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10 Distortion of Heat-TreatedComponentsMichiharu Narazaki and George E. Totten
CONTENTS
10.1 Introduction .............................................................................................................614
10.2 Basic Distortion Mechanisms...................................................................................609
10.2.1 Relief of Residual Stresses........................................................................... 609
10.2.2 Material Movement Due to Temperature Gradients during Heating
and Cooling................................................................................................. 610
10.2.3 Volume Changes during Phase Transformations ........................................ 610
10.3 Residual Stresses ...................................................................................................... 612
10.3.1 Residual Stress in Components ................................................................... 612
10.3.2 Residual Stresses Prior to Heat Treatment ................................................. 612
10.3.3 Heat Treatment after Work-Hardening Process .........................................612
10.4 Distortion during Manufacturing ............................................................................613
10.4.1 Manufacturing and Design Factors Prior to Heat Treatment That
Affect Distortion ......................................................................................... 613
10.4.1.1 Material Properties ..................................................................... 614
10.4.1.2 Homogeneity of Material............................................................ 614
10.4.1.3 Distribution of Residual Stress System....................................... 614
10.4.1.4 Part Geometry............................................................................. 614
10.4.2 Distortion during Component Heating ....................................................... 615
10.4.2.1 Shape Change Due to Relief of Residual Stress ......................... 615
10.4.2.2 Shape Change Due to Thermal Stresses......................................615
10.4.2.3 Volume Change Due to Phase Change on Heating .................... 615
10.4.3 Distortion during High-Temperature Processing ........................................ 616
10.4.3.1 Volume Expansion during Case Diffusion.................................. 616
10.4.3.2 Distortion Caused by Metal Creep ............................................. 616
10.4.4 Distortion during Quenching Process ......................................................... 617
10.4.4.1 Effect of Cooling Characteristics on Residual
Stress and Distortion from Quenching ....................................... 617
10.4.4.2 Effect of Surface Condition of Components...............................624
10.4.4.3 Minimizing Quench Distortion ...................................................625
10.4.4.4 Quench Uniformity ..................................................................... 629
10.4.4.5 Quenching Methods ....................................................................630
10.5 Distortion during Post Quench Processing .............................................................. 631
10.5.1 Straightening ............................................................................................... 631
10.5.2 Tempering ...................................................................................................631
10.5.3 Stabilization with Tempering and Subzero Treatment................................ 632
10.5.4 Metal Removal after Heat Treatment ......................................................... 633
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10.6 Measurement of Residual Stresses ...........................................................................633
10.6.1 X-Ray Diffraction Method .........................................................................634
10.6.2 Hole-Drilling Methods ................................................................................635
10.6.3 Bending and Deflection Methods................................................................ 636
10.6.4 Other Residual Stress Measurement Methods ............................................ 636
10.7 Tests for Propensity for Distortion and Cracking ................................................... 636
10.7.1 Navy C-Ring and Slotted Disk Test ........................................................... 637
10.7.2 Cylindrical Specimens ................................................................................. 637
10.7.3 Stepped Bar Test ......................................................................................... 638
10.7.4 Key-Slotted Cylindrical Bar Test ................................................................ 638
10.7.5 Disk with an Eccentric-Positioned Hole...................................................... 638
10.7.6 Finned Tubes...............................................................................................639
10.8 Prediction of Distortion and Residual Stresses ........................................................ 640
10.8.1 Governing Equations ..................................................................................642
10.8.1.1 Mixture Rule............................................................................... 642
10.8.1.2 Heat Conduction Equations and Diffusion Equation ................ 642
10.8.1.3 Constitutive Equation .................................................................643
10.8.1.4 Kinetics of Quenching Process....................................................643
10.8.1.5 Transformation Plasticity............................................................644
10.8.2 Coupling Algorithm in Simulation by Finite-Element Analysis.................. 644
10.8.3 Example of Simulation Results ...................................................................645
10.8.3.1 Prediction of Warpage of Steel Shafts with Keyway .................. 645
10.8.3.2 Prediction of Distortion during Carburized Quenching
Process of CrMo Steel Ring ...................................................... 645
10.9 Summary .................................................................................................................. 648
References .......................................................................................................................... 648
10.1 INTRODUCTION
In various manufacturing processes of steel components, heat treatment is the most sensitive
and least controllable operation because it involves unexpected and uncontrollable distortion.
