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24. Case Hardening
Numerous industrial applications
require a hard wear-resistantsurface called the case and a
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relatively soft, tough inside called the core.
There are five principal methods of case-
hardening.1.Carburizing
2.Nitriding
3.Cyaniding or carbonitriding4.Flame Hardening
5.Induction Hardening
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The first three methods change the
chemical composition, carburizing by the
addition of carbon, nitriding by the additionof nitrogen and cyaniding by addition of
both carbon and nitrogen. The last two
methods do not change the chemical
composition of the steel and areessentially shallow-hardening methods. In
flame and induction hardening the steel
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must be capable of being hardened,therefore, the carbon content must beabout 0.30 % or higher.
25. Carburizing: This is the oldest and oneof the cheapest methods of casehardening. A low-carbon steel, usually
about 0.20 % carbon or lower, is placed inan atmosphere that contains substantialamounts of carbon monoxide. The usual
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carburizing temperature is 1700 F. At this
temperature, the following reaction takes place:
where represents carbon dissolved in austenite . Themaximum amount of carbon that can be dissolved inaustenite at 1700 F is indicated on the iron-iron carbideequilibrium diagram at the line. Therefore very
quickly, a surface layer of high carbon (about 1.2 %) isbuilt up. Since the core is of low carbon content, thecarbon atoms trying to reach equilibrium will begin todiffuse inward. The rate of diffusion of carbon inaustenite, at a given temperature, is dependent upon
22 CO FeCO Fe
C p
C Fe
cm A
cm A
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The diffusion coefficient and the carbon-con-
centration gradient. Under known and
standard operating conditions, with nthesurface at a fixed carbon concentration,
the form of the carbon gradient may be
predicted, with reasonable accuracy, as a
function of elapsed time. After diffusion,has taken for the required amount of time
depending upon the case depth desired,
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the part is removed from the furnace and
cooled.If the part is furnace-cooled and
examined microscopically, the carbongradient will be visiblein the gradual
change of the structure. At the surface is
the hypereutectoid zone consisting of
pearlite with a white cementite network,followed by the eutectoid zone of only
pearlite and finally the hypoeutectoid zone
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of pearlite and ferrite, with the amount of
ferrite increasing until the core is reached.
(This is illustrated in Fig: 8-70). Analysis to
determine carbon content is made and
results can be plotted graphically as in Fig:
8-71. The relation of time and temperature
to case depth is shown in Fig: 8-72 andTable: 8-7.
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The carburizing equation given previously,
is reversible and may proceed
to the left, removing carbon from the surface
layer if the steel is heated in an atmospherecontaining carbon dioxide. This is called
decarburization.
Decarburization is a problem primarily with high-carbon steels and tool steels. The surface,
depleted of carbon, will not transform to
martensite on subsequent hardening, and the
22 CO FeCO Fe
C p
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steel will be left with a soft skin . For many
tool applications, the stresses to which the
part is subjected in service are maximum
at or near the surface, so decarburization
is harmful. Fig: 8-73 shows decarb-
urization on the surface of a high-carbon
steel. Decarburization may be preventedby using an endothermic gas atmosphere
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in the furnace to protect the surface of the
steel from oxygen, carbon dioxide and
water vapour.
Commercial carburizing may be accom-
plished by means of packed carburizing,
gas carburizing and liquid carburizing. In
packed carburizing, the work issurrounded by a carburizing compound
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in a closed container. The container is
heated to the proper temperature for the
required amount of time and then slow-
cooled. This is essentially a batch method
and does not lend itself to high production.
Commercial carburizing compunds usually
consist of hardwood charcoal, coke andabout 20 % barium carbonate as an
energizer. The carburizing compound is in
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the form of coarse particles or lumps, so
that, when covered is sealed on the
container, sufficient air will be trapped
inside to form carbon monoxide. The
principal advantages of packed carburizing
are that it does not require the use of
prepared atmosphere and that it is efficientand economical for individual processing
of small lots of parts or of large, massive
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parts. The disadvantage are that it is not well
suited in the production of thin carburized cases
that must be controlled to close tolerances. It
cannot provide the close control of surfacecarbon that can be be obtained by gas
carburising; parts cannot be direct-quenched
from the carburizing temperature; and excessive
time is consumed in the heating and cooling thecharge. Because of the inherent variation in
case depth and cost of packing materials, pack
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carburizing is not used on work requiring a case
depth of less than 0.03 in., and to tolerances are
at least 0.010 in.
