of 8 min. and the other for 1 hr. For
the finest work the longer time is preferable, while for ordinary
rough work 8 min. is sufficient, after the steel has reached the
specified temperature.
The rate of cooling after tempering seems to be immaterial, and
the piece can be cooled at any rate, providing that in large pieces
it is sufficiently slow to prevent strains.
KNOWING WHAT TAKES PLACE.--How are we to know if we have given a
piece of steel the very best possible treatment?
The best method is by microscopic examination of polished and etched
sections, but this requires a certain expense for laboratory equipment
and upkeep, which may prevent an ordinary commercial plant from
attempting such a refinement. It is highly recommended that any
firm that has any large amount of heat treatment to do, install
such an equipment, which can be purchased for from $250 to $500.
Its intelligent use will save its cost in a very short time.
The other method is by examination of fractures of small test bars.
Steel heated to its correct temperatures will show the finest possible
grain, whereas underheated steel has not had its grain structure
refined sufficiently, and so will not be at its best. On the other
hand, overheated steel will have a coarser structure, depending
on the extent of overheating.
To determine the proper quenching temperature of any particular
grade of steel it is only necessary to heat pieces to various
temperatures not more than 20°C. (36°F.) apart, quench in water,
break them, and examine the fractures. The temperature producing
the finest grain should be used for annealing and hardening.
Similarly, to determine tempering temperatures, several pieces
should be hardened, then tempered to various degrees, and cooled
in air. Samples, say six, reheated to temperatures varying by 100°
from 300 to 800°C. will show a considerable range of properties,
and the drawing temperature of the piece giving the desired results
can be used.
For drawing tempers up to 500°F. oil baths of fresh cotton seed
oil can be safely and satisfactorily used. For higher temperature
a bath of some kind of fused salt is recommended.
HINTS FOR TOOL STEEL USERS
Do not hesitate to ask for information from the maker as to the
best steel to use for a given purpose, mentioning in as much detail
as possible the use for which it is intended.
Do not heat the steel to a higher degree than that fixed in the
description of each class. Never heat the steel to more than a
cherry red without forging it or giving it a definite heat treatment.
Heating steel at even moderate temperature is liable to coarsen the
grain which can only be restored by forging or by heat treating.
Let the forging begin as soon as the steel is hot enough and never
let tool steel soak in the fire. Continue the hammering vigorously
and constantly, using lighter blows as it cools off, and stopping
when the heat becomes a very dull red or a faint brown.
Should welding be necessary care should be taken not to overheat
in order to make an easy weld. Keep it below the sparkling point
as this indicates that the steel is burnt.
Begin to forge as soon as the welds are put together, taking care
to use gentle strokes at first increasing them as the higher heat
falls, but not overdoing the hammering when the steel cools. The
hammering should be extended beyond the welding point and should
continue until the dull red or brown heat is reached.
PREVENTING CRACKS IN HARDENING
The blacksmith in the small shop, where equipment is usually very
limited, often consisting of a forge, a small open hard-coal furnace,
a barrel of water and a can of oil must have skill and experience.
With this equipment the smith is expected to, and usually can,
produce good results if proper care is taken.
In hardening carbon tool steel in water, too much cannot be said in
favor of slow, careful heating, nor against overheating if cracks
are to be avoided.
It is not wise to take the work from the hardening bath and leave
it exposed to the air if there is any heat left in it, because
it is more liable to crack than if left in the bath until cold.
In heating, plenty of time is taken for the work to heat evenly
clear through, thus avoiding strains caused by quick and improper
heating, In quenching in water, contraction is much more rapid
than was the expansion while heating, and strains begin the moment
the work touches the water. If the piece has any considerable size
and is taken from the bath before it is cold and allowed to come to
the air, expansion starts again from the inside so rapidly that the
chilled hardened surface cracks before the strains can be relieved.
Many are most successful with the hardening bath about blood warm.
When the work that is being hardened is nearly cold, it is taken
from the water and instantly put into a can of oil, where it is
allowed to finish cooling. The heat in the body of the tool will
come to the surface more slowly, thus relieving the strain and
overcoming much of the danger of cracking.
Some contend that the temper should be drawn as soon as possible
after hardening: but that if this cannot be done for some hours, the
work should be left in the oil until the tempering can be done. It
is claimed that forming dies and punch-press dies that are difficult
to harden will seldom crack if treated in this way.
Small tools or pieces that are very troublesome because of peculiar
shape should be made of steel which has been thoroughly annealed.
It is often well to mill or turn off the outer skin of the bar,
to remove metal which has been cold-worked. Then heat slowly just
through the critical range and cool in the furnace, in order to
produce a very fine grain. Tools machined from such stock, and
hardened with the utmost care, will have the best chance to survive
without warping, growth or cracking.
SHRINKING AND ENLARGING WORK
Steel can be shrunk or enlarged by proper heating and cooling.
Pins for forced fits can be enlarged several thousandths of an
inch by rapid heating to a dull red and quenching in water. The
theory is that the metal is expanded in heating and that the sudden
cooling sets the outer portion before the core can contract. In
dipping the piece is not held under water till cold but is dipped,
held a moment and removed. Then dipped again and again until cold.
Rings and drawing dies are also shrunk in a similar way. The rings
are slowly heated to a cherry red, slipped on a rod and rolled
in a shallow pan of water which cools only the outer edge. This
holds the outside while the inner heated portion is forced inward,
reducing the hole. This operation can be repeated a number of times
with considerable success.
TEMPERING ROUND DIES
A number of circular dies of carbon tool steel for use in tool
holders of turret lathes were required. No proper tempering oven
was available, so the following method was adopted and proved quite
successful.
After the dies had been hardened dead hard in water, they were
cleaned up bright. A pair of ordinary smiths' tongs was made with
jaws of heavy material and to fit nicely all around the outside of
the die, leaving a 3/32-in. space when the jaws were closed around
the die. The dies being all ready, the tongs were heated red hot, and
the dies were picked up and held by the tongs. This tempered them
from the outside in, left the teeth the temper required and the
outside slightly softer. The dies held up the work successfully
and were better than when tempered in the same bath.
THE EFFECT OF TEMPERING ON WATER-QUENCHED GAGES
The following information has been supplied by Automatic and Electric
Furnaces, Ltd., 6, Queenstreet, London, S. W.:
Two gages of 3/4 in. diameter, 12 threads per inch, were heated
in a Wild-Barfield furnace, using the pyroscopic detector, and
were quenched in cold water. They were subsequently tempered in a
salt bath at various increasing temperatures, the effective diameter
of each thread and the scleroscope hardness being measured at each
stage. The figures are in 10,000ths of an inch, and indicate the
change + or - with reference to the original effective diameter
of the gages. The results for the two gages have been averaged.
TABLE 24.--CHANGES DUE TO QUENCHING
----------------------------------------------------------------
| After |Tempering temperature, degrees Centigrade
Thread |quenching|-----------------------------------------
| | 220 | 260 | 300 | 340 | 380 | 420
------------|---------|------|------|------|------|------|------
1 | +25 | +19 | +17 | +15 | +13 | +11 | +11
2 | +18 | +12 | +11 | + 9 | + 6 | + 5 | + 5
3 | +12 | + 6 | + 5 | + 3 | 0 | 0 | 0
4 | +10 | + 4 | + 4 | + 2 | ... | 0 | - 1
5 | + 9 | + 4 | + 4 | + 2 | 0 | 0 | 0
6 | + 9 | + 4 | + 3 | + 2 | 0 | 0 | 0
7 | +10 | + 5 | + 5 | + 3 | + 2 | + 1 | + 2
8 | + 8 | + 4 | + 3 | + 2 | 0 | 0 | + 1
9 | + 9 | + 4 | + 3 | + 2 | + 1 | + 1 | + 1
10 | + 9 | + 5 | + 5 | + 3 | + 2 | + 2 | + 2
11 | + 7 | + 4 | + 4 | + 2 | + 1 | + 1 | + 1
12 | + 9 | + 5 | + 5 | + 5 | + 4 | + 4 | + 3
| | | | | | |
Scleroscope | 80 | 70 | 70 | 62 | 56 | 53 | 52
----------------------------------------------------------------
Had these gages been formed with a plain cylindrical end projecting
in front of the screw, the first two threads would have been prevented
from increasing more than the rest. The gages would then have been
fairly easily corrected by lapping after tempering at 220°C. Practically
no lapping would be required if they were tempered at 340°C. There
seems to be no advantage in going to a higher temperature than
this. The same degree of hardness could have been obtained with
considerably less distortion by quenching directly in fused salt. It
is interesting to note that when the swelling after water quenching
does not exceed 0.0012 in., practically the whole of it may be
recovered by tempering at a sufficiently high temperature, but when
the swelling exceeds this amount the steel assumes a permanently
strained condition, and at the most only 0.0014 in. can be recovered
by tempering.
TEMPERING COLORS ON CARBON STEELS
Opinions differ as to the temperature which is indicated by the
various colors, or oxides, which appear on steel in tempering.
The figures shown are from five different sources and while the
variations are not great, it is safer to take the average temperature
shown in the last column.
TABLE 25.--COLORS, TEMPERATURES, DEGREES FAHRENHEIT
----------------------------------------------------------
| _A_ | _B_ | _C_ | _D_ | _E_ | Average
------------------|-----|-----|-----|-----|-----|---------
Faint yellow | 430 | 430 | 430 | 430 | 430 | 430
Light straw | 475 | 460 | 450 | ... | 450 | 458
Dark straw | 500 | 500 | 470 | 450 | 470 | 478
Purple (reddish) | 525 | 530 | 520 | 530 | 510 | 523
Purple (bluish) | ... | 555 | 550 | 550 | 550 | 551
Blue | 575 | 585 | 560 | 580 | 560 | 572
Gray blue | ... | 600 | ... | 600 | 610 | 603
Greenish blue | ... | 625 | ... | ... | 630 | 627
----------------------------------------------------------
TABLE 26.--ANOTHER COLOR TABLE
----------------------------------------------------------
Degrees |
Fahrenheit | High temperatures judged by color
------------|---------------------------------------------
430 | Very pale yellow \
460 | Straw-yellow |
480 | Dark yellow |
500 | Brown-yellow > Visible in full daylight
520 | Brown-purple |
540 | Full purple |
560 | Full blue |
600 | Very dark blue /
752 | Red heat, visible in the dark
885 | Red heat, visible in the twilight
975 | Red heat, visible in the daylight
1,292 | Dark red
1,652 | Cherry-red
1,832 | Bright cherry-red
2,012 | Orange-red
2,192 | Orange-yellow
2,372 | Yellow-white
2,552 | White welding heat
2,732 | Brilliant white
2,912 | Dazzling white (bluish-white)
----------------------------------------------------------
These differences might easily be due to the difference in the light
at the time the colors were observed. It must also be remembered
that even a thin coating of oil will make quite a difference and
cause confusion. It is these possible sources of error, coupled
with the ever present chance of human error, that makes it advisable
to draw the temper of tools in an oil bath heated to the proper
temperature as shown by an accurate high-temperature thermometer.
Another table, by Gilbert and Barker, runs to much higher temperatures.
Beyond 2,200°, however, the eye is very uncertain.
TABLE 26.--COLORS FOR TEMPERING TOOLS
-----------------------------------------------------------------------
Approximate |
color and | Kind of tool
temperature |
--------------|--------------------------------------------------------
Yellow | Thread chasers, hollow mills (solid type) twist drills
430 to 450°F.| centering tools, forming tools, cut-off tools, profile
| cutters, milling cutters, reamers, dies, etc.
--------------|--------------------------------------------------------
Straw-yellow | Thread rolling dies, counterbores, countersinks. Shear
460°F. | blades, boring tools, engraving tools, etc.
--------------|--------------------------------------------------------
Brown-yellow | Taps, Thread dies, cutters, reamers, etc.
500°F. |
--------------|--------------------------------------------------------
Light purple | Taps, dies, rock drills, knives, punches, gages, etc.
530°F. |
--------------|--------------------------------------------------------
Dark purple | Circular saws for metal, augers, dental and surgical
550°F. | instruments, cold chisels, axes.
--------------|--------------------------------------------------------
Pale blue | Bone saws, chisels, needles, cutters, etc.
580°F. |
--------------|--------------------------------------------------------
Blue | Hack saws, wood saws, springs, etc.
600°F. |
-----------------------------------------------------------------------
CHAPTER X
HIGH-SPEED STEEL
For centuries the secret art of making tool steel was handed down
from father to son. The manufacture of tool steel is still an art
which, by the aid of science, has lost much of its secrecy; yet
tool steel is today made by practical men skilled as melters,
hammer-men, and rollers, each knowing his art. These practical
men willingly accept guidance from the chemist and metallurgists.
A knowledge of conditions existing today in the manufacture of
high-speed steel is essential to steel treaters. It is well for
the manufacturer to have steel treaters understand some of his
troubles and difficulties, so that they will better comprehend the
necessity of certain trade customs and practices, and, realizing
the manufacturer's desire to cooperate with them, will reciprocate.
The manufacturer of high-speed steel knows and appreciates the
troubles and difficulties that may sometimes arise in the heat-treating
of his product. His aim is to make a uniform steel that will best
meet the requirements of the average machine shop on general work,
and at the same time allow the widest variation in heat treatment
to give desired results.
High speed steel is one of the most complex alloys known. A
representative steel contains approximately 24 per cent of alloying
metals, namely, tungsten, chromium, vanadium, silicon, manganese,
and in addition there is often found cobalt, molybdenum, uranium,
nickel, tin, copper and arsenic.
STANDARD ANALYSIS
The selection of a standard analysis by the manufacturer is the
result of a series of compromises between various properties imparted
to the steel by the addition of different elements and there is a
wide range of chemical analyses of various brands. The steel, to
be within the range of generally accepted analysis, should contain
over 16 per cent and under 20 per cent tungsten; if of lower tungsten
content it should carry proportionately more chromium and vanadium.
The combined action of tungsten and chromium in steel gives to it the
remarkable property of maintaining its cutting edge at relatively high
temperature. This property is commonly spoken of as "red-hardness."
The percentages of tungsten and chromium present should bear a
definite relationship to each other. Chromium imparts to steel
a hardening property similar to that given by carbon, although
to a less degree. The hardness imparted to steel by chromium is
accompanied by brittleness. The chromium content should be between
3.5 and 5 per cent.
Vanadium was first introduced in high-speed steel as a "scavenger,"
thereby producing a more homogeneous product, of greater density
and physical strength. It soon became evident that vanadium used
in larger quantities than necessary as a scavenger imparted to
the steel a much greater cutting efficiency. Recently, no less an
authority than Prof. J. O. Arnold, of the University of Sheffield,
England, stated that "high-speed steels containing vanadium have
a mean efficiency of 108.9, as against a mean efficiency of 61.9
obtained from those without vanadium content." A wide range of
vanadium content in steel, from 0.5 to 1.5 per cent, is permissible.
An ideal analysis for high-speed steel containing 18 per cent tungsten
is a chromium content of approximately 3.85 per cent; vanadium, 0.85
to 1.10 per cent, and carbon, between 0.62 and 0.77 per cent.
DETRIMENTAL ELEMENTS.--Sulphur and phosphorus are two elements known
to be detrimental to all steels. Sulphur causes "red-shortness"
and phosphorus causes "cold-shortness." The detrimental effects
of these two elements counteract each other to some extent but
the content should be not over 0.02 sulphur and 0.025 phosphorus.
The serious detrimental effect of small quantities of sulphur and
phosphorus is due to their not being uniformly distributed, owing
to their tendency to segregate.
The manganese and silicon contents are relatively unimportant in
the percentages usually found in high-speed steel.
The detrimental effects of tin, copper and arsenic are not generally
realized by the trade. Small quantities of these impurities are
exceedingly harmful. These elements are very seldom determined
in customers' chemical laboratories and it is somewhat difficult
for public chemists to analyze for them.
In justice to the manufacturer, attention should be called to the
variations in chemical analyses among the best of laboratories.
Generally speaking, a steel works' laboratory will obtain results
more nearly true and accurate than is possible with a customer's
laboratory, or by a public chemist. This can reasonably be expected,
for the steel works' chemist is a specialist, analyzing the same
material for the same elements day in and day out.
The importance of the chemical laboratory to a tool-steel plant
cannot be over-estimated. Every heat of steel is analyzed for each
element, and check analyses obtained; also, every substance used
in the mix is analyzed for all impurities. The importance of using
pure base materials is known to all manufacturers despite chemical
evidence that certain detrimental elements are removed in the process
of manufacture.
The manufacture of high-speed steel represents the highest art
in the making of steel by tool-steel practice. Some may say, on
account of our increased knowledge of chemistry and metallurgy,
that the making of such steel has ceased to be an art, but has
become a science. It is, in fact an art; aided by science. The
human element in its manufacture is a decided factor, as will be
brought in the following remarks:
The heat treatment of steel in its broad aspect may be said to
commence with the melting furnace and end with the hardening and
tempering of the finished product. High-speed steel is melted by
two general types of furnace, known as crucible and electric. Steel
treaters, however, are more vitally interested in the changes that
take place in the steel during the various processes of manufacture
rather than a detailed description of those processes, which are
more or less familiar to all.
In order that good high-speed steel may be furnished in finished
bars, it must be of correct chemical analysis, properly melted and
cast into solid ingots, free from blow-holes and surface defects.
Sudden changes of temperature are to be guarded against at every
stage of its manufacture and subsequent treatment. The ingots are
relatively weak, and the tendency to crack due to cooling strains
is great. For this reason the hot ingots are not allowed to cool
quickly, but are placed in furnaces which are of about the same
temperature and are allowed to cool gradually before being placed
in stock. Good steel can be made only from good ingots.
Steel treaters should be more vitally interested in the important
changes which take place in high-speed steel during the hammering
operations than that of any other working the steel receives in
the course of its manufacture.
QUALITY AND STRUCTURE
The quality of high-speed steel is dependent to a very great extent
upon its structure. The making of the structure begins under the
hammer, and the beneficial effects produced in this stage persist
through the subsequent operations, provided they are properly carried
out. The massive carbides and tungstides present in the ingot are
broken down and uniformly distributed throughout the billet.
To accomplish this the reduction in area must be sufficient and the
hammer blows should be heavy, so as to carry the compression into
the center of the billet; otherwise, undesirable characteristics
such as coarse structure and carbide envelopes will exist and cause
the steel treater much trouble. Surface defects invisible in the
ingot may be opened up under the hammering operation, in which
event they are chipped from the hot billet.
