24. case hardening

8/2/2019 24. Case Hardening

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24. Case Hardening

Numerous industrial applications

require a hard wear-resistantsurface called the case and a



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



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



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



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



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,



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



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.



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



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



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



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



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



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



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



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



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



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)



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



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



% 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



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.



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



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



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



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



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.



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



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.



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.



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



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



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



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.



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



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.



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



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



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



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.



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.



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.



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



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),



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



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



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



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



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



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



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



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



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.



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



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



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.



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



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



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



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.



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



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



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



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.

24. case hardening

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