Gas carburizing may either batch or continuousand lends itself better to production heat
treatment. The steel is heated in contact with
carbon monoxide and/or a hydrocarbon which is
readily decomposed at the carburizing
temperature. The hydrocarbon may be methane,
propane, natural gas or vapourized fluid
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hydrocarbon. Commercial practice is to usea carrier gas, such as obtained from anendothermic generator and enrich it with
one of the hydrocarbons.It was mentioned previously that carburized
parts will usually have a thin outer layer of high carbon. There are two reasons why itmay be desirable to avoid this hyper-eutectoid layer. First, if the piece is cooled
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slowly from the carburizing temperature, a
proeutectoid cementite network will form at the
grain boundaries. On subsequent hardening,
particularly if the steel is heated below theline, some grain boundary cementite will remain
in the finished piece and is a frequent cause of
failure. Second, the hypereutectoid surface-
carbon content will increase the amount of retain austenite. Therefore, if the steel is highly
alloyed, the carbon content of the case should
cm A
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be no greater than the eutectoid content of
0.80 % carbon. By using a diffusion period
, during which the gas is turned off but thetemperature maintained, gas carburizing
allows the surface carbon to be reduced to
any desired value. Use of the diffusion
period also produces much cleaner workby dissipation of carbon deposit (soot)
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during the time when no gas is flowing. Gas
carburizing allows quicker handling by
direct quenching, lower cost, cleaner
suroundings, closer quality control, and
greater flexibility of operation compared to
packed carburizing.
Liquid carburizing is a method of case-hardening steel by placing it in bath of
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Molten cyanide so that carbon will diffuse
from the bath into the metal and produce a
case comparable to one resulting from
pack or gas carburizing. Liquid carburizing
may be distinguished from cyaniding by
the character and composition of the case
produced. The cyanide case is higher innitrogen and lower in carbon; the reverse
is true of liquid carburized cases. Cyanide
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% and operate between between 1650 and
1750 F. High-temperature salt baths are
used for producing case depths of 0.030 to
0.120 in., although it is possible to go as
high as 0.250 in. In general, liquid
carburizing is best suited to small and
medium-size parts, since large parts aredifficult to process in salt baths. The
advantages of liquid carburizing are
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1)freedom from oxidation and sooting
problems
2)Uniform case depth and carbon content3)A rapid rate of penetration, and
4)The fact that the bath provides high
thermal conductivity, thereby reducing thetime required for the steel to reach the
carburizing temperature.
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Disadvantages include:
1)Parts must be thoroughly washed after
treatment to prevent rusting2)Regular checking and adjustment of the
bath composition is necessary to obtain
uniform case depth;
3)Some shapes cannot be handled because
they either float or will cause excessive
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dragout of salt; and
4)Cyanide salts are poisonous and require
careful attention to safety.26. Heat Treatment after Carburizing
Since steel is carburized in the austenite
region, direct quenching from thecarburizing temperature will harden both
the case and core if the cooling rate is
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greater than the critical cooling rate. Direct
quenching of coarse-grain steels often
leads to brittleness and distortion, so the
treatment should be applied only to fine-
grain steels. Alloy steels are rarely used in
the direct-quenched condition because of
large amount of retained austenite in thehardened case. Fig: 8-74 shows a
diagrammatic representation of various
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hardening treatments for carburized steels
together with case and core properties.
When a carburized part is hardened, thecase will appear as a light martensite zone
followed by a darker transition zone (Fig:
8-75). The hard case or effective case is
measured from the outer edge to themiddle of the dark zone. From the nature
of carbon gradient, the hard case contains
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the portion of the case above 0.40 %
carbon and is approximately equal to two-
thirds of the total case. Hardness-traverse
measurements may also be used to
determine the depth of the effective case
since the middle of the transition zone is at
approximately Rockwell C 50.
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28. Nitriding: This is process for case
hardening of alloy steel in an atmosphere
consisting of a mixture in suitable
proportions of ammonia gas and
dissociated ammonia. The effectiveness of
process depends on the formation of
nitrides in the steel by reaction of nitrogenwith certain alloying elements. Although at
suitable temperatures and with the proper
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atmosphere all steels are capable of
forming iron nitrides, the best results
obtained in those steels that contain one
or more of the major nitride-forming
alloying elements. These are aluminum,
chromium and molybdenum. The nitrogen
must be supplied in the atomic or nascentform, molecular nitrogen will not react.
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The parts to be nitrided are placed in an
airtight container through which the
nitriding atmosphere is supplied
continuously while the temperature is
raised and held between 925 and 1050 F.
The nitriding cycle is quite long, depending
upon the case depth desired. As shown inFig: 8-77, a 60-h cycle will give a case
depth of ~0.024 in at 975 F.
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A nitrided case consists of two distinct
zones. In the outer zone the nitride-
forming elements, including iron, have
been converted to nitrides. This region,
which varies in thickness up to a maximum
of about 0.002 in., is commonly known as
the ³ white layer´ because of theappearance after the nital etch. In the
zone beneath this white layer, alloy
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nitrides only have been precipitated. A
typical microstructure, illustrated in Fig: 8-
78b, shows the white layer and underlying
nitride case. At lower magnification,
illustrated in Fig: 8-78a, the lighter core
structure can be seen beneath the nitride
case. The depth of nitride case isdetermined by the rate of diffusion of
nitrogen from the white layer to the region
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beneath. The nitriding medium, therefore,
needs to contain only sufficient active
nitrogen to maintain the white layer. Any
increase beyond this point serves to
increase the depth of white layer and does
not affect the thickness of the inner layer.