Ingots are first hammered into billets. These billets are carefully
inspected and all surface defects ground or chipped. The hammered
billets are again slowly heated and receive a second hammering,
known as "cogging." The billet resulting therefrom is known as
a "cogged" billet and is of the proper size for the rolling mill
or for the finishing hammer.
Although it is not considered good mill practice, some manufacturers
who have a large rolling mill perform the very important cogging
operation in the rolling mill instead of under the hammer. Cogging
in a rolling mill does not break up and distribute the carbides and
tungstides as efficiently as cogging under the hammer; another objection
to cogging in the rolling mill is that there is no opportunity to
chip surface defects developed as they can be under the trained eye
of a hammer-man, thereby eliminating such defects in the finished
billet.
The rolling of high-speed steel is an art known to very few. The
various factors governing the proper rolling are so numerous that
it is necessary for each individual rolling mill to work out a
practice that gives the best results upon the particular analysis
of steel it makes. Important elements entering into the rolling
are the heating and finishing temperatures, draft, and speed of
the mill. In all of these the element of time must be considered.
High-speed steel should be delivered from the rolling mill to the
annealing department free from scale, for scale promotes the formation
of a decarbonized surface. In preparation of bars for annealing,
they are packed in tubes with a mixture of charcoal, lime, and
other material. The tubes are sealed and placed in the annealing
furnace and the temperature is gradually raised to about 1,650°F.,
and held there for a sufficient length of time, depending upon the
size of the bars. After very slow cooling the bars are removed
from the tubes. They should then show a Brinnell number of between
235 and 275.
The inspection department ranks with the chemical and metallurgical
departments in safeguarding the quality of the product. It inspects
all finished material from the standpoint of surface defects, hardness,
size and fracture. It rejects such steel as is judged not to meet
the manufacturer's standard. The inspection and metallurgical
departments work hand in hand, and if any department is not functioning
properly it will soon become evident to the inspectors, enabling
the management to remedy the trouble.
The successful manufacture of high-speed steel can only be obtained
by those companies who have become specialists. The art and skill
necessary in the successful working of such steel can be attained
only by a man of natural ability in his chosen trade, and trained
under the supervision of experts. To become an expert operator
in any department of its manufacture, it is necessary that the
operator work almost exclusively in the production of such steel.
As to the heat treatment, it is customary for the manufacturer
to recommend to the user a procedure that will give to his steel
a high degree of cutting efficiency. The recommendations of the
manufacturer should be conservative, embracing fairly wide limits,
as the tendency of the user is to adhere very closely to the
manufacturer's recommendations. Unless one of the manufacturer's
expert service men has made a detailed study of the customer's
problem, the manufacturer is not justified in laying down set rules,
for if the customer does a little experimenting he can probably
modify the practice so as to produce results that are particularly
well adapted to his line of work.
The purpose of heat-treating is to produce a tool that will cut so
as to give maximum productive efficiency. This cutting efficiency
depends upon the thermal stability of the complex hardenites existing
in the hardened and tempered steel. The writer finds it extremely
difficult to convey the meaning of the word "hardenite" to those that
do not have a clear conception of the term. The complex hardenites
in high-speed steel may be described as that form of solid solution
which gives to it its cutting efficiency. The complex hardenites are
produced by heating the steel to a very high temperature, near the
melting point, which throws into solution carbides and tungstides,
provided they have been properly broken up in the hammering process
and uniformly distributed throughout the steel. By quenching the
steel at correct temperature this solid solution is retained at
atmospheric temperature.
It is not the intention to make any definite recommendations as to
heat-treating of high-speed steel by the users. It is recognized
that such steel can be heat-treated to give satisfactory results
by different methods. It is, however, believed that the American
practice of hardening and tempering is becoming more uniform. This
is due largely to the exchange of opinions in meetings and elsewhere.
The trend of American practice for hardening is toward the following:
_First_, slowly and carefully preheat the tool to a temperature
of approximately 1,500°F., taking care to prevent the formation
of excessive scale.
_Second_, transfer to a furnace, the temperature of which is
approximately 2,250 to 2,400°F., and allow to remain in the furnace
until the tool is heated uniformly to the above temperature.
_Third_, cool rapidly _in oil_, dry air blast, or lead bath.
_Fourth_, draw back to a temperature to meet the physical requirements
of the tool, and allow to cool in air.
It was not very long ago that the desirability of drawing hardened
high-speed steel to a temperature of 1,100° was pointed out, and it
is indeed encouraging to learn that comparatively few treaters have
failed to make use of this fact. Many treaters at first contended
that the steel would be soft after drawing to this temperature and
it is only recently, since numerous actual tests have demonstrated
its value, that the old prejudice has been eliminated.
High-speed steel should be delivered only in the annealed condition
because annealing relieves the internal strains inevitable in the
manufacture and puts it in vastly improved physical condition. The
manufacturer's inspection after annealing also discloses defects
not visible in the unannealed state.
The only true test for a brand of high-speed steel is the service that
it gives by continued performance month in and month out under actual
shop conditions. The average buyer is not justified in conducting a
test, but can well continue to purchase his requirements from a
reputable manufacturer of a brand that is nationally known. The
manufacturer is always willing to cooperate with the trade in the
conducting of a test and is much interested in the information
received from a well conducted test. A test, to be valuable, should be
conducted in a manner as nearly approaching actual working conditions
in the plant in which the test is made as is practical. In conducting
a test a few reputable brands should be allowed to enter. All tools
entered should be of exactly the same size and shape. There is much
difference of opinion as to the best practical method of conducting
a test, and the decision as to how the test should be conducted
should be left to the customer, who should cooperate with the
manufacturers in devising a test which would give the best basis
for conclusions as to how the particular brands would perform under
actual shop conditions.
The value of the file test depends upon the quality of the file and
the intelligence and experience of the person using it. The file
test is not reliable, but in the hands of an experienced operator,
gives some valuable information. Almost every steel treater knows
of numerous instances where a lathe tool which could be touched
with a file has shown wonderful results as to cutting efficiency.
Modern tool-steel practice has changed from that of the past, not
by the use of labor-saving machinery, but by the use of scientific
devices which aid and guide the skilled craftsman in producing a
steel of higher quality and greater uniformity. It is upon the
intelligence, experience, and skill of the individual that quality
of tool steel depends.
HARDENING HIGH-SPEED STEELS
We will now take up the matter of hardening high-speed steels. The
most ordinary tools used are for lathes and planers. The forging
should be done at carbon-steel heat. Rough-grind while still hot
and preheat to about carbon-steel hardening heat, then heat quickly
in high-speed furnace to white heat, and quench in oil. If a very
hard substance is to be cut, the point of tool may be quenched in
kerosene or water and when nearly black, finish cooling in oil.
Tempering must be done to suit the material to be cut. For cutting
cast iron, brass castings, or hard steel, tempering should be done
merely to take strains out of steel.
On ordinary machinery steel or nickel steel the temper can be drawn
to a dark blue or up to 900°F. If the tool is of a special form
or character, the risk of melting or scaling the point cannot be
taken. In these cases the tool should be packed, but if there is
no packing equipment, a tool can be heated to as high heat as is
safe without risk to cutting edges, and cyanide or prussiate of
potash can be sprinkled over the face and then quenched in oil.
Some very adverse criticism may be heard on this point, but experience
has proved that such tools will stand up very nicely and be perfectly
free from scales or pipes. Where packing cannot be done, milling
cutters, and tools to be hardened all over, can be placed in muffled
furnace, brought to 2,220° and quenched in oil. All such tools,
however, must be preheated slowly to 1,400 to 1,500° then placed in
a high-speed furnace and brought up quickly. Do not soak high-speed
steel at high heats. Quench in oil.
We must bear in mind that the heating furnace is likely to expand
tools, therefore provision must be made to leave extra stock to
take care of such expansion. Tools with shanks such as counter
bores, taps, reamers, drills, etc., should be heated no further
than they are wanted hard, and quench in oil. If a forge is not
at hand and heating must be done, use a muffle furnace and cover
small shanks with a paste from fire clay or ground asbestos. Hollow
mills, spring threading dies, and large cutting tools with small
shanks should have the holes thoroughly packed or covered with
asbestos cement as far as they are wanted soft.
CUTTING-OFF STEEL FROM BAR
To cut a piece from an annealed bar, cut off with a hack saw, milling
cutter or circular saw. Cut clear through the bar; do not nick or
break. To cut a piece from an unannealed bar, cut right off with
an abrasive saw; do not nick or break. If of large cross-section,
cut off hot with a chisel by first slowly and uniformly heating
the bar, at the point to be cut, to a good lemon heat, 1,800 to
1,850°F. and cut right off while hot; do not nick or break. Allow
the tool length and bar to cool before reheating for forging.
LATHE AND PLANER TOOLS
FORGING.--Gently warm the steel to remove any chill, is particularly
desirable in the winter, then heat slowly and carefully to a scaling
heat, that is a lemon heat (1,800 to 2,000°F.), and forge uniformly.
Reheat the tool for further forging directly the steel begins to
stiffen under the hammer. Under no circumstances forge the steel
when the temperature falls below a dark lemon to an orange color
about 1,700°F. Reheat as often as is necessary to finish forging
the tool to shape. Allow the tool to cool after forging by burying
the tool in dry ashes or lime. Do not place on the damp ground
or in a draught of air.
The heating for forging should be done preferably in a pipe or
muffle furnace but if this is not convenient use a good clean fire
with plenty of fuel between the blast pipe and the tool. Never
allow the tool to soak after the desired forging heat has been
reached. Do not heat the tool further back than is necessary to
shape the tool, but give the tool sufficient heat. See that the
back of the tool is flatly dressed to provide proper support under
the nose of the tool.
HARDENING HIGH-SPEED STEEL.--Slowly reheat the cutting edge of
the tool to a cherry red, 1,400°F., then force the blast so as
to raise the temperature quickly to a full white heat, 2,200 to
2,250°F., that is, until the tool starts to sweat at the cutting
face. Cool the point of the tool in a dry air blast or preferably
in oil, further cool in oil keeping the tool moving until the tool
has become black hot.
To remove hardening strains reheat the tool to from 500 to 1,100°F.
Cool in oil or atmosphere. This second heat treatment adds to the
toughness of the tool and therefore to its life.
GRINDING TOOLS.--Grind tools to remove all scale. Use a quick-cutting,
dry, abrasive wheel. If using a wet wheel, be sure to use plenty
of water. Do not under any circumstances force the tool against
the wheel so as to draw the color, as this is likely to set up
checks on the surface of the tool to its detriment.
FOR MILLING CUTTERS AND FORMED TOOLS
FORGING.--Forge as before.--ANNEALING.--Place the steel in a pipe,
box or muffle. Arrange the steel so as to allow at least 1 in.
of packing, consisting of dry powder ashes, powdered charcoal,
mica, etc., between the pieces and the walls of the box or pipe.
If using a pipe close the ends. Heat slowly and uniformly to a
cherry red, 1,375 to 1,450°F. according to size. Hold the steel at
this temperature until the heat has thoroughly saturated through
the metal, then allow the muffle box and tools to cool very slowly
in a dying furnace or remove the muffle with its charge and bury
in hot ashes or lime. The slower the cooling the softer the steel.
The heating requires from 2 to 10 hr. depending upon the size of
the piece.
HARDENING AND TEMPERING.--It is preferable to use two furnaces
when hardening milling cutters and special shape tools. One furnace
should be maintained at a uniform temperature from 1,375 to 1,450°F.
while the other should be maintained at about 2,250°F. Keep the
tool to be hardened in the low temperature furnace until the tool
has attained the full heat of this furnace. A short time should be
allowed so as to be assured that the center of the tool is as hot
as the outside. Then quickly remove the tool from this preheating
furnace to the full heat furnace. Keep the tool in this furnace only
as long as is necessary for the tool to attain the full temperature
of this furnace. Then quickly remove and quench in oil or in a
dry air blast. Remove before the tool is entirely cold and draw
the temper in an oil bath by raising the temperature of the oil
to from 500 to 750°F. and allow this tool to remain, at this
temperature, in the bath for at least 30 min., insuring uniformity
of temper; then cool in the bath, atmosphere or oil.
If higher drawing temperatures are desired than those possible
with oil, a salt bath can be used. A very excellent bath is made
by mixing two parts by weight of crude potassium nitrate and three
parts crude sodium nitrate. These will melt at about 450°F. and
can be used up to 1,000°F. Before heating the steel in the salt
bath, slowly preheat, preferably in oil. Reheating the hardened
high-speed steel to 1,000°F. will materially increase the life
of lathe tools, but milling and form cutters, taps, dies, etc.,
should not be reheated higher than 500 to 650°F., unless extreme
hardness is required, when 1,100 to 1,000°F., will give the hardest
edge.
INSTRUCTIONS FOR WORKING HIGH-SPEED STEEL
Owing to the wide variations in the composition of high-speed steels
by various makers, it is always advisable to follow the directions
of each when using his brand of steel. In the absence of specific
directions the following general suggestions from several makers
will be found helpful.
The Ludlum Steel Company recommend the following:
CUTTING-OFF.--To cut a piece from an annealed bar, cut off with
a hack saw, milling cutter or circular saw. Cut clear through the
bar; do not nick or break. To cut a piece from an unannealed bar,
cut right off with an abrasive saw; do not nick or break. If of
large cross-section, cut off hot with a chisel by first slowly
and uniformly heating the bar, at the point to be cut, to a good
lemon heat, 1,800°-1,850°F. and cut right off while hot; do not nick
or break. Allow the tool length and bar to cool before reheating
for forging.
LATHE AND PLANER TOOLS
TO FORGE.--Gently warm the steel to remove any chill is particularly
desirable in the winter. Then heat slowly and carefully to a scaling
heat, that is a lemon heat (1,800°-2,000°F.), and forge uniformly.
Reheat the tool for further forging directly the steel begins to
stiffen under the hammer. Under no circumstances forge the steel
when the temperature falls below a dark lemon to an orange color:
about 1,700°F. Reheat as often as is necessary to finish forging
the tool to shape. Allow the tool to cool after forging by burying
the tool in dry ashes or lime. Do not place on the damp ground
or in a draught of air.
The heating for forging should be done preferably in a pipe or
muffle furnace, but if this is not convenient use a good clean
fire with plenty of fuel between the blast pipe and the tool. Never
allow the tool to soak after the desired forging heat has been
reached. Do not heat the tool further back than is necessary to
shape the tool, but give the tool sufficient heat. See that the
back of the tool is flatly dressed to provide proper support under
the nose of the tool.
HARDENING.--Slowly reheat the cutting edge of the tool to a cherry
red, 1,400°F., then force the blast so as to raise the temperature
quickly to a full white heat, 2,200°-2,250°F., that is, until the
tool starts to sweat at the cutting face. Cool the point of the
tool in a dry air blast or preferably in oil; further cool in oil,
keeping the tool moving until the tool has become black hot.
To remove hardening strains reheat the tool to from 500° to 1,100°F.
Cool in oil or atmosphere. This second heat treatment adds to the
toughness of the tool and therefore to its life.
GRINDING.--Grind tools to remove all scale. Use a quick cutting,
dry, abrasive wheel. If using a wet wheel, be sure to use plenty
of water. Do not under any circumstances force the tool against
the wheel so as to draw the color, as this is likely to set up
checks on the surface of the tool to its detriment.
The Firth-Sterling Steel Company say:
INSTEAD OF PRINTING ANY RULES ON THE HARDENING AND TEMPERING OF
FIRTH-STERLING STEELS WE WISH TO SAY TO OUR CUSTOMERS: TRUST THE
STEEL TO THE SKILL AND THE JUDGEMENT OF YOUR TOOLSMITH AND TOOL
TEMPERER.
The steel workers of today know by personal experience and by
inheritance all the standard rules and theories on forging, hardening
and tempering of all fine tool steels. They know the importance of
slow, uniform heating, and the danger of overheating some steels,
and underheating others.
The tempering of tools and dies is a science taught by heat, muscle
and brains.
The tool temperer is the man to hold responsible for results. The
tempering of tools has been his life work. He may find suggestions
on the following pages interesting, but we are always ready to
trust the treatment of our steels to the experienced man at the
fire.
HEAT TREATMENT OF LATHE, PLANER AND SIMILAR TOOLS
FIRE.--For these tools a good fire is one made of hard foundry
coke, broken in small pieces, in an ordinary blacksmith forge with
a few bricks laid over the top to form a hollow fire. The bricks
should be thoroughly heated before tools are heated. Hard coal
may be used very successfully in place of hard coke and will give
a higher heat. It is very easy to give Blue Chip the proper heat
if care is used in making up the fire.
FORGING.--Heat slowly and uniformly to a good forging heat. Do
not hammer the steel after it cools below a bright red. Avoid as
much as possible heating the body of the tool, so as to retain
the natural toughness in the neck of the tool.
HARDENING.--Heat the point of the tool to an extreme white heat
(about 2,200°F.) until the flux runs. This heat should be the highest
possible short of melting the point. Care should be taken to confine
the heat as near to the point as possible so as to leave the annealing
and consequent toughness in the neck of the tool and where the tool
is held in the tool post.
COOL in an air blast, the open air or in oil, depending upon the
tools or the work they are to do.
For roughing tools temper need not be drawn except for work where
the edge tends to crumble on account of being too hard.
For finishing tools draw the temper to suit the purpose for which
they are to be used.
GRIND thoroughly on dry wheel (or wet wheel if care is used to prevent
checking).
HEAT TREATMENT OF MILLING CUTTERS, DRILLS, REAMERS, ETC.
THE FIRE.--Gas and electric furnaces designed for high heats are
now made for treating high-speed steels. We recommend them for
treating all kinds of Blue Chip tools and particularly the above
class. After tools reach a yellow heat in the forge fire they must
not be allowed to touch the fuel or come in contact with the blast
or surrounding air.
HEATING.--Tools of this kind should be heated to a mellow white
heat, or as hot as possible without injuring the cutting edges
(2,000 to 2,200°F.). For most work the higher the heat the better
the tool. Where furnaces are used, we recommend preheating the
tools to a red heat in one furnace before putting them in a white
hot furnace.
COOLING.--We recommend quenching all of the above tools in oil
when taken from the fire. We have found fish oil, cottonseed oil,
Houghton's No. 2 soluble oil and linseed oil satisfactory. The
high heat is the important thing in hardening Blue Chip tools.