To assure high quality and reliability of steel components, manufacturers perform heat
treatments. As long as parts have been heat-treated, distortion has been a concern. As greater
dimensional accuracy is required for components, distortion becomes even more of a prob-
lem. The main industrial concern is therefore to account for distortion during design and
manufacturing. Recent studies and contacts with industry have often highlighted the frustra-
tions experienced by manufacturers trying to control dimensions consistently.
It is known that almost every step in the manufacturing process can affect the final shape
of the part. If it could be accurately predicted what the new shape of a part would be after
heat treatment, then this could be included in the design during manufacturing. However,
there are so many variables interacting in so many ways that the problem is often beyond the
present capacity for analysis, and thus distortion cannot be accurately predicted. This leads to
a definition of heat treatment distortion: Distortion is an unexpected or an inconsistent
change in size or shape caused by variations in manufacturing process conditions.
Although distortion of parts may become noticeable after heat treatment, the root
cause may lie in another manufacturing process that is contributing to variability, such as
variable residual stress, due to differences in machining. However, heat treatment of steel
often requires that the steel be heated to high temperatures, held at that temperature for long
periods, and then rapidly cooled by quenching. These processes are necessary to generate
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high mechanical properties but they can also cause parts to change shape in unpredictable
ways unless conditions are closely controlled.
This chapter will provide an overview of the effects of various factors on distortion,
residual stress, and cracking of steel components. The subjects that will be discussed include:
. Basic distortion mechanisms
. Residual stresses
. Distortion during manufacturing
. Distortion during postquenching processing
. Measurement of residual stresses
. Tests for propensity for distortion and cracking
. Prediction of residual stress and distortion
10.2 BASIC DISTORTION MECHANISMS
The shape and size changes of a part during heat treating can be attributed to three
fundamental causes:
. Residual stresses that cause shape change when they exceed the material yield strength.
This will occur on heating when the strength properties decline.. Stresses caused by differential expansion due to thermal gradients. These stresses will
increase with the thermal gradient and will cause plastic deformation as the yield
strength is exceeded.. Volume changes due to transformational phase change. These volume changes will be
contained as residual stress systems until the yield strength is exceeded.
10.2.1 RELIEF OF RESIDUAL STRESSES
If a part has locked-in residual stresses, these stresses can be relieved by heating the part until
the locked-in stresses exceed the strength of the material. A typical stressstrain curve
obtained from a tension test is shown in Figure 10.1. Initial changes in shape are elastic but
under increased stress they occur in the plastic zone and are permanent. Upon heating, the
stresses are gradually relieved by changes in the shape of the part due to plastic flow. This is a
continuous process and as the temperature of the part is increased, the material yield stress
Stress, s =
Elastic Plastic
Uniformelongation
NeckingFracture
Offset Strain, e =
Ultimatetensile
strengthyield
stress
P
A0
L L0L0
FIGURE 10.1 Various features of a typical stressstrain curve obtained from a tension test.
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decreas es as shown in Figu re 10.2 [1]. It is a functi on not only of tempe ratur e but also of time,
becau se the mate rial will creep unde r lower applie d stresses . It is ap parent that stresses can
never be reduced to zero, becau se the material wi ll always pos sess some level of yield strength
below which the residual stresses cannot be reduced.