The double-stage process, is also knownas the Floe process, has the advantage of
reducing the thickness of the white nitride
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layer. In the first stage of the double-stage
process, the ammonia dissociation is held
at 20 % for a period of 5 to 10 h at 975 F.
During this period the white layer is
established and the useful nitride starts to
form by diffusion of nitrogen out of it. In the
second stage, the ammonia dissociation isincreased to 83 to 86 %, and the temp-
erature is usually raised to 1025 to 1050 F.
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During the second stage, the gas
composition is such that it maintains only a
thin white layer on the finished part. A
typical structure of the case produced by
this method is shown in Fig: 8-79.
The white layer is brittle and tends to chip or
spall from the surface if it has a thicknessin excess of 0.0005 in. Thicker white
layers produced by the single-stage
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process must be removed by grinding or
lapping after nitriding.
This very thin white layer obtained by thismethod, usually from 0.0002 to 0.0004 in.,
in depth, does not chip or pit, and the
frictional characteristics of the surface are
excellent. This layer also has good wear-inproperties and may be expected to
improve corrosion resistance.
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Hardest cases ~R/C 70 are obtained with
the aluminum alloy steels known as
Nitroalloys. These are medium-carbon
steels containing also chromium and
molybdenum. For some applications
where lower hardness is acceptable,
medium-carbon standard steels containingchromium and molybdenum (AISI 4100,
4300 series) are used. Nitriding has also
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been applied to stainless steels and tool
steels for certain applications
Some complex parts which cannot be case-hardened satisfactorily by carburizing have
been nitrided without difficulty. Wear
resistance is an outstanding characteristic
of the nitrided case and is responsible for it selection in most applications. The
hardness of nitrided case is unaffected by
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heating to temperatures below the original
nitriding temperature. Substantial
hardness is retained to at least 1150 F in
marked contrast with carburized case,
which begins to lose its hardness at
relatively low temperatures. Fatigue
resistance is also an important advantage.Tool marks and surface scratches have
little effect on the fatigue properties of
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nitrided steels. Although it is sometimes
indicated that nitriding improves the
corrosion resistance of a steel, this is true
only if the white layer is not removed.
Corrosion resistance of stainless steels is
reduced considerably by nitriding, a factor
which must be taken into account whennitrided stainless steel are used in
corrosive atmospheres.
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Disadvantages of nitriding include the long
cycles usually required, the brittle case,
use of special alloy steels if maximum
hardness is to be obtained, cost of
ammonia atmosphere, and the technical
control required. Nitriding is used
extensively for aircraft engine parts suchas cams, cylinder liners, valve stems,
shafts and piston rods.
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31. Residual Stresses: These are stresses
that remain in the part after force has
disappeared. Residual stresses always
arise from a nonuniform plastic
deformation. In the case of heat treatment,
this nonuniform plastic deformation may
be caused by a temperature gradient or phase change or usually, a combination of
both factors during cooling.
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Consider the effect of temperature gradient
alone. Under the effect of size and mass,
that during quenching the surface is
cooled more rapidly than the inside. This
results in a temperature gradient across
the cross section of the piece or the
temperature difference between thesurface and the centre.
If the stress exceeds the ultimate strength of
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the material, cracking will occur. This is
what usually happens when glass is
subjected to a large temperature
difference. In case of steel, however,
thermal stresses alone very rarely leading
to cracking. If the stress is below the yield
strength of the steel, the stress will beborne elastically.
Austenite, being f.c.c. (face centred cubic),
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is a denser structure than any of its
transformation products. Therefore when
austenite changes to ferrite, pearlite,
bainite and martensite, an expansion
occurs. The austenite-to-martensite
expansion is the largest and amounts to a
volume increase of about 4.6 % . Themartensite expansion will be greater the
lower the temperature. Fig: 8-83 shows
f M
s M
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the changes in length, during cooling of a small-
diameter cylinder as measured in a dilatometer.
The piece is austenitic at the elevated
temperature, and normal contraction of theaustenite takes place until the temperature
is reached. Between the and the the
transformation of austenite to martensite causes
an expansion in length. After thetemperature, the martensite undergoes normal
contraction.
s M
s M f M
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Let us now consider the combined effect of
temperature gradient and phase change
for the two possibilities: (1) through-
hardened steel and (2) shallow-hardened
steel
Fig: 8-84 shows the surface and centre
cooling curves superimposed on the I-Tdiagram for the through-hardened steel.