If a white hot tool is allowed to cool in the open air it will be
hard, but the air scales the tool.
DRAWING THE TEMPER.--Tools of this class should be drawn considerably
more than water-hardening steel for the same purpose.
HEAT TREATMENT OF PUNCHES AND DIES, SHEARS, TAPS, ETC.
HEATING.--The degree to which tools of the above classes should
be heated depends upon the shape, size and use for which they are
intended. Generally, they should not be heated to quite as high a
heat as lathe tools or milling cutters. They should have a high
heat, but not enough to make the flux run on the steel (by pyrometer
1,900 to 2,100°F.).
COOLING.--Depending on the tools, some should be dipped in oil
all over, some only part way, and others allowed to cool down in
the air naturally, or under air blast. In cooling, the toughness
is retained by allowing some parts to cool slowly and quenching
parts that should be hard.
DRAWING THE TEMPER.--As in cooling, some parts of these tools will
require more drawing than others, but, on the whole, they must
be drawn more than water hardening tools for the same purpose or
to about 500°F. all over, so that a good file will just "touch"
the cutting or working parts.
BARIUM CHLORIDE PROCESS.--This is a process developed for treating
certain classes of tools, such as taps, forming tools, etc. It is
being successfully used in many large plants. Briefly the treatment
is as follows:
In this treatment the tools are first preheated to a red heat,
but small tools may be immersed without preheating. The barium
chloride bath is kept at a temperature of from 2,000 to 2,100°F.,
and tools are held in it long enough to reach the same temperature.
They are then dipped in oil. The barium chloride which adheres
to the tools is brushed off, leaving the tools as dean as before
heating.
A CHROMIUM-COBALT STEEL
The Latrobe Steel Company make a high-speed steel without tungsten,
its red-hardness properties depending on chromium and cobalt instead
of tungsten. It is known as P. R. K-33 steel. It does not require
the high temperature of the tungsten steels, hardening at 1,830 to
1,850°F. instead of 2,200° or even higher, as with the tungsten.
This steel is forged at 1,900 to 2,000°F. and must not be worked
at a lower temperature than 1,600°F. It requires soaking in the
fire more than the tungsten steels. It can be normalized by heating
slowly and thoroughly to 1,475°F., holding this for from 10 to 20
min. according to the size of the piece and cooling in the open
air, protected from drafts.
A peculiarity of this steel is that it becomes non-magnetic at or
above 1,960°F. and the magnetic quality is not restored by cooling.
Normalizing as above, however, restores the magnetic qualities. This
enables the user to detect any tools which have been overheated,
with a horseshoe magnet.
It is sometimes advantageous to dip tools, before heating for hardening,
in ordinary fuel or quenching oil. The oil leaves a thin film of
carbon which tends to prevent decarbonization, giving a very hard
surface.
For other makes of high-speed steel used in lathe and planer tools
the makers recommend that the tools be cut from the bar with a
hack saw or else heated and cut with a chisel. The heating should
be very slow until the steel reaches a red after which it can be
heated more rapidly and should only be forged at a high heat. It
can be forged at very high heats but care should be taken not to
forge at a low heat. The heating should be uniform and penetrate
clear to the center of the bar before forging is begun. Reheat
as often as necessary to forge at the proper heat.
After forging cool in lime before attempting to harden. Do not
attempt to harden with the forging heat as was sometimes done with
the carbon tools.
For hardening forged tools, heat slowly up to a bright red and
then rapidly until the point of the tool is almost at a melting
heat. Cool in a blast of cold, dry air. For large sizes of steel,
cool in linseed oil or in fish oil as is most convenient. If the
tools are to be used for finishing cuts heat to a bright yellow
and quench in oil. Grind for use on a sand wheel or grindstone
in preference to an emery or an artificial abrasive wheel.
For hardening milling and similar cutters, preheat to a bright
red, place the cutter on a round bar of suitable size, and revolve
it quickly over a very hot fire. Heat as high as possible without
melting the points of the teeth and cool in a cold blast of dry
air or in fish oil.
Light fragile cutters, twist drills, taps and formed cutters may
be heated almost white and then dipped in fish oil for hardening.
Where possible it is better to give an even higher heat and cool
in the blast of cold, dry air as previously recommended.
SUGGESTIONS FOR HANDLING HIGH-SPEED STEELS
The following suggestions for handling high-speed steels are given
by a maker whose steel is probably typical of a number of different
makes, so that they will be found useful in other cases as well.
These include hints as to forging as well as hardening, together
with a list of "dont's" which are often very useful. This applies
to forging, hardening of lathe, slotting, planing and all similar
tools.
[Illustration: FIG. 84.--All-steel, 5/8 in. square, 1/2 X 1 in.,
and larger is usually mild finished, and can be cut in a hack saw.
If cut off hot, be sure to heat the butt end slowly and thoroughly
in a clean fire. Rapid and insufficient heating invariably cracks
the steel. If you want to stamp the end with the name of the steel,
it is necessary that this is done at a good high orange color heat,
as it is otherwise apt to split the steel. (Take your time, do
not hurry.)]
HARDENING HIGH-SPEED STEEL
In forging use coke for fuel in the forge. Heat steel slowly and
thoroughly to a lemon heat. Do not forge at a lower heat. Do not
let the steel cool below a bright cherry red while forging. After
the tool is dressed, reheat to forging heat to remove the forging
strain, and lay on the floor until cold. Then have the tool rough
ground on a dry emery wheel.
[Illustration: FIG. 85.--Be sure to have a full yellow heat at the
dotted line. Remember this is a boring mill tool and will stand
out in the tool-post, and if you do not have a high thorough lemon
heat, your tool will snap off at the dotted line. (Ninety-five
per cent of all tools which break, have been forged at too low a
heat or at a heat not thorough to the center.)]
[Illustration: FIG. 86.--Keep your high lemon forging heat up.
If you forge under a steam hammer, take light blows. Do not jam
your tool into shape. Put frequently back into the fire. Never
let the high lemon color go down and beyond the dotted line.]
For built-up and bent tools special care should be taken that the
forging heat does not go below a bright cherry. For tools 3/4 by
1-1/2 or larger where there is a big strain in forging, such as
bending at angles of about 45 deg. and building the tools up, they
should be heated to at least 1,700°F. Slowly and without much blast.
For a 3/4 by 1-1/2 tool it should take about 10 min. with the correct
blast in a coke fire. Larger tools in proportion. They can then be
bent readily, but no attempt should be made to forge the steel
further without reheating to maintain the bright cherry red. This
is essential, as otherwise the tools crack in hardening or while
in use.
[Illustration: FIG. 87.--Be sure that the tool is absolutely straight
at the bottom, so as to lie flat in the tool-post.]
[Illustration: FIG. 88.--This is the finished forged tool, and
let this grow cold by itself, the slower the better. It is well to
cool the tool slowly in hot ashes, to remove all forging strain.
You can now grind the tool dry on a sharp emery wheel. The more
you now finish the tool in grinding, the less there is to come
off after hardening.]
In hardening place the tool in a coke fire (hollow fire if possible)
with a slow blast and heat gradually up to a white welding heat
on the nose of the tool. Then dip the white hot part only into
thin oil or hold in a strong cold air blast. When hardening in
oil do not hold the tool in one place but keep it moving so that
it cools as quickly as possible. It is not necessary to draw the
temper after hardening these tools.
[Illustration: FIG. 89.--This tool is ground, ready for hardening.
Never harden from the forging heat.]
[Illustration: FIG. 90.--Heat the nose of the tool only up to dotted
line, very slowly and thoroughly to an absolutely white welding
heat, so that it shows a trifle fused around the edges, and be
very sure that this fusing has gone thoroughly through the nose,
otherwise the fusing effect will be taken off after the second
grinding. Note the difference of the nose between this and Fig. 86.]
[Illustration: FIG. 91.--Shows unnecessary roasting and drossing.
Such hardening requires a great amount of grinding and is not good.
After hardening grind carefully on a wet emery wheel, and be sure
that the wheel is sharp with a plentiful supply of water. Do not
force the grinding, otherwise the cold water striking the steel
heated up by friction, will crack the nose. Be sure that the grinding
wheel is sharp.]
In grinding all tools should be ground as lightly as possible on
a soft wet sandstone or on a wet emery wheel, and care should be
taken not to create any surface cracks, which are invariably the
result of grinding too forcibly. The foregoing illustrations, Figs.
84 to 91, with their captions, will be found helpful.
Special points of caution to be observed when hardening high-speed
steel.
DON'T use a green coal fire; use coke, or build a hollow fire.
DON'T have the bed of the fire free from coal.
DON'T hurry the heating for forging. The heating has to be done
very slowly and the forging heat has to be kept very high (a full
lemon color) heat and the tool has to be continually brought back
into the fire to keep the high heat up. When customers complain
about seams and cracks, in 9 cases out of 10, this has been caused
by too low a forging heat, and when the blacksmith complains about
tools cracking, it is necessary to read this paragraph to him.
DON'T try to jam the tool into shape under a steam hammer with one
or two blows; take easy blows and keep the heat high.
DON'T have the tool curved at the bottom; it must lie perfectly
flat in the tool post.
DON'T harden from your forging heat; let the tool grow cold or
fairly cold. After forging you can rough grind the tool dry, but
not too forcibly.
DON'T, for hardening, get more than the nose white hot.
DON'T get the white heat on the surface only.
DON'T hurry your heating for hardening; let the heat soak thoroughly
through the nose of the tool.
DON'T melt the nose of the tool.
DON'T, as a rule, dip the nose into water; this should be done
only for extremely hard material. It is dangerous to put the nose
into water for fear of cracking and when you do put the nose into
water put just 1/2 in. only of the extreme white hot part into the
water and don't keep it too long in the water; just a few seconds,
and then harden in oil. We do not recommend water hardening.
DON'T grind too forcibly.
DON'T grind dry after hardening.
DON'T discolor the steel in grinding.
DON'T give too much clearance on tools for cutting cast iron.
DON'T start on cast iron with a razor edge on the tool. Take an
oil stone and wipe three or four times over the razor edge.
DON'T use tool holder steel from bars without hardening the nose
of each individual tool bit.
AIR-HARDENING STEELS.--These steels are recommended for boring,
turning and planing where the cost of high-speed seems excessive.
They are also recommended for hard wood knives, for roughing and
finishing bronze and brass, and for hot bolt forging dies. This
steel cannot be cut or punched cold but can be shaped and ground
on abrasive wheels of various kinds.
It should be heated slowly and evenly for forging and kept as evenly
heated at a bright red as possible. It should not be forged after
it cools to a dark red.
After the tool is made, heat it again to a bright red and lay it
down to cool in a dry place or it can be cooled in a cold, dry
air blast. Water must be kept away from it while it is hot.
CHAPTER XI
FURNACES
There are so many standard furnaces now on the market that it is
not necessary to go into details of their design and construction
and only a few will be illustrated. Oil, gas and coal or coke are
most common but there is a steady growth of the use of electric
furnaces.
[Illustration: FIG. 92.--Standard lead pot furnace.]
TYPICAL OIL-FIRED FURNACES.--Several types of standard oil-fired
furnaces are shown herewith. Figure 92 is a lead pot furnace, Fig.
93 is a vertical furnace with a center column. This column reduces
the cubical contents to be heated and also supports the cover.
[Illustration: FIG. 93.--Furnace with center column.]
A small tool furnace is shown in Fig. 94, which gives the construction
and heat circulation. A larger furnace for high-speed steel is
given in Fig. 95. The steel is supported above the heat, the lower
flame passing beneath the support.
For hardening broaches and long reamers and taps, the furnace shown
in Fig. 96 is used. Twelve jets are used, these coming in radially
to produce a whirling motion.
[Illustration: FIG. 94.--Furnace for cutting tools.]
[Illustration: FIG. 95.--High-speed steel furnace.]
Oil and gas furnaces may be divided into three types: the open
heating chamber in which combustion takes place in the chamber
and directly over the stock; the semimuffle heating chamber in
which combustion takes place beneath the floor of the chamber from
which the hot gases pass into the chamber through suitable openings;
and the muffle heating chamber in which the heat entirely surrounds
the chamber but does not enter it. The open furnace is used for
forging, tool dressing and welding. The muffle furnace is used for
hardening dies, taps, cutters and similar tools of either carbon
or high-speed steel. The muffle furnace is for spring hardening,
enameling, assaying and work where the gases of combustion may
have an injurious effect on the material.
[Illustration: FIG. 96.--Furnace for hardening broaches.]
[Illustration: FIG. 97.--Forging and welding furnace.]
[Illustration: FIG. 98.--Semi-muffle furnace.]
[Illustration: FIG. 99.--Muffle furnace.]
Furnaces of these types of oil-burning furnaces are shown in Figs.
97, 98, and 99; these being made by the Gilbert & Barker Manufacturing
Company. The first has an air curtain formed by jets from the large
pipe just below the opening, to protect the operator from heat.
[Illustration: FIG. 100.--Gas fired furnace.]
[Illustration: FIG. 101.--Car door type of annealing furnace.]
Oil furnaces are also made for both high- and low-pressure air,
each having its advocates. The same people also make gas-fired
furnaces.
Several types of furnaces for various purposes are illustrated
in Fig. 100 and 101. The first is a gas-fired hardening furnace
of the surface-combustion type.
A large gas-fired annealing furnace of the Maxon system is shown
in Fig. 101. This is large enough for a flat car to be run into
as can be seen. It shows the arrangement of the burners, the track
for the car and the way in which it fits into the furnace. These
are from the designs of the Industrial Furnace Corporation.
Before deciding upon the use of gas or oil, all sides of the problem
should be considered. Gas is perhaps the nearest ideal but is as a
rule more expensive. The tables compiled by the Gilbert & Barker
Manufacturing Company and shown herewith, may help in deciding
the question.
TABLE 27.--SHOWING COMPARISON OF OIL FUEL WITH VARIOUS GASEOUS FUELS
Heat units
per thousand
cubic feet
Natural gas 1,000,000
Air gas (gas machine) 20 cp 815,500
Public illuminating gas, average 650,000
Water gas (from bituminous coal) 377,000
Water and producer gas, mixed 175,000
Producer gas 150,000
Since a gallon of fuel oil (7 lb.) contains 133,000 heat units, the
following comparisons may evidently be made. At 5 cts. a gallon,
the equivalent heat units in oil would equal:
Per thousand
cubic feet
Natural gas at $0.375
Air gas, 20 cp at 0.307
Public illuminating gas, average at 0.244
Water gas (from bituminous coal) at 0.142
Water and producer gas, mixed at 0.065
Producer gas at 0.057
Comparing oil and coal is not always simple as it depends on the
work to be done and the construction of the furnaces. The variation
rises from 75 to 200 gal. of oil to a ton of coal. For forging
and similar work it is probably safe to consider 100 gal. of oil
as equivalent to a ton of coal.
Then there is the saving of labor in handling both coal and ashes,
the waiting for fires to come up, the banking of fires and the dirt
and nuisance generally. The continuous operation possible with
oil adds to the output.
When comparing oil and gas it is generally considered that 4-1/2
gal. of fuel oil will give heat equivalent to 1,000 cu. ft. of
coal gas.
The pressure of oil and air used varies with the system installed.
The low-pressure system maintains a pressure of about 8 oz. on the
oil and draws in free air for combustion. Others use a pressure
of several pounds, while gas burners use an average of perhaps
1-1/2 lb. of air to give best results.
The weights and volumes of solid fuels are: Anthracite coal, 55 to
65 lb. per cubic foot or 34 to 41 cubic feet per ton; bituminous
coal, 50 to 55 lb. per cubic foot or 41 to 45 cubic feet per ton;
coke, 28 lb. per cubic foot or 80 cubic feet per ton--the ton being
calculated as 2,240 lb. in each case.
A novel carburizing furnace that is being used by a number of people,
is built after the plan of a fireless cooker. The walls of the
furnace are extra heavy, and the ports and flues are so arranged
that when the load in the furnace and the furnace is thoroughly
heated, the burners are shut off and all openings are tightly sealed.
The carburization then goes on for several hours before the furnace
is cooled below the effective carburizing range, securing an ideal
diffusion of carbon between the case and the core of the steel
being carburized. This is particularly adaptable where simple steel
is used.
PROTECTIVE SCREENS FOR FURNACES
Workmen needlessly exposed to the flames, heat and glare from furnaces
where high temperatures are maintained suffer in health as well as
in bodily discomfort. This shows several types of shields designed
for the maximum protection of the furnace worker.
Bad conditions are not necessary; in almost every case means of relief
can be found by one earnestly seeking them. The larger forge shops
have adopted flame shields for the majority of their furnaces. Years
ago the industrial furnaces (particularly of the oil-burning variety)
were without shields, but the later models are all shield-equipped.
These shields are adapted to all of the more modern, heat-treating
furnaces, as well as to those furnaces in use for working forges;
and attention should be paid to their use on the former type since
the heat-treating furnaces are constantly becoming more numerous
as manufacturers find need of them in the many phases of munitions
making or similar work.
The heat that the worker about these furnaces must face may be
divided in general into two classes: there is first that heat due
to the flame and hot gases that the blast in the furnaces forces
out onto a man's body and face. In the majority of furnaces this
is by far the most discomforting, and care must be taken to fend
it and turn it behind a suitable shield. The second class is the
radiant heat, discharged as light from the glowing interior of
the furnace. This is the lesser of the two evils so far as general
forging furnaces are concerned, but it becomes the predominating
feature in furnaces of large door area such as in the usual
case-hardening furnaces. Here the amount of heat discharged is
often almost unbearable even for a moment. This heat can be taken
care of by interposing suitable, opaque shields that will temporarily
absorb it without being destroyed by it, or becoming incandescent.
Should such shields be so constructed as to close off all of the
heat, it might be impossible to work around the furnace for the
removal of its contents, but they can be made movable, and in such
a manner as to shield the major portion of the worker's body.
First taking up the question of flame shields, the illustration,
Fig. 102, is a typical installation that shows the main features
for application to a forging machine or drop-hammer, oil-burning
furnace, or for an arched-over, coal furnace where the flame blows
out the front. This shield consists of a frame covered with sheet
metal and held by brackets about 6 in. in front of the furnace.
It will be noted that slotted holes make this frame adjustable
for height, and it should be lowered as far as possible when in
use, so that the work may just pass under it and into the furnace
openings.