10.2.2 MATERIAL MOVEMENT DUE TO T EMPERATURE GRADIENTS DURINGHEATING AND COOLING
Wh en parts are heated during heat treatment , a thermal grad ient e xists across the cross-
sectio n of the compo nent. If a section is heated so that a portio n of the compo nent becomes
hotter than the surroundi ng material , the hotter material expand s and occu pies a greater
volume than the adjacent mate rial an d will thus be exposed to applied stresses that will cause
a shape chan ge when they exceed mate rial strength. Thes e movem ents can be relat ed to
heati ng rate and secti on thickne ss of the co mponent.
10.2.3 V OLUME C HANGES DURING P HASE TRANSFORMATIONS
Wh en a steel part is heated, it transform s to au stenite with an accompan ying reductio n in
volume as shown in Figu re 10.3 [2]. W hen steel is slowly coo led, it unde rgoes a crystal
struc ture (size) change as it trans form s from a less densely packed au stenite (face-cent ered
cub ic or fcc) to a more densely pack ed body -cent ered cub ic (bcc) structure of ferr ite. At faster
coo ling rates, the form ation of ferr ite is suppress ed, an d mart ensite, which is an even less
den sely pack ed body-cent ered tetrago nal (bct) structure than austeni te, is form ed. This resul ts
in a volume tric expan sion at the Ms tempe ratur e, as shown in Figure 10 .3. If these volume
changes cause stresses that are constrained within the strength of material, a residual stress
system is created. If the stresses cannot be contained, then material movement will occur,
which will cause cracking under extreme conditions.
The expansi on is relat ed to the composi tion of steel. Figure 10.4 shows that the crystal
lattice of austenite expands with increasing carbon content [3]. It has been reported that,
typically, when a carbideferrite mixture is converted to martensite, the resulting expansion
due to increasing carbon content is approximately 0.002 in./in. at 0.25% C and 0.007 in./in. at
1.2%C [3]. The fractional increase in size when austenite is converted to martensite is
approximately 0.014 in./in. for eutectoid compositions. This illustrates the effect of carbon
structure and steel transformation on residual stresses and distortion leading to dimensional
changes.
200 400 600 800800 1000 1200
50
40
30
20
10
0
Carbonmanganese steel
Low-alloy steel
Creep-resistantaustenitic steel
0100
0.2%
Pro
of s
tress
, MPa
200
200
300 400
400
Temperature, 8C
Temperature, 8F
500 600
600
700
0.2%
Pro
of s
tress
, tsi
FIGURE 10.2 Variation of yield strength with temperature for three generic classes of steel. (From D.A.Canonico, in ASM Handbook, Vol. 4, ASM International, Materials Park, OH, 1991, pp. 3334.)
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While these physical changes are well known, the situation is more complex when all three
events occur simultaneously. In addition, other events such as heating rate, quenching, and
inconsistent material composition cause further complications that are discussed later in this
chapter.
0
Line
ar e
xpan
sion,
%
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
100 200 300Temperature, 8C
400 500 600 700 800 900
0 200Temperature, 8F
400 600
Austenite
Very slowcooling
Pearlite
Austenite
Ms
Martensiteforms
Rapidquenching
High-temperaturetransformation
800 1000 1200 1400 1600
FIGURE 10.3 Steel expansion and contraction on heating and cooling. (From C.E. Bates, G.E. Totten,and R.L. Brennan, in ASM Handbook, Vol. 4, ASM International, Materials Park, OH, 1991, pp. 67
120.)
a
3.61
3.59
3.57
3.55
3.04
2.96
2.88
00 0.4 0.8
Carbon, wt%
Aust
enite
, M
arte
nsite
,
1.2 1.61.000
1.020
1.040
1.060
1.080
c/a
c/a
c
a
FIGURE 10.4 Carbon content versus lattice parameters of (retained) austenite and martensite at roomtemperature. a at the top of the graph is the lattice parameter of fcc austenite. a and c in the lower half of
the graph are the lattice parameters for tetragonal martensite. The ratio of c/a for martensite as a
function of carbon content is also given. (From S. Mocarski, Ind. Heat., 41(5), 1974, 5870.)
