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_