Immediately below the furnace openings, and close to the furnace
frame will be noted a blast pipe carrying air from the forge-shop
fan. This has a row of small holes drilled in its upper side for
the entire length, and these direct a curtain of cold air vertically
across the furnace openings, forcing all of the flame, or a greater
portion of it, to rise behind the shield. Since the shield extends
above the furnace top there is no escape for this flame until it has
passed high enough to be of no further discomfort to the workman.
In this case fan-blast air is used for cooling, and this is cheaper
and more satisfactory because a great volume may be used. However,
where high-pressure air is used for atomizing the oil at the burner,
and nothing else is available, this may be employed--though naturally
a comparatively small pipe will be needed, in which minute holes
are drilled, else the volume of air used will be too great for
the compressor economically to supply. Steam may also be employed
for like service.
[Illustration: FIGS. 102 to 108.--Protective devices for furnace
fronts.]
The latest shields of this type are all made double, as illustrated,
with an inner sheet of metal an inch or two inside of the front.
In the illustration, _A_, Fig. 102, this inner sheet is smaller,
but some are now built the same size as the front and bolted to
it with pipe spacers between. The advantage of the double sheet
is that the inner one bears the brunt of the flame, and, if needs
be, burns up before the outer; while, if due to a heavy fire it
should be heated red at any point, the outer sheet will still be
much cooler and act as an additional shield to the furnace man.
HEAVY FORGING PRACTICE.--In heavy forging practice where the metal
is being worked at a welding heat, the amount of flame that will
issue from an open-front furnace is so great that a plain, sheet-steel
front will neither afford sufficient protection nor stand up in
service. For such a place a water-cooled front is often used. The
general type of this front is illustrated in Fig. 103, and appears to
have found considerable favor, for numbers of its kind are scattered
throughout the country.
In this case the shield is placed at a slight angle from the vertical,
and along the top edge is a water pipe with a row of small holes
through which sprays of water are thrown against it. This water runs
down in a thin sheet over the shield, cooling it, and is collected
in a trough connected with a run-off pipe at the bottom. The lower
blast-pipe arrangement is similar to the one first described.
There are several serious objections to this form of shield that
should lead to its replacement by a better type; the first is that
with a very hot fire, portions in the center may become so rapidly
heated that the steam generated will part the sheet of water and
cause it to flow from that point in an inverted V, and that section
will then quickly become red hot. Another feature is that after
the water and fire are shut down for the night the heat of the
furnace can be great enough to cause serious warping of the surface
of the shield so that the water will no longer cover it in a thin,
uniform sheet.
After rigging up a big furnace with a shield of this type several
years ago, its most serious object was found in the increase of
the water bill of the plant. This was already of large proportions,
but it had suddenly jumped to the extent of several hundred dollars.
Investigation soon disclosed the fact that this water shield was one
of the main causes of the added cost of water. A little estimating
of the amount of water that can flow through a 1/2-in. pipe under
30-lb. pressure, in the course of a day, will show that this amount
at 10 cts. per 1,000 gal., can count up rather rapidly.
Figure 103 is a section through a portion of the furnace front and
shield showing all of the principal parts. This shield consists
essentially of a very thin tank, about 2-1/2 in. between walls,
and filled with water. Like other shields it is fitted with an
adjustment, that it may be raised and lowered as the work demands.
The tank having an open top, the water as it absorbs heat from
the flame will simply boil away in steam; and only a small amount
will have to be added to make up for that which has evaporated. The
water-feed pipe shown at _F_ ends a short distance above the top
of the tank so that just how much water is running in may readily
be seen.
An overflow pipe is provided at _O_ which aids in maintaining the
water at the proper height, as a sufficient quantity can always be
permitted to run in, to avoid any possibility of the shield ever
boiling dry; at the same time the small excess can run off without
danger of an overflow. The shield illustrated in Fig. 104 has been
in constant use for over two years, giving greater satisfaction
than any other of which the writer has known. It might also be
noted that this shield was made with riveted joints, the shop not
having a gas-welding outfit. To flange over the edges and then
weld them with an acetylene torch would be a far more economical
procedure, and would also insure a tight and permanent joint.
The water-cooled front shown in Fig. 105 is an absurd effort to
accomplish the design of a furnace that will provide cool working
conditions. This front was on a bolt-heating furnace using hard
coal for fuel; and it may be seen that it takes the place of all
of the brickwork that should be on that side. Had this been nothing
more than a very narrow water-cooled frame, with brickwork below
and supporting bricks above, put in like the tuyeres in a foundry
cupola, the case would have been somewhat different, for then it
would have absorbed a smaller proportion of the heat.
A blacksmith who knows how a piece of cold iron laid in a small
welding furnace momentarily lowers the temperature, will appreciate
the enormous amount of extra heat that must be maintained in the
central portion of this furnace to make up for the constant chilling
effect of the cold wall. Moreover, since there would have been
serious trouble had steam generated in this front, a steady stream
of water had to be run through it constantly to insure against
an approach to the boiling point. This is illustrated because of
its absurdity, and as a warning of something to avoid.
Water-cooled, tuyere openings, as mentioned above, which support
brick side-walls of the furnace, have proved successful for coal
furnaces used for forging machine and drop-hammer heating, since
they permit a great amount of work to be handled through their
openings without wearing away as would a brick arch. Great care
should be exercised properly to design them so that a minimum amount
of the cold tuyere will be in contact with the interior of the
furnace, and all interior portions possible should be covered by
the bricks. However, a discussion of these points will hardly come
in the flame-shield class, although they can be made to do a great
deal toward relieving the excessive heat to be borne by the furnace
worker.
FLANGE SHIELDS FOR FURNACES.--Such portable flame shields as the
one illustrated in Fig. 106 may prove serviceable before furnaces
required for plate work, where the doors are often only opened
for a moment at a time. This shield can be placed far enough in
front of the furnace, that it will be possible to work under it
or around it, in removing bulky work from the furnace, and yet
it will afford the furnace tender some relief from the excessive
glare that will come out the wide-opened door. To have this shield
of light weight so that it may be readily pushed aside when not
wanted, the frame may be made up of pipe and fittings, and a piece
of thin sheet steel fastened in the panel by rings about the frame.
About the most disagreeable task in a heat-treating shop is the
removal of the pots from the case-hardening furnaces; these must
be handled at a bright red heat in order that their contents may be
dumped into the quenching tank with a minimum-time contact with the
air, and before they have cooled sufficiently to require reheating.
Facing the heat before the large open doors of the majority of
these furnaces, in a man-killing task even when the weather is
moderately cool. The boxes soon become more or less distorted,
and then even the best of lifting devices will not remove a hot
pot without several minutes labor in front of the doors.
In Fig. 107 is shown a method of arranging a shield on one type of
charging and removing truck. This shield cannot afford more than
a partial protection to the body of the furnace tender, because
he must be able to see around it, and in some cases even push it
partly through the door of the furnace, but even small as it is it
may still afford some welcome protection. The great advantage in
this case of having the shield on the truck instead of stationary
in front of the furnace, is that it still affords protection as
long as the hot pot is being handled through the shop on its way
to the quenching tank.
It might be interesting to many engaged in the heat-treating or
case hardening of steel parts, to make a special note of the design
of the truck that is illustrated in connection with the shield;
the general form is shown although the actual details for the
construction of such a truck are lacking; these being simple, may be
readily worked out by anyone wishing to build one. This is considered
to be one of the quickest and easiest operated devices for the
removal of this class of work from the furnace. To be sure it may
only be used where the floor of the furnace has been built level
with the floor of the room, but many of the modern furnaces of
this class are so designed.
The pack-hardening pots are cast with legs, from two to three inches
high, to permit the circulation of the hot gases, and so heat more
quickly. Between these legs and under the body of the pot, the two
forward prongs of the truck are pushed, tilting the outer handle
to make these prongs as low as possible. The handle is then lowered
and, as it has a good leverage, the pot is easily raised from the
floor, and the truck and its load rolled out.
HEATING OF MANGANESE STEEL.--Another form of heat-treating furnace
is that which is used for the heating of manganese and other alloy
steels, which after having been brought to the proper heat are drawn
from the furnace into an immediate quenching tank. With manganese
steel in particular, the parts are so fragile and easily damaged
while hot that it is frequent practice to have a sloping platform
immediately in front of the furnace door down which the castings
may slide into a tank below the floor level. Such a furnace with
a quenching tank in front of its door is shown in Fig. 108.
These tanks are covered with plates while charging the furnace
and the cold castings are placed in a moderately cool furnace.
Since some of these steels must not be charged into a furnace where
the heat is extreme but should be brought up to their final heat
gradually, there is little discomfort during the charging process.
When quenching, however, from a temperature of 1,800° to 1,900°,
it is extremely unpleasant in front of the doors. The swinging
shield is here adapted to give protection for this work. As will
be noted it is hung a sufficient distance in front of the doors,
that it may not interfere with the castings as they come from the
furnace, and slide down into the tank.
To facilitate the work, and avoid the necessity of working with
the bars outside the edges of the shield, the slot-like hole is
cut in the center of the shield, and through this the bars or rakes
for dragging out the castings are easily inserted and manipulated.
The advantage of such a swinging shield is that it may be readily
moved from side to side, or forward and back as occasion requires.
FURNACE DATA
In order to give definite information concerning furnaces, fuels
etc., the following data is quoted from a paper by Seth A. Moulton
and W. H. Lyman before the Steel Heat Treaters Society in September,
1920.
This considers a factory producing 30,000 lb. of automobile gears
per 24 hr. The transmission gears will be of high-carbon steel and
the differential of low-carbon steel, carburized. The heat-treating
equipment required is:
1. Annealing furnaces 1,400 to 1,600°F.
2. Carburizing furnaces 1,700 to 1,800°F.
3. Hardening furnaces 1,450 to 1,550°F.
4. Drawing furnaces 350 to 950°F.
All of the forging blanks are annealed before machining, about
three-quarters of the machined gears and parts are carburized,
all the carburized gears are given a double treatment for core and
case, all gears and parts are hardened and all parts are drawn.
The possible sources of heat supply and their values are as follows:--
1. Oil 140,000 B.t.u. per gallon
2. Natural gas 1,100 B.t.u. per cubic foot
3. City gas 650 B.t.u. per cubic foot
4. Water gas 300 B.t.u. per cubic foot
5. Producer gas 170 B.t.u. per cubic foot
6. Coal 12,000 B.t.u. per pound
7. Electric current 3,412 B.t.u. per kilowatt-hour
For the heat treatment specified only comparatively low temperatures
are required. No difficulty will be experienced in attaining the
desired maximum temperature of 1,800°F. with any of the heating
medium above enumerated; but it should be noted that the producer
gas with a B.t.u. content of 170 per cubic foot and the electric
current would require _specially_ designed furnaces to obtain higher
temperatures than 1800°F.
TABLE 28.--COMPARATTVE OPERATING COSTS
Assuming
Cost of oil- and gas-fired furnaces
installed as $100.00 per square foot of hearth
Cost of coal-fired furnace installed as 150.00 per square foot of hearth
Cost of electric furnace 100 kw.
capacity installed as 90.00 per kilowatt
Cost of electric furnace 150 kw.
capacity installed as 70.00 per kilowatt
Output 3,000 lb. charge, 8 hr. heat carburizing, 2 hr. heating
only. Annual service 7,200 hr. Fixed charges including interest,
depreciation, taxes, insurance and maintenance 15 per cent. Extra
operating labor for coal-fired furnace 60 cts. per hour, one man
four furnaces.
COST OF VARIOUS TYPES OF FURNACES
-------------------------------------------------------------------------------
| Class fuel | Fuel per | Unit fuel|Installation|Efficiency| Fixed |Cost per
| | charge | cost | cost | per cent |charges| charge
-|------------|------------|----------|------------|----------|-------|--------
| 1 | 2 | 3 | 4 | 5 | 6 | 7
-|------------|------------|----------|------------|----------|-------|--------
Carburizing
-|------------|------------|----------|------------|----------|-------|--------
1|Oil | 52.0 gal. |$0.15 gal.| $2,400.00 | 12.6 | $.40 | $8.20
2|Natural gas | 4.4 M | 0.50 M | 2,400.00 | 18.8 | 0.40 | 2.60
3|City gas | 8.3 M | 0.80 M | 2,400.00 | 17.0 | 0.40 | 7.04
4|Water gas | 18.7 M | 0.40 | 2,400.00 | 16.4 | 0.40 | 7.88
5|Producer gas| 37.3 M | 0.10 M | 2,400.00 | 14.5 | 0.40 | 4.13
6|Coal |814.0 lb. | 6.00 ton | 3,600.00 | 9.4 | 0.60 | 3.98
7|Electricity |500.0 kw-hr.| 0.015 kw.| 9,000.00 | 53.0 | 1.50 | 9.00
-|------------|------------|----------|------------|----------|-------|--------
Heating
-|------------|------------|----------|------------|----------|-------|--------
1|Oil | 30.8 gal. | 0.15 gal.| 2,400.00 | 21.4 | 0.10 | 4.72
2|Natural gas | 2.61 M | 0.50 M | 2,400.00 | 32.0 | 0.10 | 1.40
3|City gas | 4.9 M | 0.80 M | 2,400.00 | 28.8 | 0.10 | 4.02
4|Water gas | 11.1 M | 0.40 M | 2,400.00 | 27.6 | 0.10 | 4.54
5|Producer gas| 22.1 M | 0.10 M | 2,400.00 | 24.6 | 0.10 | 2.31
6|Coal |348.0 lb. | 6.00 ton | 3,600.00 | 22.0 | 0.15 | 1.38
7|Electricity |329.0 kw-hr.| 0.015 kw.| 10,500.00 | 81.75 | 0.44 | 5.38
-------------------------------------------------------------------------------
This shows but two of the operations and for a single furnace.
The total costs for all operations on the 30,000 lb. of gears per
24 hr. is shown in Table 29.
TABLE 29.--COMPARATIVE ANNUAL PRODUCTION COSTS FOR 30,000 POUNDS
OUTPUT IN 24 HOURS
----------------------------------------------------------
No. | Equipment | Installation
| | cost
-----|-------------------------------------|--------------
1 | 2 | 3
| |
I | Oil | $179,000.00
II | Oil and electric | 213,000.00
III | Natural gas | 117,000.00
IV | (A) Natural gas containing furnaces | 120,000.00
V | Natural gas and electric | 181,000.00
VI | City gas | 122,000.00
VII | City gas and electric | 182,000.00
VIII| Water gas | 214,000.00
IX | Water gas and electric | 238,000.00
X | Producer gas | 246,000.00
XI | Producer gas and electric | 255,000.00
XII | Coal and electric | 194,000.00
XIII| Electric | 257,000.00
----------------------------------------------------------
---------------------------------------------------------------------
| Annual operating expenses | | Cost
No. |----------------------------------------| Total | per lb.
| Fixed | Heat | Labor | | metal,
| charges | | | | cents
-----|------------|-------------|-------------|-------------|--------
1 | 4 | 5 | 6 | 7 | 8
| | | | |
I | $26,850.00 | $156,000.00 | $105,000.00 | $287,850.00 | $3.19
II | 31,950.00 | 142,770.00 | 97,000.00 | 271,720.00 | 3.02
III | 17,550.00 | 44,250.00 | 97,000.00 | 158,800.00 | 1.78
IV | 18,000.00 | 41,000.00 | 94,000.00 | 153,000.00 | 1.70
V | 27,150.00 | 73,820.00 | 90,000.00 | 190,970.00 | 2.13
VI | 18,300.00 | 123,200.00 | 94,000.00 | 235,500.00 | 2.62
VII | 27,300.00 | 128,820.00 | 90,000.00 | 246,020.00 | 2.74
VIII| 18,600.00 | 104,000.00 | 94,000.00 | 216,600.00 | 2.41
IX | 27,450.00 | 117,420.00 | 90,000.00 | 234,870.00 | 2.62
X | 18,900.00 | 69,300.00 | 90,000.00 | 178,200.00 | 1.98
XI | 27,750.00 | 92,520.00 | 90,000.00 | 210,270.00 | 2.34
XII | 29,100.00 | 87,220.00 | 90,000.00 | 206,320.00 | 2.30
XIII| 38,550.00 | 135,000.00 | 84,000.00 | 257,550.00 | 2.86
---------------------------------------------------------------------
NOTE.--Producer plant fixed charges are included in the cost of
gas and are charged as "heat" in column 5, so they are omitted
from column 4.
CHAPTER XII
PYROMETRY AND PYROMETERS
A knowledge of the fundamental principles of pyrometry, or the
measurement of temperatures, is quite necessary for one engaged
in the heat treatment of steel. It is only by careful measurement
and control of the heating of steel that the full benefit of a
heat-treating operation is secured.
Before the advent of the thermo-couple, methods of temperature
measurement were very crude. The blacksmith depended on his eyes
to tell him when the proper temperature was reached, and of course
the "color" appeared different on light or dark days. "Cherry"
to one man was "orange" to another, and it was therefore almost
impossible to formulate any treatment which could be applied by
several men to secure the same results.
One of the early methods of measuring temperatures was the "iron
ball" method. In this method, an iron ball, to which a wire was
attached, was placed in the furnace and when it had reached the
temperature of the furnace, it was quickly removed by means of
the wire, and suspended in a can containing a known quantity of
water; the volume of water being such that the heat would not cause
it to boil. The rise in temperature of the water was measured by a
thermometer, and, knowing the heat capacity of the iron ball and
that of the water, the temperature of the ball, and therefore the
furnace, could be calculated. Usually a set of tables was prepared
to simplify the calculations. The iron ball, however, scaled, and
changed in weight with repeated use, making the determinations
less and less accurate. A copper ball was often used to decrease
this change, but even that was subject to error. This method is
still sometimes used, but for uniform results, a platinum ball,
which will not scale or change in weight, is necessary, and the
cost of this ball, together with the slowness of the method, have
rendered the practice obsolete, especially in view of modern
developments in accurate pyrometry.
PYROMETERS
Armor plate makers sometimes use the copper ball or Siemens' water
pyrometer because they can place a number of the balls or weights on
the plate in locations where it is difficult to use other pyrometers.
One of these pyrometers is shown in section in Fig. 109.