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10.3 RESIDUAL STRESSES
10.3.1 RESIDUAL STRESS IN COMPONENTS
Residual stresses are present in parts after any process that strains the material. Heavy metal
working such as forging, rolling, and extrusion causes stresses that remain in the metal if the
process is performed below the hot-working temperature. If a material is hot-worked,
stresses are continually removed. Processes such as cutting, grinding, and shot peening also
cause residual stress formation but to much shallower depth. While compressive residual
stresses are desirable in a finished component to enable it to resist applied stress systems, the
stresses that exist during manufacture will be relieved during heating with consequent move-
ment in the material as the stress system readjusts.
Residual stresses result not only from heat treatment but also from cold-working through
metalworking, machining processes, and so forth. Within any steel parts there is a balanced
stress system consisting of tensile and compressive residual stresses. If the finished part has the
compressive stresses at the surface, these stresses increase the strength of the part under
normal tensile loading and are thus beneficial. Processes like shot peening are also used to
increase surface compressive stresses to improve performance and compensate for structural
defects. This type of residual stress is intentional and is part of the design. The problem arises
when a metal part has a residual stress system prior to heat treatment. Then an unpredictable
shape change will occur.
10.3.2 RESIDUAL STRESSES PRIOR TO HEAT TREATMENT
Parts for heat treatment should have not only correct dimensions but also a consistent
residual stress pattern. Ideally, the part should be absolutely stress-free so that movement
due to stress-relief can be disregarded, but in practice some final machining passes are
necessary before heat treatment. The best compromise is to completely stress-relieve the
part before the final machining. Several stress-relieving treatments may be necessary during
initial machining to prevent dimensions from going out of control. If a part with a preexisting
stress system is machined and has thus had some of the stresses removed, the system will
constantly rebalance itself by changing its stress pattern.
Any forming or machining processes leave stress systems that will be relieved by a dimen-
sional change during heat treatment. Thus, if the part is heavily stressed prior to heat treatment,
the shape will change due to this factor alone. Processing should be designed so that virtually
stress-free parts are heat-treated. Variations in heat treatment parameters such as case carbon
level and processing temperatures will also cause final shape and size differences.
10.3.3 HEAT TREATMENT AFTER WORK-HARDENING PROCESS
After metalworking, forgings or rolled products are often given an annealing or normalizing
heat treatment to reduce hardness so that the steel may be in the best condition for machining.
These processes also reduce residual stresses in the steel.
Annealing and normalizing are terms used interchangeably, but they do have specific
meaning. Both terms imply heating the steel above the transformation range. The difference
lies in the cooling method. Annealing requires a slow cooling rate, whereas normalized parts
are cooled faster in still, room-temperature air. Annealing can be a lengthy process but
produces relatively consistent results, whereas normalizing is much faster (and therefore
favored from a cost point of view) but can lead to variable results depending on the position
of the part in the batch and the variation of the section thickness in the part that is stress-
relieved.
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Normalizing always involves transforming the steel to the austenitic condition by heating
to about 508C (1008F) above the AC3 temperature as defined in the ironcarbon phase
diagram. Cooling then usually occurs in air, and the actual cooling rate depends on the
mass which is cooled.
This treatment (normalizing) have three main purposes:
. To control hardness for machinability purposes.
. To control structure. Heating to above the austenitizing temperature range will allow
the material to recrystallize on cooling and to form a fine-grain structure having
superior mechanical properties.. To remove residual stresses on heating. However, if cooling is not controlled, a new
stress system may result after cooling.
Stress-relief heat treating involves controlled heating to a temperature below AC1, holding for
the required time, and then cooling at a rate to avoid the introduction of thermal stresses. The
stress relaxation involves microscopic creep and the results will be dependent on both time
and temperature as correlated by the LarsonMiller equation
Thermal effect T( log t 20)=1000
where T is temperature in Rankine degrees and t is time in hours [1].