Since the centre-cooling rate exceeds the
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critical cooling rate, the part will be fully
martensitic across its diameter. During the
first stage, to time , the stresses present
are due to the temperature gradient. The
surface, prevented from contracting as
much as it should by the centre, will be in
tension while the centre will be incompression. During the second stage,
between times and the surface,
1t
1t
1t
2t
2t
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having reached the temperature,
transforms to martensite and expands.
The centre, however, is undergoing
normal contraction due to cooling. The
centre contracting will prevent the surface
from expanding as much as it should, and
the surface will tend to be in compressionwhile the centre will tend to be in tension.
After the surface has reached room
s M
2t
2t
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temperature and will be a hard, brittle,
martensitic structure. During the third
stage, the centre finally reaches the
temperature and begins to expand,forming martensite. The centre, as it
expands, will try to pull the surface along
with it, putting the surface in tension. Thestress condition in the three stages is
summarized below:
s M
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To initiate and propagate a crack it is
necessary for tensile stress to be present.
Let us examine the three stages with
regard to the danger of cracking. In thefirst stage, the surface is in tension,
however, it is austenitic and if the stress is
high enough rather than cracking it willdeform plastically, relieving the stress. In
the second stage, the centre is in tension
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and is austenitic, so that the tendency is to
produce plastic deformation rather than
cracking. In the last stage the surface is
again in tension. Now, however, thesurface is hard, unyielding martensite. As
the centre expands, there is little likelihood
of plastic deformation. It is during thisstage that the greatest danger of cracking
exists. Depending upon, the difference in
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time between the transformation of the
surface and centre, the cracking may
occur soon after the quench or sometimes
many hours later. Fig: 8-85 showsschematically the type of failure that may
occur. The crack will take place in the
tension layers and will be widest at thesurface.
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One heat-treating rule which minimizes the
danger of cracking is parts should be
tempered immediately after hardening.
Tempering will give the surface martensitesome ductility before the centre
transforms.
Another very effective method of minimizingdistortion and cracking is by martempering
or marquenching illustrated in Fig: 8-86. It
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is carried out by heating to proper
autenitizing temperature, quenching
rapidly in liquid-salt bath held just above
the temperature, and holding for aperiod of time. This allows the surface and
centre to reach the same temperature; air
cooling to room temperature then follows.Since air cooling from just above the
martensite-formation range introduces
s M s M
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very little temperature gradient, the
martensite will be formed at nearly the
same time throughout the piece. This
martempering minimizes residual stressesand greatly reduces the danger of
distortion and cracking. The heat
treatment is completed by tempering themartensite to the desired hardness.
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Fig: 8-87 shows the surface and centre
cooling curves superimposed on the I-T
diagram for the shallow-hardened steel.
During the first stage, up to time , thestresses present are due only to the
temperature gradient, and as in the
through ±hardened condition, the surfacewill be in tension and centre will be in
compression. During the second stage,
2t
1t
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between times and both the surface
and centre will transform. The surface will
transform to martensite while the centre
will transform to softer product, likepearlite. The entire piece is expanding, but
since the expansion resulting from the
formation of martensite is greater than thatresulting from the formation of pearlite, the
surface tends to expand more than the
1t 2
t
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centre. This tends to put the centre in
tension and the surface is in compression.
After the centre will contract on cooling
from the transformation temperature toroom temperature. The surface, being
martensitic and having reached room
temperature earlier, will prevent the centrefrom contracting as much as it should.
This will result in higher tensile stresses in
2t
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expanding, the tensile stress will be small.
During the third stage, the surface, as a
hard rigid shell of martensite, will prevent
the centre from contracting as it cools toroom temperature. The tensile stresses in
the centre may reach a high value, and
since the centre is pearlite of relatively lowtensile strength, it is during this stage that
the greatest danger of cracking exists.
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Fig: 8-88 shows schematically the type of
failure that may occur in a shallow-
hardened steel. The tensile crack is
internal and cannot come to the surfacebecause of the compressive stress in the
surface layers. Since they are internal,
these cracks are difficult to detect. X-raytesting or in some cases Magnaflux
inspection may show the presence of
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internal fissures. Very often these parts are
placed in service without knowledge of
internal quenching cracks. As soon as
there is the slightest bit of tensile stress inthe surface due to the external load, the
crack will come through and the part will
fail.In many applications, the tensile stress
developed by the external force is
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maximum at or near the surface. For these
applications, shallow-hardened or case-
hardened parts are preferred, since the
surface residual stresses are usuallycompressive. In order for the surface to
be in tension, the residual compressive
must first be brought to zero. Thiseffectively increases the strength of the
surface. The same beneficial effect and
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greatly increased life have been found for
leaf springs where residual surface
compressive stresses were induced by
shot peening before the springs wereplaced in service.