SIEMENS' WATER PYROMETER.--It consists of a cylindrical copper vessel
provided with a handle and containing a second smaller copper vessel
with double walls. An air space _a_ separates the two vessels, and
a layer of felt the two walls of the inner one, in order to retard
the exchange of temperature with the surroundings. The capacity
of the inner vessel is a little more than one pint. A mercury
thermometer _b_ is fixed close to the wall of the inner vessel,
its lower part being protected by a perforated brass tube, whilst
the upper projects above the vessel and is divided as usual on the
stem into degrees, Fahrenheit or Centigrade, as desired. At the
side of the thermometer there is a small brass scale _c_, which
slides up and down, and on which the high temperatures are marked
in the same degrees as those in which the mercury thermometer is
divided; on a level with the zero division of the brass scale a
small pointer is fixed, which traverses the scale of the thermometer.
[Illustration: FIG. 109.--Siemens' copper-ball pyrometer.]
Short cylinders _d_, of either copper, iron or platinum, are supplied
with the pyrometer, which are so adjusted that their heat capacity at
ordinary temperature is equal to one-fiftieth of that of the copper
vessel filled with one pint of water. As, however, the specific heat
of metals increases with the temperature, allowance is made on the
brass sliding scales, which are divided according to the metal used
for the pyrometer cylinder _d_. It will therefore be understood that
a different sliding scale is required for the particular kind of
metal of which a cylinder is composed. In order to obtain accurate
measurements, each sliding scale must be used only in conjunction
with its own thermometer, and in case the latter breaks a new scale
must be made and graduated for the new thermometer.
The water pyrometer is used as follows:
Exactly one pint (0.568 liter) of clean water, perfectly distilled
or rain water, is poured into the copper vessel, and the pyrometer
is left for a few minutes to allow the thermometer to attain the
temperature of the water.
The brass scale _c_ is then set with its pointer opposite the
temperature of the water as shown by the thermometer. Meanwhile
one of the metal cylinders has been exposed to the high temperature
which is to be measured, and after allowing sufficient time for
it to acquire that temperature, it is rapidly removed and dropped
into the pyrometer vessel without splashing any of the water out.
The temperature of the water will rise until, after a little while,
the mercury of the thermometer has become stationary. When this
is observed the degrees of the thermometer are read off, as well
as those on the brass scale _c_ opposite the top of the mercury.
The sum of these two values together gives the temperature of the
flue, furnace or other heated space in which the metal cylinder
had been placed. With cylinders of copper and iron, temperatures up
to 1,800°F. (1,000°C.) can be measured, but with platinum cylinders
the limit is 2,700°F. (1,500°C.).
For ordinary furnace work either copper or wrought-iron cylinders
may be used. Iron cylinders possess a higher melting point and have
less tendency to scale than those of copper, but the latter are
much less affected by the corrosive action of the furnace gases;
platinum is, of course, not subject to any of these disadvantages.
The weight to which the different metal cylinders are adjusted is
as follows:
Copper 137.0 grams
Wrought-iron 112.0 grams
Platinum 402.6 grams
In course of time the cylinders lose weight by scaling; but tables
are provided giving multipliers for the diminished weights, by
which the reading on the brass scale should be multiplied.
THE THERMO-COUPLE
With the application of the thermo-couple, the measurement of
temperatures, between, say, 700 and 2,500°F., was made more simple
and precise. The theory of the thermo-couple is simple; it is that
if two bars, rods, or wires of different metals are joined together
at their ends, when heated so that one junction is hotter than the
other, an electromotive force is set up through the metals, which
will increase with the increase of the _difference_ of temperature
between the two junctions. This electromotive force, or voltage, may
be measured, and, from a chart previously prepared, the temperature
determined. In most pyrometers, of course, the temperatures are
inscribed directly on the voltmeter, but the fact remains that
it is the voltage of a small electric current, and not heat, that
is actually measured.
There are two common types of thermo-couples, the first making use
of common, inexpensive metals, such as iron wire and nichrome wire.
This is the so-called "base metal" couple. The other is composed of
expensive metals such as platinum wire, and a wire of an alloy of
platinum with 10 per cent of rhodium or iridium. This is called
the "rare metal" couple, and because its component metals are less
affected by heat, it lasts longer, and varies less than the base
metal couple.
The cold junction of a thermo-couple may be connected by means
of copper wires to the voltmeter, although in some installations
of base metal couples, the wires forming the couple are themselves
extended to the voltmeter, making copper connections unnecessary.
From the foregoing, it may be seen that accurately to measure the
temperature of the hot end of a thermo-couple, we _must know the
temperature of the cold end_, as it is the _difference_ in the
temperatures that determines the voltmeter readings. This is absolutely
essential for precision, and its importance cannot be over-emphasized.
When pyrometers are used in daily operation, they should be checked
or calibrated two or three times a month, or even every week. Where
there are many in use, it is good practice to have a master pyrometer
of a rare metal couple, which is used only for checking up the
others. The master pyrometer, after calibrating against the melting
points of various substances, will have a calibration chart which
should be used in the checking operation.
It is customary now to send a rare metal couple to the Bureau of
Standards at Washington, where it is very carefully calibrated
for a nominal charge, and returned with the voltmeter readings
of a series of temperatures covering practically the whole range
of the couple. This couple is then used only for checking those
in daily use.
Pyrometer couples are more or less expensive, and should be cared
far when in use. The wires of the couple should be insulated from
each other by fireclay leads or tubes, and it is well to encase them
in a fireclay, porcelain, or quartz tube to keep out the furnace
gases, which in time destroy the hot junction. This tube of fireclay,
or porcelain, etc., should be protected against breakage by an
iron or nichrome tube, plugged or welded at the hot end. These
simple precautions will prolong the life of a couple and maintain
its precision longer.
Sometimes erroneous temperatures are recorded because the "cold
end" of the couple is too near the furnace and gets hot. This always
causes a temperature reading lower than the actual, and should be
guarded against. It is well to keep the cold end cool with water,
a wet cloth, or by placing it where coal air will circulate around
it. Best of all, is to have the cold junction in a box, together
with a thermometer, so that its temperature may definitely be known.
If this temperature should rise 20°F. on a hot day, a correction of
20°F. should be added to the pyrometer reading, and so on. In the
most up-to-date installations, this cold junction compensation is
taken care of automatically, a fact which indicates its importance.
Optical pyrometers are often used where it is impracticable to
use the thermo-couple, either because the temperature is so high
that it would destroy the couple, or the heat to be measured is
inaccessible to the couple of ordinary length. The temperatures of
slag or metal in furnaces or running through tap-holes or troughs
are often measured with optical pyrometers.
In one type of optical pyrometer, the observer focuses it on the
metal or slag and moves an adjustable dial or gage so as to get
an exact comparison between the color of the heat measured with
the calor of a lamp or screen in the pyrometer itself. This, of
course, requires practice, and judgment, and brings in the personal
equation. With care, however, very reliable temperature measurements
may be made. The temperatures of rails, as they leave the finishing
pass of a rolling mill, are measured in this way.
Another type of optical pyrometer is focused on the body, the
temperature of which is to be measured. The rays converge in the
telescope on metal cells, heating them, and thereby generating a
small electric current, the voltage of which is read an a calibrated
voltmeter similar to that used with the thermo-couple. The best
precision is obtained when an optical pyrometer is used each time
under similar conditions of light and the same observer.
Where it is impracticable to use either thermo-couples or optical
pyrometers, "sentinels" may be used. There are small cones or cylinders
made of salts or other substances of known melting points and covering
a wide range of temperatures.
If six of these "sentinels," melting respectively at 1,300°, 1,350°,
1,400°, 1,450°, 1,500°, and 1,550°F., were placed in a row in a
furnace, together with a piece of steel to be treated, and the
whole heated up uniformly, the sentinels would melt one by one and
the observer, by watching them through an opening in the furnace,
could tell when his furnace is at say 1,500° or between 1,500° and
1,550°, and regulate the heat accordingly.
A very accurate type of pyrometer, but one not so commonly used as
those previously described, is the resistance pyrometer. In this
type, the temperature is determined by measuring the resistance to an
electric current of a wire which is at the heat to be measured. This
wire is usually of platinum, wound around a quartz tube, the whole
being placed in the furnace. When the wire is at the temperature of
the furnace, it is connected by wires with a Wheatstone Bridge, a
delicate device for measuring electrical resistance, and an electric
current is passed through the wire. This current is balanced by
switching in resistances in the Wheatstone Bridge, until a delicate
electrical device shows that no current is flowing. The resistance
of the platinum wire at the heat to be measured is thus determined
on the "Bridge," and the temperature read off on a calibration
chart, which shows the resistance at various temperatures.
These are the common methods used to-day for measuring temperatures,
but whatever method is used, the observer should bear in mind that
the greatest precision is obtained, and hence the highest efficiency,
by keeping the apparatus in good working order, making sure that
conditions are the same each time, and calibrating or checking
against a standard at regular intervals.
THE PYROMETER AND ITS USE
In the heat treatment of steel, it has become absolutely necessary
that a measuring instrument be used which will give the operator an
exact reading of heat in furnace. There are a number of instruments
and devices manufactured for this purpose but any instrument that
will not give a direct reading without any guess work should have
no place in the heat-treating department.
A pyrometer installation is very simple and any of the leading
makers will furnish diagrams for the correct wiring and give detailed
information as to the proper care of, and how best to use their
particular instrument. There are certain general principles, however,
that must be observed by the operators and it cannot be too strongly
impressed upon them that the human factor involved is always the
deciding factor in the heat treatment of steel.
A pyrometer is merely an aid in the performance of doing good work,
and when carefully observed will help in giving a uniformity of
product and act as a check on careless operators. The operator
must bear in mind that although the reading on the pyrometer scale
gives a measure of the temperature where the junction of the two
metals is located, it will not give the temperature at the center
of work in the furnace, unless by previous tests, the heat for
penetrating a certain bulk of material has been decided on, and
the time necessary for such penetration is known.
Each analysis of plain carbon or alloy steel is a problem in itself.
Its critical temperatures will be located at slightly different
heats than for a steel which has a different proportion of alloying
elements. Furthermore, it takes time for metal to acquire the heat
of the furnace. Even the outer surface lags behind the temperature
of the furnace somewhat, and the center of the piece of steel lags
still further. It is apparent, therefore, that temperature, although
important, does not tell the whole story in heat treatment. _Time_
is also a factor.
Time at temperature is also of great importance because it takes
time, after the temperature has been reached, for the various internal
changes to take place. Hence the necessity for "soaking," when
annealing or normalizing. Therefore, a clock is as necessary to
the proper pyrometer equipment as the pyrometer itself.
For the purpose of general work where a wide range of steels or
a variable treatment is called for, it becomes necessary to have
the pyrometer calibrated constantly, and when no master instrument
is kept for this purpose the following method can be used to give
the desired results:
CALIBRATION OF PYROMETER WITH COMMON SALT
An easy and convenient method for standardization and one which
does not necessitate the use of an expensive laboratory equipment
is that based upon determining the melting point of common table
salt (sodium chloride). While theoretically salt that is chemically
pure should be used (and this is neither expensive nor difficult
to procure), commercial accuracy may be obtained by using common
table salt such as is sold by every grocer. The salt is melted in
a clean crucible of fireclay, iron or nickel, either in a furnace
or over a forge-fire, and then further heated until a temperature
of about 1,600 to 1,650°F. is attained. It is essential that this
crucible be clean because a slight admixture of a foreign substance
might noticeably change the melting point.
The thermo-couple to be calibrated is then removed from its protecting
tube and its hot end is immersed in the salt bath. When this end
has reached the temperature of the bath, the crucible is removed
from the source of heat and allowed to cool, and cooling readings
are then taken every 10 sec. on the milli-voltmeter or pyrometer. A
curve is then plotted by using time and temperature as coördinates,
and the temperature of the freezing point of salt, as indicated
by this particular thermocouple, is noted, _i.e._, at the point
where the temperature of the bath remains temporarily constant
while the salt is freezing. The length of time during which the
temperature is stationary depends on the size of the bath and the
rate of cooling, and is not a factor in the calibration. The melting
point of salt is 1,472°F., and the needed correction for the instrument
under observation can be readily applied.
It should not be understood from the above, however, that the salt-bath
calibration cannot be made without plotting a curve; in actual
practice at least a hundred tests are made without plotting any curve
to one in which it is done. The observer, if awake, may reasonably
be expected to have sufficient appreciation of the lapse of time
definitely to observe the temperature at which the falling pointer
of the instrument halts. The gradual dropping of the pointer before
freezing, unless there is a large mass of salt, takes place rapidly
enough for one to be sure that the temperature is constantly falling,
and the long period of rest during freezing is quite definite.
The procedure of detecting the solidification point of the salt
by the hesitation of the pointer without plotting any curve is
suggested because of its simplicity.
COMPLETE CALIBRATION OF PYROMETERS.--For the complete calibration
of a thermo-couple of unknown electromotive force, the new couple
may be checked against a standard instrument, placing the two bare
couples side by side in a suitable tube and taking frequent readings
over the range of temperatures desired.
If only one instrument, such as a millivoltmeter, is available,
and there is no standard couple at hand, the new couple may be
calibrated over a wide range of temperatures by the use of the following
standards:
Water, boiling point 212°F.
Tin, under charcoal, freezing point 450°F.
Lead, under charcoal, freezing point 621°F.
Zinc, under charcoal, freezing point 786°F.
Sulphur, boiling point 832°F.
Aluminum, under charcoal, freezing point 1,216°F.
Sodium chloride (salt), freezing point 1,474°F.
Potassium sulphate, freezing point 1,958°F.
A good practice is to make one pyrometer a standard; calibrate it
frequently by the melting-point-of-salt method, and each morning
check up every pyrometer in the works with the standard, making the
necessary corrections to be used for the day's work. By pursuing
this course systematically, the improved quality of the product
will much more than compensate for the extra work.
The purity of the substance affects its freezing or melting point.
The melting point of common salt is given in one widely used handbook
at 1,421°F., although chemically pure sodium chloride melts at
1,474°F. as shown above. A sufficient quantity for an extended
period should be secured. Test the melting point with a pyrometer
of known accuracy. Knowing this temperature it will be easy to
calibrate other pyrometers.
PLACING OF PYROMETERS.--When installing a pyrometer, care should be
taken that it reaches directly to the point desired to be measured,
that the cold junction is kept cold, and that the wires leading to
the recording instrument are kept in good shape. The length of
these lead wires have an effect; the longer they are, the lower
the apparent temperature.
When pyrometers placed in a number of furnaces are connected up
in series, and a multiple switch is used for control, it becomes
apparent that pyrometers could not be interchanged between furnaces
near and far from the instrument without affecting the uniformity
of product from each furnace.
Calibration can best be done without disturbing the working pyrometer,
by inserting the master instrument into each furnace separately, place
it alongside the hot junction of the working pyrometer, and compare
the reading given on the indicator connected with the multiple
switch.
Protection tubes should be replaced when cracked, as it is important
that no foreign substance is allowed to freeze in the tube, so
that the enclosed junction becomes a part of a solid mass joined
in electrical contact with the outside protecting tube. Wires over
the furnaces must be carefully inspected from time to time, as no
true reading can be had on an instrument, if insulation is burned
off and short circuits result.
If the standard calibrating instrument used contains a dry battery,
it should be examined from time to time to be sure it is in good
condition.
THE LEEDS AND NORTHRUP POTENTIOMETER SYSTEM
The potentiometer pyrometer system is both flexible and substantial
in that it is not affected by the jar and vibration of the factory
or the forge shop. Large or small couples, long or short leads
can be used without adjustment. The recording instrument may be
placed where it is most convenient, without regard to the distance
from the furnace.
ITS FUNDAMENTAL PRINCIPLE.--The potentiometer is the electrical
equivalent of the chemical balance, or balance arm scales. Measurements
are made with balance scales by varying known weights until they
equal the unknown weight. When the two are equal the scales stand
at zero, that is, in the position which they occupy when there is
no weight on either pan; the scales are then said to be balanced.
Measurements are made with the potentiometer by varying a known
electromotive force until it equals the unknown; when the two are
equal the index of the potentiometer, the galvanometer needle,
stands motionless as it is alternately connected and disconnected.
The variable known weights are units separate from the scales, but
the potentiometer provides its own variable known electromotive
force.
The potentiometer provides, first, a means of securing a known
variable electromotive force and, second, suitable electrical
connections for bringing that electromotive force to a point where
it may be balanced against the unknown electromotive force of the
couple. The two are connected with opposite polarity, or so that
the two e.m.f.s oppose one another. So long as one is stronger
than the other a current will flow through the couple; when the
two are equal no current will flow.
Figure 107 shows the wiring of the potentiometer in its simplest
form. The thermo-couple is at _H_, with its polarity as shown by
the symbols + and -. It is connected with the main circuit of the
potentiometer at the fixed point _D_ and the point _G_.
[Illustration: FIG. 110.--Simple potentiometer.]
A current from the dry cell _Ba_ is constantly flowing through the
main, or so-called potentiometer circuit, _ABCDGEF_. The section
_DGE_ of this circuit is a slide wire, uniform in resistance throughout
its length. The scale is fixed on this slide wire. The current
from the cell _Ba_ as it flows through _DGE_, undergoes a fall
in potential, setting up a difference in voltage, that is, an
electromotive force, between _D_ and _E_. There will also be
electromotive force between _D_ and all other points on the slide
wire. The polarity of this is in opposition to the polarity of the
thermo-couple which connects into the potentiometer at _D_ and
at _G_. By moving _G_ along the slide wire a point is found where
the voltage between _D_ and _G_ in the slide wire is just equal to
the voltage between _D_ and _G_ generated by the thermo-couple. A
galvanometer in the thermo-couple circuit indicates when the balance
point is reached, since at this point the galvanometer needle will
stand motionless when its circuit is opened and closed.
[Illustration: FIG. 111.--Standard cell potentiometer.]
The voltage in the slide wire will vary with the current flowing
through it from the cell _Ba_ and a means of standardizing this is
provided. _SC_, Fig. 111, is a cadmium cell whose voltage is constant.
It is connected at two points _C_ and _D_ to the potentiometer
circuit whenever the potentiometer current is to be standardized.
At this time the galvanometer is thrown in series with _SC_. The
variable rheostat _R_ is then adjusted until the current flowing
is such that as it flows through the standard resistance _CD_,
the fall in potential between _C_ and _D_ is just equal to the
voltage of the standard cell _SC_. At this time the galvanometer
will indicate a balance in the same way as when it was used with
a thermo-couple. By this operation the current in the slide wire
_DGE_ has been standardized.
[Illustration: FIG. 112.--Hand adjusted cold-end compensator.]
DEVELOPMENT OF THE WIRING SCHEME OF THE COLD-END COMPENSATOR.--The
net voltage generated by a thermo-couple depends upon the temperature
of the hot end and the temperature of the cold end. Therefore, any
method adopted for reading temperature by means of thermo-couples
must in some way provide a means of correcting for the temperature of
the cold end. The potentiometer may have either of two very simple
devices for this purpose. In one form the operator is required
to set a small index to a point on a scale corresponding to the
known cold junction temperature. In the other form an even more
simple automatic compensator is employed. The principle of each is
described in the succeeding paragraphs, in which the assumption is
made that the reader already understands the potentiometer principle
as described above.
As previously explained the voltage of the thermo-couple is measured
by balancing it against the voltage drop _DG_ in the potentiometer.
As shown in Fig. 111, the magnitude of the balancing voltage is
controlled by the position of _G_. Make _D_ movable as shown in Fig.
112 and the magnitude of the voltage _DG_ may be varied either from
the point _D_ or the point _G_. This gives a means of compensating
for cold end changes by setting the slider _D_. As the cold end
temperature rises the net voltage generated by the couple decreases,
assuming the hot end temperature to be constant. To balance this
decreased voltage the slider _D_ is moved along its scale to a new
point nearer _G_. In other words, the slider _D_ is moved along
its scale until it corresponds to the known temperature of the cold
end and then the potentiometer is balanced by moving the slider
_G_. The readings of _G_ will then be direct.
[Illustration: FIG. 113.--Another type of compensator.]
The same results will be obtained if a slide wire upon which _D_
bears is in parallel with the slide wire of _G_, as shown in Fig.
113.
AUTOMATIC COMPENSATOR.--It should be noted that the effect of moving
the contact _D_, Fig. 113, is to vary the ratio of the resistances
on the two sides of the point _D_ in the secondary slide wire. In
the recording pyrometers, an automatic compensator is employed.
This automatic compensator varies the ratio on the two sides of
the point _D_ in the following manner:
The point _D_, Fig. 114, is mechanically fixed; on one side of
_D_ is the constant resistance coil _M_, on the other the nickel
coil _N_. _N_ is placed at or near the cold end of the thermo-couple
(or couples). Nickel has a high temperature coefficient and the
electrical proportions of _M_ and _N_ are such that the resistance
change of _N_, as it varies with the temperature of the cold end,
has the same effect upon the balancing voltage between _D_ and
_G_ that the movement of the point _D_, Fig. 114, has in the
hand-operated compensator.
Instruments embodying these principles are shown in Figs. 115 to
117. The captions making their uses clear.
[Illustration: FIG. 114.--Automatic cold-end compensator.]
PLACING THE THERMO-COUPLES
The following illustrations from the Taylor Instrument Company
show different applications of the thermo-couples to furnaces of
various kinds. Figure 118 shows an oil-fired furnace with a simple
vertical installation. Figure 119 shows a method of imbedding the
thermo-couple in the floor of a furnace so as to require no space
in the heating chamber.
[Illustration: FIG. 115.--Potentiometer ready for use.]
Various methods of applying a pyrometer to common heat-treatment
furnaces are shown in Figs. 120 to 122.
[Illustration: FIG. 116.--Eight-point recording pyrometer-Carpenter
Steel Co.]
LEEDS AND NORTHRUP OPTICAL PYROMETER
The principles of this very popular method of measuring temperature
are sketched in Fig. 123.
[Illustration: FIG. 117.--Multiple-point thermocouple
recorder--Bethlehem Steel Co.]
[Illustration: FIG. 118.--Tycos pyrometer in oil-fired furnace.]
The instrument is light and portable, and can be sighted as easily
as an opera glass. The telescope, which is held in the hand, weighs
only 25 oz.; and the case containing the battery, rheostat and
milliammeter, which is slung from the shoulder, only 10 lb.
[Illustration: FIG. 119.--Thermocouple in floor of furnace.]
[Illustration: FIG. 120.--Pyrometer in gas furnace.]
A large surface to sight at is not required. So long as the image
formed by the objective is broader than the lamp filament, the
temperature can be measured accurately.
[Illustration: FIG. 121.--Tycos multiple indicating pyrometer and
recorder.]
[Illustration: FIG. 122.--Pyrometer in galvanizing tank.]
Distance does not matter, as the brightness of the image formed
by the lens is practically constant, regardless of the distance
of the instrument from the hot object.
[Illustration: FIG. 123.--Leeds & Northrup optical pyrometer.]
The manipulation is simple and rapid, consisting merely in the turning
of a knurled knob. The setting is made with great precision, due to
the rapid change in light intensity with change in temperature and
to the sensitiveness of the eye to differences of light intensity.
In the region of temperatures used for hardening steel, for example,
different observers using the instrument will agree within 3°C.
[Illustration: FIG. 124.--Too low.
FIG. 125.--Too high.
FIG. 126.--Correct.]
Only brightness, not color, of light is matched, as light of only
one color reaches the eye. Color blindness, therefore, is no hindrance
to the use of this method. The use of the instrument is shown in
Fig. 127.
OPTICAL SYSTEM AND ELECTRICAL CIRCUIT OF THE LEEDS & NORTHRUP OPTICAL
PYROMETER.--For extremely high temperature, the optical pyrometer is
largely used. This is a comparative method. By means of the rheostat
the current through the lamp is adjusted until the brightness of
the filament is just equal to the brightness of the image produced
by the lens _L_, Fig. 123, whereupon the filament blends with or
becomes indistinguishable in the background formed by the image
of the hot object. This adjustment can be made with great accuracy
and certainty, as the effect of radiation upon the eye varies some
twenty times faster than does the temperature at 1,600°F., and some
fourteen times faster at 3,400°F. When a balance has been obtained,
the observer notes the reading of the milliammeter. The temperature
corresponding to the current is then read from a calibration curve
supplied with the instrument.
[Illustration: FIG. 127.--Using the optical pyrometer.]
As the intensity of the light emitted at the higher temperatures
becomes dazzling, it is found desirable to introduce a piece of red
glass in the eye piece at _R_. This also eliminates any question
of matching colors, or of the observer's ability to distinguish
colors. It is further of value in dealing with bodies which do
not radiate light of the same composition as that emitted by a
black body, since nevertheless the intensity of radiation of any
one color from such bodies increases progressively in a definite
manner as the temperature rises. The intensity of this one color
can therefore be used as a measure of temperature for the body
in question. Figures 124 to 126 show the way it is read.
CORRECTION FOR COLD-JUNCTION ERRORS
The voltage generated by a thermo-couple of an electric pyrometer is
dependent on the difference in temperature between its hot junction,
inside the furnace, and the cold junction, or opposite end of the
thermo-couple to which the copper wires are connected. If the
temperature or this cold junction rises and falls, the indications
of the instrument will vary, although the hot junction in the furnace
may be at a constant temperature.
A cold-junction temperature of 75°F., or 25°C., is usually adopted
in commercial pyrometers, and the pointer on the pyrometer should
stand at this point on the scale when the hot junction is not heated.
If the cold-junction temperature rises about 75°F., where base metal
thermo-couples are used, the pyrometer will read approximately 1°
low for every 1° rise in temperature above 75°F. For example, if the
instrument is adjusted for a cold-junction temperature of 75°, and
the actual cold-junction temperature is 90°F., the pyrometer will
read 15° low. If, however, the cold-junction temperature falls below
75°F., the pyrometer will read high instead of low, approximately
1° for every 1° drop in temperature below 75°F.
With platinum thermo-couples, the error is approximately 1/2° for
1° change in temperature.
CORRECTION BY ZERO ADJUSTMENT.--Many pyrometers are supplied with
a zero adjuster, by means of which the pointer can be set to any
actual cold-junction temperature. If the cold junction of the
thermo-couple is in a temperature of 100°F., the pointer can be
set to this point on the scale, and the readings of the instrument
will be correct.
COMPENSATING LEADS.--By the use of compensating leads, formed of
the same material as the thermo-couple, the cold junction can be
removed from the head of the thermo-couple to a point 10, 20 or 50
ft. distant from the furnace, where the temperature is reasonably
constant. Where greater accuracy is desired, a common method is
to drive a 2-in. pipe, with a pointed closed end, some 10 to 20
ft. into the ground, as shown in Fig. 128. The compensating leads
are joined to the copper leads, and the junction forced down to
the bottom of the pipe. The cold junction is now in the ground,
beneath the building, at a depth at which the temperature is very
constant, about 70°F., throughout the year. This method will usually
control the cold-junction temperature within 5°F.
Where the greatest accuracy is desired a compensating box will
overcome cold-junction errors entirely. It consists of a case enclosing
a lamp and thermostat, which can be adjusted to maintain any desired
temperature, from 50 to 150°F. The compensating leads enter the box
and copper leads run from the compensating box to the instrument,
so that the cold junction is within the box. Figure 129 shows a
Brown compensating box.
[Illustration: FIG. 128.--Correcting cold-junction error.]
If it is desired to maintain the cold junction at 100°: the thermostat
is set at this point, and the lamp, being wired to the 110- or
220-volt lighting circuit, will light and heat the box until 100°
is reached, when the thermostat will open the circuit and the light
is extinguished. The box will now cool down to 98°, when the circuit
is again closed, the lamp lights, the box heats up, and the operation
is repeated.
[Illustration: FIG. 129.--Compensating box.]
BROWN AUTOMATIC SIGNALING PYROMETER
In large heat-treating plants it has been customary to maintain
an operator at a central pyrometer, and by colored electric lights
at the furnaces, signal whether the temperatures are correct or
not. It is common practice to locate three lights above each
furnace-red, white and green. The red light burns when the temperature
is too low, the white light when the temperature is within certain
limits--for example, 20°F. of the correct temperature--and the
green light when the temperature is too high.
[Illustration: FIG. 130.--Brown automatic signaling pyrometer.]
Instruments to operate the lights automatically have been devised and
one made by Brown is shown in Fig. 130. The same form of instrument is
used for this purpose to automatically control furnace temperatures,
and the pointer is depressed at intervals of every 10 sec. on contacts
corresponding to the red, white and green lights.
[Illustration: FIG. 131.--Automatic temperature control.]
AN AUTOMATIC TEMPERATURE CONTROL PYROMETER
Automatic temperature control instruments are similar to the Brown
indicating high resistance pyrometer with the exception that the
pointer is depressed at intervals of every 10 sec. upon contact-making
devices. No current passes through the pointer which simply depresses
the upper contact device tipped with platinum, which in turn comes
in contact with the lower contact device, platinum-tipped, and the
circuit is completed through these two contacts. The current is very
small, about 1/10 amp., as it is only necessary to operate the relay
which in turn operates the switch or valve. A small motor is used to
depress the pointer at regular intervals. The contact-making device
is adjustable throughout the scale range of the instrument, and an
index pointer indicates the point on the instrument at which the
temperature is being controlled. The space between the two contacts
on the high and low side, separated by insulating material, is
equivalent to 1 per cent of the scale range. A control of temperature
is therefore possible within 1 per cent of the total scale range.
Figure 131 shows this attached to a small furnace.
[Illustration: FIG. 132.--Portable thermocouple testing molten brass.]
PYROMETERS FOR MOLTEN METAL
Pyrometers for molten metal are connected to portable thermocouples
as in Fig. 132. Usually the pyrometer is portable, as shown in
this case, which is a Brown. Other methods of mounting for this
kind of work arc shown in Figs. 133 and 134. The bent mountings
are designed for molten metal, such as brass or copper and are
supplied with either clay, graphite or carborundum tubes. Fifteen
feet of connecting wire is usually supplied.
The angle mountings, Fig. 134, are recommended for baths such as
lead or cyanide. The horizontal arm is usually about 14 in. long,
and the whole mounting is easily taken apart making replacements
very easy. Details of the thermo-couple shown in Fig. 132 are given
in Fig. 135. This is a straight rod with a protector for the hand
of the operator. The lag in such couples is less than one minute.
These are Englehard mountings.
PROTECTORS FOR THERMO-COUPLES
Thermo-couples must be protected from the danger of mechanical
injury. For this purpose tubes of various refractory materials
are made to act as protectors. These in turn are usually protected
by outside metal tubes. Pure wrought iron is largely used for this
purpose as it scales and oxidizes very slowly. These tubes are
usually made from 2 to 4 in. shorter than the inner tubes. In lead
baths the iron tubes often have one end welded closed and are used
in connection with an angle form of mounting.
[Illustration: FIG. 133.--Bent handle thermocouple with protector.]
Where it is necessary for protecting tubes to project a considerable
distance into the furnace a tube made of nichrome is frequently used.
This is a comparatively new alloy which stands high temperatures
without bending. It is more costly than iron but also much more
durable.
When used in portable work and for high temperatures, pure nickel
tubes are sometimes used. There is also a special metal tube made
for use in cyanide. This metal withstands the intense penetrating
characteristics of cyanide. It lasts from six to ten months as
against a few days for the iron tube.
The inner tubes of refractory materials, also vary according to
the purposes for which they are to be used. They are as follows:
MARQUARDT MASS TUBES for temperatures up to 3,000°F., but they will
not stand sudden changes in temperature, such as in contact with
intermittent flames, without an extra outer covering of chamotte,
fireclay or carborundum.
[Illustration: FIG. 134.--Other styles of bent mounting.]
FUSED SILICA TUBES for continuous temperatures up to 1,800°F. and
intermittently up to 2,400°F. The expansion at various temperatures
is very small, which makes them of value for portable work. They
also resist most acids.
CHAMOTTE TUBES are useful up to 2,800°F. and are mechanically strong.
They have a small expansion and resist temperature changes well,
which makes them good as outside protectors for more fragile tubes.
They cannot be used in molten metals, or baths of any kind nor
in gases of an alkaline nature. They are used mainly to protect
a Marquardt mass or silica tube.
CARBORUNDUM TUBES are also used as outside protection to other
tubes. They stand sudden changes of temperature well and resist
all gases except chlorine, above 1,750°F. Especially useful in
protecting other tubes against molten aluminum, brass, copper and
similar metals.
CLAY TUBES are sometimes used in large annealing furnaces where they
are cemented into place, forming a sort of well for the insertion of
the thermo-couple. They are also used with portable thermo-couples
for obtaining the temperatures of molten iron and steel in ladles.
Used in this way they are naturally short-lived, but seem the best
for this purpose.
[Illustration: FIG. 135.--Straight thermocouple and guard.]
CORUNDITE TUBES are used as an outer protection for both the Marquardt
mass and the silica tubes for kilns and for glass furnaces. Graphite
tubes are also used in some cases for outer protections.
CALORIZED TUBES are wrought-iron pipe treated with aluminum vapor
which often doubles or even triples the life of the tube at high
temperature.
These tubes come in different sizes and lengths depending on the
uses for which they are intended. Heavy protecting outer tubes
may be only 1 in. in inside diameter and as much as 3 in. outside
diameter, while the inner tubes, such as the Marquardt mass and
silica tubes are usually about 3/4 in. outside and 3/8 in. inside
diameter. The length varies from 12 to 48 in. in most cases.
Special terminal heads are provided, with brass binding posts for
electrical connections, and with provisions for water cooling when
necessary.
APPENDIX
TABLE 32.--Temperature Conversion Tables.
TABLE 33.--Comparison Between Degrees Centigrade and Degrees Fahrenheit.
TABLE 34.--Weight of Round, Octagon and Square Carbon Tool Steel
per Foot.
TABLE 35.--Weight of Round Carbon Tool Steel 12 In. in Diameter
and Larger, per Foot.
TABLE 36.--Decimal Equivalents of a foot.
TEMPERATURE CONVERSION TABLES
By ALBERT SAUVEUR
--------------------------------------------------------------------------
-459.4 to 0 | 0 to 100 | 100 to 1000
-----------------|------------------------------|-------------------------
C. F. | C. F. | C. F. | C. F.| C. F.
-----------------|---------------|--------------|-----------|-------------
-273 -459.4 |-17.8 0 32 |10.0 50 122.0| 38 100 212|260 500 932
-268 -450 |-17.2 1 33.8|10.6 51 123.8| 43 110 230|266 510 950
-262 -440 |-16.7 2 35.6|11.1 52 125.6| 49 120 248|271 520 968
-257 -430 |-16.1 3 37.4|11.7 53 127.4| 54 130 266|277 530 986
-251 -420 |-15.6 4 39.2|12.2 54 129.2| 60 140 284|282 540 1004
-246 -410 |-15.0 5 41.0|12.8 55 131.0| 66 150 302|288 550 1022
-240 -400 |-14.4 6 42.8|13.3 56 132.8| 71 160 320|293 560 1040
-234 -390 |-13.9 7 44.6|13.9 57 134.6| 77 170 336|299 570 1058
-229 -380 |-13.3 8 46.4|14.4 58 136.4| 82 180 358|304 580 1076
-223 -370 |-12.8 9 48.2|15.0 59 138.2| 88 190 374|310 590 1094
-218 -360 |-12.2 10 50.0|15.6 60 140.0| 93 200 392|316 600 1112
-212 -350 |-11.7 11 51.8|16.1 61 141.8| 99 210 410|321 610 1130
-207 -340 |-11.1 12 53.6|16.7 62 143.6|100 212 413|327 620 1148
-201 -330 |-10.6 13 55.4|17.2 63 145.4|104 220 428|332 630 1166
-196 -320 |-10.0 14 57.2|17.8 64 147.2|110 230 446|338 640 1184
-190 -310 | -9.44 15 59.0|18.3 65 149.0|116 240 464|343 650 1202
-184 -300 | -8.89 16 61.8|18.9 66 150.8|121 250 482|349 660 1220
-179 -290 | -8.33 17 63.6|19.4 67 152.6|127 260 500|354 670 1238
-173 -280 | -7.78 18 65.4|20.0 68 154.4|132 270 518|360 680 1256
-169 -273 -459.4| -7.22 19 67.2|20.6 69 156.2|138 280 536|366 690 1274
-168 -270 -454 | -6.67 20 68.0|21.1 70 158.0|143 290 554|371 700 1292
-162 -260 -436 | -6.11 21 69.8|21.7 71 159.8|149 300 572|377 710 1310
-157 -250 -418 | -5.56 22 71.6|22.2 72 161.6|154 310 590|382 720 1328
-151 -240 -400 | -5.00 23 73.4|22.8 73 163.4|160 320 608|388 730 1346
-146 -230 -382 | -4.44 24 75.2|23.3 74 165.2|166 330 626|393 740 1364
-140 -220 -364 | -3.89 25 77.0|23.9 75 167.0|171 340 644|399 750 1382
-134 -210 -346 | -3.33 26 78.8|24.4 76 168.8|177 350 662|404 760 1400
-129 -200 -328 | -2.78 27 80.6|25.0 77 170.6|182 360 680|410 770 1418
-123 -190 -310 | -2.22 28 82.4|25.6 78 172.4|188 370 698|416 780 1436
-118 -180 -292 | -1.67 29 84.2|26.1 79 174.2|193 380 716|421 790 1454
-112 -170 -274 | -1.11 30 86.0|26.7 80 176.0|199 390 734|427 800 1472
-107 -160 -256 | -0.56 31 87.8|27.2 81 177.8|204 400 752|432 810 1490
-101 -150 -238 | 0 32 89.6|27.8 82 179.6|210 410 770|438 820 1508
-95.6 -140 -220 | 0.56 33 91.4|28.3 83 181.4|216 420 788|443 830 1526
-90.0 -130 -202 | 1.11 34 93.2|28.9 84 183.2|221 430 806|449 840 1544
-84.4 -120 -184 | 1.67 35 95.0|29.4 85 185.0|227 440 824|454 850 1562
-78.9 -110 -166 | 2.22 36 96.8|30.0 86 186.8|232 450 842|460 860 1580
-73.3 -100 -148 | 2.78 37 98.6|30.6 87 188.6|238 460 860|466 870 1598
-67.8 -90 -130 | 3.33 38 100.4|31.1 88 190.4|243 470 878|471 880 1616
-62.2 -80 -112 | 3.89 39 102.2|31.7 89 192.2|249 480 896|477 890 1634
-56.7 -70 -94 | 4.44 40 104.0|32.2 90 194.0|254 490 914|482 900 1652
-51.1 -60 -76 | 5.00 41 105.8|32.8 91 195.8| |488 910 1670
-45.6 -50 -58 | 5.56 42 107.6|33.3 92 197.6| |493 920 1688
-40.0 -40 -40 | 6.11 43 109.4|33.9 93 199.4| |499 930 1706
-34.4 -30 -22 | 6.67 44 111.2|34.4 94 201.2| |504 940 1724
-28.9 -20 4 | 7.22 45 113.0|35.0 95 203.0| |510 950 1742
-23.3 -10 14 | 7.78 46 114.8|35.6 96 204.8| |516 960 1760
-17.8 0 32 | 8.33 47 116.6|36.1 97 206.6| |521 970 1778
| 8.89 48 118.4|36.7 98 208.4| |527 980 1796
| 9.44 49 120.2|37.2 99 210.2| |532 990 1814
| |37.8 100 212.0| |538 1000 1832
--------------------------------------------------------------------------
----------------------------------------------------------------
1000 to 2000 | 2000 to 3000
-------------------------------|--------------------------------
C. F. | C. F. | C. F. | C. F.
--------------|----------------|----------------|---------------
538 1000 1832 | 816 1500 2732 | 1093 2000 3632 | 1371 2500 4534
543 1010 1850 | 821 1510 2750 | 1099 2010 3650 | 1377 2510 4552
549 1020 1868 | 827 1520 2768 | 1104 2020 3668 | 1382 2520 4560
554 1030 1886 | 832 1530 2786 | 1110 2030 3686 | 1388 2530 4588
560 1040 1904 | 838 1540 2804 | 1116 2040 3704 | 1393 2540 4606
566 1050 1922 | 843 1550 2822 | 1121 2050 3722 | 1399 2550 4622
571 1060 1940 | 849 1560 2840 | 1127 2060 3740 | 1404 2560 4640
577 1070 1958 | 854 1570 2858 | 1132 2070 3758 | 1410 2570 4658
582 1080 1976 | 860 1580 2876 | 1138 2080 3776 | 1416 2580 4676
588 1090 1994 | 866 1590 2894 | 1143 2090 3794 | 1421 2590 4694
593 1100 2012 | 871 1600 2912 | 1149 2100 3812 | 1427 2600 4712
599 1110 2030 | 877 1610 2930 | 1154 2110 3830 | 1432 2610 4730
604 1120 2048 | 882 1620 2948 | 1160 2120 3848 | 1438 2620 4748
610 1130 2066 | 888 1630 2966 | 1166 2130 3866 | 1443 2630 4766
616 1140 2084 | 893 1640 2984 | 1171 2140 3884 | 1449 2640 4784
621 1150 2102 | 899 1650 3002 | 1777 2150 3902 | 1454 2650 4802
627 1160 2120 | 904 1660 3020 | 1182 2160 3920 | 1460 2660 4820
632 1170 2138 | 910 1670 3038 | 1188 2170 3938 | 1466 2670 4838
638 1180 2156 | 916 1680 3056 | 1193 2180 3956 | 1471 2680 4854
643 1190 2174 | 921 1690 3074 | 1199 2190 3974 | 1477 2690 4876
649 1200 2192 | 927 1700 3092 | 1204 2200 3992 | 1482 2700 4892
654 1210 2210 | 932 1710 3110 | 1210 2210 4010 | 1488 2710 4910
660 1220 2228 | 938 1720 3128 | 1216 2220 4028 | 1493 2720 4928
666 1230 2246 | 943 1730 3146 | 1221 2230 4046 | 1499 2730 4946
671 1240 2264 | 949 1740 3164 | 1227 2240 4064 | 1504 2740 4964
677 1250 2282 | 954 1750 3182 | 1232 2250 4082 | 1510 2750 4982
682 1260 2300 | 960 1760 3200 | 1238 2260 4100 | 1516 2760 5000
688 1270 2318 | 966 1770 3218 | 1243 2270 4118 | 1521 2770 5018
693 1280 2336 | 971 1780 3236 | 1249 2280 4136 | 1527 2780 5036
699 1290 2354 | 977 1790 3254 | 1254 2290 4154 | 1532 2790 5054
704 1300 2372 | 982 1800 3272 | 1260 2300 4172 | 1538 2800 5072
710 1310 2390 | 988 1810 3290 | 1266 2310 4190 | 1543 2810 5090
716 1320 2408 | 993 1820 3308 | 1271 2320 4208 | 1549 2820 5108
721 1330 2426 | 999 1830 3326 | 1277 2330 4226 | 1554 2830 5126
727 1340 2444 | 1004 1840 3344 | 1282 2340 4244 | 1560 2840 5144
732 1350 2462 | 1010 1850 3362 | 1288 2350 4262 | 1566 2850 5162
738 1360 2480 | 1016 1860 3380 | 1293 2360 4280 | 1571 2860 5180
743 1370 2498 | 1021 1870 3398 | 1299 2370 4298 | 1577 2870 5198
749 1380 2516 | 1027 1880 3416 | 1304 2380 4316 | 1582 2880 5216
754 1390 2534 | 1032 1890 3434 | 1310 2390 4334 | 1588 2890 5234
760 1400 2552 | 1038 1900 3452 | 1316 2400 4352 | 1593 2900 5252
766 1410 2570 | 1043 1910 3470 | 1321 2410 4370 | 1599 2910 5270
771 1420 2588 | 1049 1920 3488 | 1327 2420 4388 | 1604 2920 5288
777 1430 2606 | 1054 1930 3506 | 1332 2430 4406 | 1610 2930 5306
782 1440 2624 | 1060 1940 3524 | 1338 2440 4424 | 1616 2940 5324
788 1450 2642 | 1066 1950 3542 | 1343 2450 4442 | 1621 2950 5342
793 1460 2660 | 1071 1960 3560 | 1349 2460 4460 | 1627 2960 5360
799 1470 2678 | 1077 1970 3578 | 1354 2470 4478 | 1632 2970 5378
804 1480 2696 | 1082 1980 3596 | 1360 2480 4496 | 1638 2980 5396
810 1490 2714 | 1088 1990 3614 | 1366 2490 4514 | 1643 2990 5414
| 1093 2000 3632 | | 1649 3000 5432
---------------------------------------------------------------
NOTE.--The numbers in bold face type refer to the temperature either
in degrees Centigrade or Fahrenheit which it is desired to convert
into the other scale. If converting from Fahrenheit degrees to
Centigrade degrees the equivalent temperature will be found in
the left column, while if converting from degrees Centigrade to
degrees Fahrenheit, the answer will be found in the column on the
right. These tables are a revision of those by Sauveur & Boylston,
metallurgical engineers, Cambridge, Mass. Copyright, 1920.
INTERPOLATION FACTORS
C. F. C. F.
0.56 1 1.8 | 3.33 6 10.8
1.11 2 3.6 | 3.89 7 12.6
1.67 3 5.4 | 4.44 8 14.4
2.22 4 7.2 | 5.00 9 16.2
2.78 5 9.0 | 5.56 10 18.0
Those using pyrometers will find this and the preceding conversion
table of great convenience:
TABLE 33.--COMPARISON BETWEEN DEGREES CENTIGRADE AND DEGREES FAHRENHEIT
-------------------------------------------------------------------------
Degrees | Degrees | Degrees | Degrees | Degrees | Degrees | Degrees
---------|---------|---------|---------|---------|---------|-------------
F.| C. | F.| C. | F.| C. | F.| C. | F.| C. | F.| C. | F.| C.
---|-----|---|-----|---|-----|---|-----|---|-----|---|-----|-----|-------
-40|-40.0| 3|-16.1| 46| 7.7| 89| 31.6|132| 55.5|175| 79.4| 275| 135.0
-39|-39.4| 4|-15.5| 47| 8.3| 90| 32.2|133| 56.1|176| 80.0| 300| 148.8
-38|-38.8| 5|-15.0| 48| 8.8| 91| 32.7|134| 56.6|177| 80.5| 325| 162.7
-37|-38.3| 6|-14.4| 49| 9.3| 92| 33.3|135| 57.2|178| 81.1| 350| 176.6
-36|-37.7| 7|-13.8| 50| 10.0| 93| 33.9|136| 57.7|179| 81.6| 375| 190.5
-35|-37.2| 8|-13.3| 51| 10.5| 94| 34.4|137| 58.3|180| 82.2| 400| 204.4
-34|-36.6| 9|-12.7| 52| 11.1| 95| 35.0|138| 58.8|181| 82.7| 425| 218.3
-33|-36.1| 10|-12.2| 53| 11.6| 96| 35.5|139| 59.4|182| 83.3| 450| 232.2
-32|-35.5| 11|-11.6| 54| 12.2| 97| 36.1|140| 60.0|183| 83.8| 475| 246.1
-31|-35.0| 12|-11.1| 55| 12.7| 98| 36.6|141| 60.5|184| 84.4| 500| 260.0
-30|-34.4| 13|-10.5| 56| 13.3| 99| 37.2|142| 61.1|185| 85.0| 525| 273.8
-29|-33.9| 14|-10.0| 57| 13.8|100| 37.7|143| 61.6|186| 85.5| 550| 287.7
-28|-33.3| 15| -9.3| 58| 14.4|101| 38.3|144| 62.2|187| 86.1| 575| 301.6
-27|-32.7| 16| -8.8| 59| 15.0|102| 38.8|145| 62.7|188| 86.6| 600| 315.5
-26|-32.2| 17| -8.3| 60| 15.5|103| 39.4|146| 63.3|189| 87.2| 625| 329.4
-25|-31.6| 18| -7.7| 61| 16.1|104| 40.0|147| 63.8|190| 87.7| 650| 343.3
-24|-31.1| 19| -7.2| 62| 16.6|105| 40.5|148| 64.4|191| 88.3| 675| 357.2
-23|-30.5| 20| -6.6| 63| 17.2|106| 41.1|149| 65.0|192| 88.8| 700| 371.1
-22|-30.0| 21| -6.1| 64| 17.7|107| 41.6|150| 65.5|193| 89.4| 725| 385.0
-21|-29.4| 22| -5.5| 65| 18.3|108| 42.2|151| 66.1|194| 90.0| 750| 398.8
-20|-28.8| 23| -5.0| 66| 18.8|109| 42.7|152| 66.6|195| 90.5| 775| 412.7
-19|-28.3| 24| -4.4| 67| 19.4|110| 43.3|153| 67.2|196| 91.1| 800| 426.6
-18|-27.7| 25| -3.8| 68| 20.0|111| 43.8|154| 67.7|197| 91.6| 825| 440.5
-17|-27.2| 26| -3.3| 69| 20.5|112| 44.4|155| 68.3|198| 92.2| 850| 454.4
-16|-26.6| 27| -2.7| 70| 21.1|113| 45.0|156| 68.8|199| 92.7| 875| 468.3
-15|-26.1| 28| -2.2| 71| 21.6|114| 45.5|157| 69.4|200| 93.3| 900| 482.2
-14|-25.5| 29| -1.6| 72| 22.2|115| 46.1|158| 70.0|201| 93.8| 925| 496.1
-13|-25.0| 30| -1.1| 73| 22.7|116| 46.6|159| 70.5|202| 94.4| 950| 510.0
-12|-24.4| 31| -0.5| 74| 23.3|117| 47.2|160| 71.1|203| 95.0| 975| 523.8
-11|-23.8| 32| -0.0| 75| 23.8|118| 47.7|161| 71.6|204| 95.5|1,000| 537.7
-10|-23.3| 33| +0.5| 76| 24.4|119| 48.3|162| 72.2|205| 96.1|1,100| 593.3
-9|-22.7| 34| 1.1| 77| 25.0|120| 48.8|163| 72.7|206| 96.6|1,200| 648.8
-8|-22.2| 35| 1.6| 78| 25.5|121| 49.4|164| 73.3|207| 97.2|1,300| 704.4
-7|-21.6| 36| 2.2| 79| 26.1|122| 50.0|165| 73.8|208| 97.7|1,400| 760.0
-6|-21.1| 37| 2.7| 80| 26.6|123| 50.5|166| 74.4|209| 98.3|1,500| 815.5
-5|-20.5| 38| 3.3| 81| 27.2|124| 51.1|167| 75.0|210| 98.8|1,600| 871.1
-4|-20.0| 39| 3.8| 82| 27.7|125| 51.6|168| 75.5|211| 99.4|1,700| 926.6
-3|-19.4| 40| 4.4| 83| 28.3|126| 52.2|169| 76.1|212|100.0|1,800| 982.2
-2|-18.8| 41| 5.0| 84| 28.8|127| 52.7|170| 76.6|213|100.5|1,900|1,037.7
-1|-18.3| 42| 5.5| 85| 29.4|128| 53.3|171| 77.2|214|101.1|2,000|1,093.3
0|-17.7| 43| 6.1| 86| 30.0|129| 53.8|172| 77.7|215|101.6|2,100|1,148.8
+1|-17.2| 44| 6.6| 87| 30.5|130| 54.4|173| 78.3|225|107.2|2,200|1,204.4
2|-16.6| 45| 7.2| 88| 31.1|131| 55.0|174| 78.8|250|121.1|2,300|1,260.0
-------------------------------------------------------------------------
9 x degrees C.
Degrees Fahrenheit = -------------- + 32
5
5 x (degrees F. - 32)
Degrees Centigrade = ---------------------
9
Three other useful tables are also given on the following pages.
TABLE 34.--WEIGHT OF ROUND, OCTAGON AND SQUARE CARBON TOOL STEEL PER FOOT
------------------------------------------------------------------------
Size | | | | Size | | |
in | Round |Octagon | Square | in | Round | Octagon | Square
inches | | | | inches | | |
--------|--------|--------|--------|--------|--------|---------|--------
1/16 | 0.010 | 0.011 | 0.013 | 2-1/2 | 16.79 | 17.71 | 21.37
1/8 | 0.042 | 0.044 | 0.053 | 2-5/8 | 18.51 | 19.52 | 23.56
3/16 | 0.094 | 0.099 | 0.120 | 2-3/4 | 20.31 | 21.42 | 25.86
1/4 | 0.168 | 0.177 | 0.214 | 2-7/8 | 22.20 | 23.41 | 28.27
5/16 | 0.262 | 0.277 | 0.334 | 3 | 24.17 | 25.50 | 30.78
3/8 | 0.378 | 0.398 | 0.481 | 3-1/8 | 26.23 | 27.66 | 33.40
7/16 | 0.514 | 0.542 | 0.655 | 3-1/4 | 28.37 | 29.92 | 36.12
1/2 | 0.671 | 0.708 | 0.855 | 3-3/8 | 30.59 | 32.27 | 38.95
9/16 | 0.850 | 0.896 | 1.082 | 3-1/2 | 32.90 | 34.70 | 41.89
5/8 | 1.049 | 1.107 | 1.336 | 3-5/8 | 35.29 | 37.23 | 44.94
11/16 | 1.270 | 1.339 | 1.616 | 3-3/4 | 37.77 | 39.84 | 48.09
3/4 | 1.511 | 1.594 | 1.924 | 3-7/8 | 40.33 | 42.54 | 51.35
13/16 | 1.773 | 1.870 | 2.258 | 4 | 42.97 | 45.34 | 54.72
7/8 | 2.056 | 2.169 | 2.618 | 4-1/4 | 48.51 | 51.17 | 61.77
15/16 | 2.361 | 2.490 | 3.006 | 4-1/2 | 54.39 | 57.37 | 69.25
1 | 2.686 | 2.833 | 3.420 | 4-3/4 | 60.60 | 63.92 | 77.16
1-1/8 | 3.399 | 3.585 | 4.328 | 5 | 67.15 | 70.83 | 85.50
1-1/4 | 4.197 | 4.427 | 5.344 | 5-1/4 | 74.03 | 78.08 | 94.26
1-3/8 | 5.078 | 5.356 | 6.646 | 5-1/2 | 81.25 | 85.70 | 103.45
1-1/2 | 6.044 | 6.374 | 7.695 | 5-3/4 | 88.80 | 93.67 | 113.07
1-5/8 | 7.093 | 7.481 | 9.031 | 6 | 96.69 | 101.99 | 123.12
1-3/4 | 8.226 | 8.674 | 10.474 | 7 | 131.61 | 138.82 | 167.58
1-7/8 | 9.443 | 9.960 | 12.023 | 8 | 171.90 | 181.32 | 218.88
2 | 10.744 | 11.332 | 13.680 | 9 | 217.57 | 229.48 | 277.02
2-1/8 | 12.129 | 12.793 | 15.443 | 10 | 268.60 | 283.31 | 342.00
2-1/4 | 13.598 | 14.343 | 17.314 | 11 | 325.01 | 342.80 | 413.82
2-3/8 | 15.151 | 15.981 | 19.291 | 12 | 386.79 | 407.97 | 492.48
------------------------------------------------------------------------
High-speed steel, being more dense than carbon steel, weighs from
10 to 11 per cent more than carbon steel. This should be added
to figures given in the table.
TABLE 35.--WEIGHT OF ROUND, CARBON TOOL STEEL 12 IN. IN DIAMETER
AND LARGER, PER FOOT
--------------------------------------------------------------------
Diameter, | Weight | Diameter, | Weight | Diameter, | Weight
inches | per foot | inches | per foot | inches | per foot
-----------|----------|-----------|----------|-----------|----------
12 | 386.790 | 15-7/8 | 677.527 | 19-3/4 | 1,049.010
12-1/8 | 395.518 | 16 | 687.600 | 19-7/8 | 1,061.705
12-1/4 | 404.246 | 16-1/8 | 699.017 | 20 | 1,074.400
12-3/8 | 412.974 | 16-1/4 | 710.435 | 20-1/8 | 1,088.502
12-1/2 | 421.702 | 16-3/8 | 721.852 | 20-1/4 | 1,102.605
12-5/8 | 430.430 | 16-1/2 | 733.270 | 20-3/8 | 1,116.707
12-3/4 | 439.158 | 16-5/8 | 744.687 | 20-1/2 | 1,130.810
12-7/8 | 447.886 | 16-3/4 | 756.105 | 20-5/8 | 1,144.912
13 | 456.615 | 16-7/8 | 767.522 | 20-3/4 | 1,159.015
13-1/8 | 465.343 | 17 | 778.940 | 20-7/8 | 1,173.118
13-1/4 | 474.071 | 17-1/8 | 790.358 | 21 | 1,187.220
13-3/8 | 482.799 | 17-1/4 | 801.777 | 21-1/8 | 1,201.322
13-1/2 | 491.527 | 17-3/8 | 813.195 | 21-1/4 | 1,215.425
13-5/8 | 500.255 | 17-1/2 | 824.614 | 21-3/8 | 1,229.527
13-3/4 | 508.983 | 17-5/8 | 836.030 | 21-1/2 | 1,243.630
13-7/8 | 517.711 | 17-3/4 | 847.447 | 21-5/8 | 1,257.732
14 | 526.440 | 17-7/8 | 858.863 | 21-3/4 | 1,271.835
14-1/8 | 536.512 | 18 | 870.280 | 21-7/8 | 1,285.937
14-1/4 | 546.585 | 18-1/8 | 883.105 | 22 | 1,300.040
14-3/8 | 556.657 | 18-1/4 | 895.920 | 22-1/8 | 1,315.485
14-1/2 | 566.730 | 18-3/8 | 908.740 | 22-1/4 | 1,330.930
14-5/8 | 576.802 | 18-1/2 | 921.560 | 22-3/8 | 1,346.375
14-3/4 | 586.875 | 18-5/8 | 934.380 | 22-1/2 | 1,361.820
14-7/8 | 596.947 | 18-3/4 | 947.200 | 22-5/8 | 1,377.265
15 | 607.020 | 18-7/8 | 960.020 | 22-3/4 | 1,392.710
15-1/8 | 617.092 | 19 | 972.840 | 22-7/8 | 1,408.155
15-1/4 | 627.165 | 19-1/8 | 985.035 | 23 | 1,423.600
15-3/8 | 637.237 | 19-1/4 | 998.230 | 23-1/8 | 1,454.490
15-1/2 | 647.310 | 19-3/8 |1,010.925 | 23-1/4 | 1,485.380
15-5/8 | 657.382 | 19-1/2 |1,023.620 | 23-3/8 | 1,516.270
15-3/4 | 667.455 | 19-5/8 |1,036.315 | 24 | 1,547.160
--------------------------------------------------------------------
To find the weight of discs made of carbon steel, in diameters
up to and including 12 in., without any allowance for finishing
multiply the per foot weight of round bar steel, shown herewith
by the decimal equivalent of a foot given in the following table:
TABLE 36.--DECIMAL EQUIVALENTS OF A FOOT
---------------------------------------------------------------------
In. | 0 | 1/8 | 1/4 | 3/8 | 1/2 | 5/8 | 3/4 | 7/8
-----|-------|-------|-------|-------|-------|-------|-------|-------
0 | 0.000 | 0.010 | 0.021 | 0.031 | 0.042 | 0.052 | 0.063 | 0.073
1 | 0.083 | 0.094 | 0.104 | 0.115 | 0.125 | 0.135 | 0.146 | 0.156
2 | 0.167 | 0.177 | 0.188 | 0.198 | 0.208 | 0.219 | 0.229 | 0.240
3 | 0.250 | 0.260 | 0.270 | 0.281 | 0.292 | 0.302 | 0.313 | 0.323
4 | 0.333 | 0.344 | 0.354 | 0.364 | 0.375 | 0.385 | 0.396 | 0.406
5 | 0.416 | 0.427 | 0.437 | 0.448 | 0.458 | 0.469 | 0.479 | 0.480
6 | 0.500 | 0.510 | 0.520 | 0.531 | 0.542 | 0.552 | 0.563 | 0.573
7 | 0.583 | 0.594 | 0.604 | 0.615 | 0.625 | 0.635 | 0.646 | 0.656
8 | 0.666 | 0.677 | 0.687 | 0.698 | 0.708 | 0.719 | 0.729 | 0.740
9 | 0.750 | 0.760 | 0.770 | 0.781 | 0.792 | 0.802 | 0.813 | 0.823
10 | 0.833 | 0.844 | 0.854 | 0.865 | 0.875 | 0.885 | 0.896 | 0.906
11 | 0.916 | 0.927 | 0.937 | 0.948 | 0.953 | 0.969 | 0.979 | 0.990
---------------------------------------------------------------------
EXAMPLE.--If the weight of a carbon steel disc 7 in. diameter,
1-5/8 in. thick is desired, turn to page 233, where the per foot
weight of 7 in. round is given as 131.6 lb. Multiply this by the
decimal equivalent of 1-5/8 in., or 0.135, as shown in the above
table, and the product will be the net weight of the disc.
131.61 lb. = the weight of 1 ft. of 7 in. round.
0.135 = the per foot decimal equivalent of 1-5/8 in:
------------
65805
39483
13161
------------
17.76735 lb. = weight of disc 7 in. diam. 1-5/8 in. thick without any
allowance for finishing.
AUTHORITES QUOTED
A
ADDIS, W H.
AMERICAN MACHINISTS' HANDBOOK
AMERICAN STEEL TREARERS' SOCIETY
AMERICAN GEAR MFRS. ASSO.
AUTOMATIC AND ELECTRIC FURNACES LTD.
ARNOLD, PROF. J. O.
B
BURLEIGH, R. W.
BORDEN, B.
BOKER, HERMAN & Co.
BROWN INSTRUMENT Co.
BROWN-LIPE-CHAPLIN Co.
C
CAMPBELL, H. H.
CARHART, H. A.
CLAYTON, C. Y.
CURTIS AIRPLANE Co.
E
ENGLEHARD, CHARLES
ENSAW, HOWARD
F
FIRTH-STERLING STEEL Co.
FIRTH, THOMAS & SONS
FOWLER, HENRY
G
GILBERT & BARKER
H
HAYWAHD, C. R.
HOWE, DR. H. M.
HOOVER STEEL BALL CO.
HEATHCOTE, H. L.
HARRIS, MATTHEW
HUNTER, J. V.
J
JANITZKY, E. J.
JOHNSTON, A. B.
JUTHE, K. A.
L
LATROBE STEEL CO.
LUDLUM STEEL CO.
LEEDS & NORTHRUP CO.
LYMAN, W. H.
M
MANSFIELD, C. A.
MIDVALE STEEL Co.
McKENNA, ROY C.
MOULTON, SETH A.
N
NILES, BEMENT, POND
P
PARKER, S. W.
POOLE, C. R.
R
RAWDON, H. S.
S
S. A. E. (SOCIETY AUTOMOTIVE ENGINEERS)
SAUVEUR, ALBERT
SPRINGFIELD ARMORY
SELLACK, T. G.
SMITH, A. J.
SHIRLEY, ALFRED J.
T
TAYLOR INSTRUMENT Co.
THUM, E. E.
TIEMANN, H. P.
U
U. S. BALL BEARING Co.
UNITED STEEL Co.
UNDERWOOD, CHARLES N.
V
VAN DE VENTER, JOHN H.
W
WALP, H. O.
WOOD, HAROLD F.
WHEELOCK, LOVEJOY & Co.
INDEX
A
ABC of iron and steel
Absorption of carbon, rate of
Air hardening steels
Analysis of high speed steel
Allotropic modifications
Alloy steel, annealing
properties of
Alloys and their effect
in high speed steel
in steel, value of
upon steel
Alpha iron
Annealing
care in
furnace
high-chromium steel
high speed tools
in bone
methods
proper
rifle components
rust-proof steel
steels
temperature
Arrests
Austentite
Automotive industry, application of Liberty engine materials to
temperature control
Axles, heat treatment of
B
Balls, making steel
Barium chloride process
Baths for tempering
Bessemer converter
Beta iron
Blending compounds
Blister steel
Blue brittleness
Bone, annealing in
Boxes for case hardening or carburizing
Breaking test gears
Brinell hardness
Broach hardening furnace
Brown automatic pyrometer
Burning
C
Calorized tubes
Carbon
content at various temperatures
content of case hardened work
in cast iron, ix
in tool steel
introduction of
penetration of
steel
steel forgings, Liberty engine
steel tools
steels, S. A. E.
steels, temper colors
strengthens iron
tool steel, forging
Carbonizing, _see_ Carburizing
Carborundum tubes
Carburization, preventing
Carburizing by gas
boxes
compounds
gas consumption by
local
material
nickel steel
or case hardening
pots for
Carburizing, process of
short method
sleeves
with charcoal
_See_ Case hardening
Car door type of furnace
Case, depth of
Case hardening boxes
cast iron
local
or surface carburizing
treatments for various steels
_see_ Carburizing
Cast iron, carbon in
case hardening
Cementite
Center column furnace
Centigrade table
Chamotte tubes
Chart of carbon penetration
heat treatment
shape
Chrome steel
Chrome-nickel steel
steel, forging
Chrome-vanadium steel
Chromium
steels, S. A. E.
Chromium-cobalt steel
Chromium-vanadium steel, S. A. E.
Classification of steel
Clay tubes
Cold end compensator
junction errors
shortness
worked steel
Color in tempering
Colors on carbon steels
Combination tank
Comparison of fuels
Compensating leads
Compensator for cold ends
automatic
Composition of steel
Compound, blending
separating from work
Compounds for carburizing
Connecting rods, Liberty motor
Continuous heating furnace
Converter, Bessemer
Cooling curves
Cooling quenching oil, roof system
rate of, for gear-forgings
Copper, effect of, in medium carbon steel
Copper-plating to prevent carburizing
Corrosion of high-chromium steel
of rust-proof steel
Corundite tubes
Cost of operating furnaces
Cracks in hardening, preventing
Crankshaft, Liberty motor
Critical point
Crucible or tool steel
Cutting off high speed steel
Cyanide bath for tool steel
D
Decarbonizing of outer surface
preventing
Depth of case
Detrimental elements in steel
Dies, drop forging
quenching
soft spots in
tempering round
Drawing
ends of gear teeth
Drop forging dies
Ductility
E
Effect of alloys
of different carburizing material
of size of piece
of copper in medium carbon steel
Elastic limit
Electric process of steel making
Electrode
Elements, chemical
Elongation
Endurance limit
Energizer, 81
Enlarging steel
Equipment for heat treating
Eutectoid
F
Fahrenheit temperature table
Fatigue test
Ferrite
File test
Flame shields
Flange shields for furnaces
Forging furnace
high speed tools
improper
of steel
practice, heavy
rifle barrels
Forgings, carbon steel Liberty engine
Formed tools, high speed
Fractures, examining by
Furnace, continuous heating
crucible
data
electric
Heroult
open hearth
records
Furnaces
annealing
broach hardening
car door type
center column
cost of operating
data on
forging, heavy
fuels for
gas fired
high speed steel
lead pot
manganese steel
muffle
oil fired
operating costs
screens for
tool
Furnaces, water cooled fronts
Fuels, comparison of
for furnaces
G
Gages, changes due to quenching
tempering
Gamma iron
Gas, carburizing by
consumption for carburizing
fired furnace
illuminating, for carburizing
Gear blanks, heat treatment of
forgings, rate of cooling for Liberty engine
hardening machine
steel, transmission
teeth, drawing ends of
Gears, Liberty engine
Gleason tempering machine
Grade of steel
Grain, refining
size
Graphitic carbon
Grinding high speed steel
H
Hair lines in forgings
Hardening
carbon steel for tools
cracks, preventing
dies
gears
high speed steel
high speed tools
of high-chromium steel
of rust-proof steel
room, modern
Hardness tests
Heating, effect of size
for forging
Heat, judging by color
treating departments
equipment
forgings
inspection of
Liberty motor
Heat treating, of axles
of chisels
of gears
of high speed steel
of steel
S. A. E.
Heat treatment
Heroult furnace
High-chromium steel
annealing of
corrosion of
hardening of
Highly stressed parts of Liberty engine
High speed steel, analysis of
annealing
cutting off
forging
furnace
hardening
heat treatment of
instructions for
manufacture
pack hardening
structure of
Hints for steel users
I
Illuminating gas for carburizing
Impact test
Improper forging
Influence of size on heating
Inspection of heat treatment
Internal stresses, relieving
Introduction of carbon
J
Jewelers' tools
Judging heat of steel by color
L
Latent heat
Lathe and planer tools
tools, high speed
Latrobe temper list
Lead bath
pot furnace
Leeds & Northrup potentiometer
optical pyrometer
Liberty engine, highly stressed parts of
Liberty engine materials, application to automotive industry
motor connecting rods
motor, crankshaft
motor piston pin
Local case hardening
Luting mixture
M
Machineability of steel
Machinery steel, annealing
Magnet test
Making steel in electric furnace
Manganese
steel
furnace
Manufacture of high speed steel
Marquardt mass tubes
Martensite
Medium carbon steel, effect of copper on
Metallography
Microphotographs
Microscopic examination
Milling cutters, high speed
Mixture for luting
Modern hardening room
Molten metal pyrometers
Molybdenum
Muffle furnace
N
Nickel
Nickel-chromium steel
steels, S. A. E.
Nickel, influence of, on steel
steel
affinity for carbon
steels, S. A. E.
Non-homogeneous melting
Non-shrinking steels
Normalizing
O
Oil bath for tempering
cooling on roof
fired furnace
hardening steel, forging
steels
temperature of quenching
Open hearth furnace
Operating costs of furnaces
Outer surface decarbonizer
Over-heated steel, restoring
Overheating
dies
P
Pack-hardening
high speed steel
Packing work for carburizing
Paste for hardening dies
Pearlite
Penetration of carbon
carbon, chart of
in case hardening
Phosphorus
Pickling Liberty motor forgings
Pig iron
Piston pin, Liberty motor
Placing pyrometers
Planer tools, high speed
"Points" of carbon in steel
Potentiometer, Leeds & Northrup
Pots for carburizing
Press for testing gears
Preventing carburization
cracks in hardening
Properties of alloy steels
of alloy steels, table
of steel
Protective screens for furnaces
Puddled iron
Punches and chisels, steels for
Pyrometers
calibration
copper ball
indicating
inspection
iron ball
molten metal
optical
placing
recording
Siemens
testing
water
Q
Quality and structure of high speed steel
of steel
Quenching,
after carburizing
dies in tank
obsolete method
oil, temperature of
tank
tool steel
R
Rate of absorption of carbon
Recording temperatures
Red shortness
Refining the grain
Regenerative open hearth furnace
Restoring overheated steel
Rifle barrels, forging
components, annealing
Roof system of cooling oil
Rust-proof steel
annealing of
corrosion of
hardening of
S
S. A. E. carbon steels
chromium steels
chromium-vanadium
heat treatments
nickel-chromium steels
nickel steels
screw stock
silico-manganese steel
standard steels
Salt bath for tempering
Scleroscope test
Scratch hardness
Screens for furnaces
Screw stock, S. A. E.
Sensible heat
Sentinels, melting of
Separating work from compound
Shields for furnace doors
Shore Scleroscope
Short method of carburizing
Shrinking steel
Silica tubes
Silico-manganese steels, S. A. E.
Silicon
Silversmiths' tools
Size of piece, effect of
Slags
Sleeves, carburizing
hardening and shrinking
shrinking
Solid solution
Sorbite
Specimens, test
Standard S. A. E. steels
Steel,
balls, stock for
bolts, making
composition of
deoxidation
for chisels and punches
forging of
give it a chance
heat treatment of
high speed
making
Bessemer process
crucible process
electric furnace process
open hearth
tools, carbon, in
users' hints
Structure of high speed steel
Sulphur
T
Tables, air, oil and water hardened steel
alloy steels, properties of
carbon content
carbon steels
case hardening
changes due to quenching
chromium steels
chromium-vanadium steels
colors and temperature
composition of steels
cost of furnaces
effect of size
fuels, comparison of
high-chromium steel
nickel-chromium steels
nickel steels
operating cost of furnaces
production cost of furnaces
S. A. E. steels
screw stock
silico-manganese steels
stock for balls
temperature conversion
tempering temperatures
weight of steel
Tank for quenching
dies
Taylor instruments
Temper, colors of
list, Latrobe
of steel
Temperature recorders
tables
Temperatures for tempering
Tempering colors on carbon steels
gages
high speed tools
machine, Gleason
round dies
temperatures
theory of
Tempers of carbon steel
Tensile test
Testing heat treatment
Tests of steel
Test specimens
Theory of tempering
Thermocouple
base metal
cold end
placing
protectors
rare metal
Time for hardening
Tool furnace, small
Tool or crucible steel, annealing
Tool steel, cyanide bath for
quenching
Tools, carbon in different
carbon steel
of high speed steel
sulphur in
tempers of various
transformation points
Transmission gear steel
Treatments for various steels
Troosite
Tubes, calorized
carborundum
Chamotte
clay
Marquardt mass
silica
Tungsten steel
U
Ultimate strength
Users of steel, hints for
V
Vanadium steel
W
Water annealing
cooled furnace fronts
Weight of steel bars
Working instructions for high speed steel
Wrought iron, ix
Y
Yield Point
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