Resistance of a steel to stress-relief is related to the yield strength at the treatment
temperature. The temperature should be selected at the point where the material yield
strength corresponds to an acceptable level of residual stress remaining in the part. After
treatment, uniform cooling is absolutely necessary. Otherwise the thermal stresses can cause a
new system of residual stresses.
10.4 DISTORTION DURING MANUFACTURING
The causes of distortion of steel parts will be considered during five separate stages of
manufacturing and processing:
. Prior to heat treatment
. During heat-up for heat treatment
. At treatment temperature, i.e., during carburizing, nitriding, etc.
. During quenching and cooling
. During postquenching processing
10.4.1 MANUFACTURING AND DESIGN FACTORS PRIOR TO HEAT TREATMENTTHAT AFFECT DISTORTION
Manufacturing and design factors that will affect distortion prior to heat treatment may be
summarized as:
. Material properties
. Homogeneity of properties across the cross section of the material
. Residual stress system magnitude and distribution
. Part geometry
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10.4.1.1 Material Properties
Material properties affect distortion response in several ways. As discussed above, the
strength properties have important effects on the response to stress-relieving treatments, on
the movement during differential thermal expansion, on the and on the residual stresses
caused during quenching. The composition also is related to hardenability, which determines
the phase changes during quenching. These properties can vary according to actual compos-
ition of the steel used. The composition specification allows a range for each element, which
means, in practice, that each batch of steel is unique and will respond slightly differently.
10.4.1.2 Homogeneity of Material
The first variable that must be considered is the material source, starting with the steel
supplier. Compositional variations across the section of the cast ingot can cause different
responses during heat treatment. Processing of the steel into the form required by the
manufacturing process can cause further variations and may leave high levels of residual
stress, which may be removed partially by normalizing or another stress-relieving process. As
these heat treatments are usually conducted on large batches, they produce variable results
from part to part, which causes different responses in subsequent processing. Steel supplied to
the manufacturers of precision parts is typically either forgings or rolled products, which are
made from ingots or continuously cast products. In rolling and forging, the steel is heated to
the 105012008C (190022008F) range and then worked by hammering, pressing, or rolling to
break down the cast structure and produce a homogeneous cross section in both composition
and structure. However, the effects of earlier processing are never totally eradicated, and they
cause variable responses in hardenability, microstructure after heat treatment, residual stress
levels, and consequently distortion.
10.4.1.3 Distribution of Residual Stress System
If the source of steel supply is consistent and the steel is processed under the same way every
time, these effects cause consistent, predictable residual stress behavior that is acceptable.
However, if the steel is coming from different melt shops, rolling mills, and forgers with
different processing schedules, heat treatment and residual stress responses can vary, often
without apparent explanation. Most steel is hot-rolled, and after rolling, it is allowed to cool
in air on a hot bed. This causes a difference in cooling due to conduction of heat from the
bottom of the bar and convection cooling from the top. If the bar is allowed to cool
completely in this position, the top of the bar will have residual tensile stresses that will
tend to bend the bar and make straightening necessary. Straightening can produce very high
levels of residual stresses, and further stress-relieving treatment must be performed.
10.4.1.4 Part Geometry
Nonuniform heating and quenching can be caused by changes in section thickness in the same
component. When a part is designed, most designers recognize the need to keep section sizes
as uniform as possible to minimize temperature gradients and the tendency to produce high
stresses due to differential expansion and contraction during heating and quenching. If a part
is made with features such as gear teeth, however, it is unavoidable that these areas will have
higher surface-to-volume ratios than the rest of the parts and that gear teeth will often tend to
heat and cool faster than the rest of the section. As a result, the base of the tooth will be
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restrained by the rim , an d this area will tend to go into compres sion during heati ng and into
tensio n during co oling. Simi lar e ffects will take place elsewhere in the pa rt wher ever there is a
change of section .
10.4.2 DISTORTION DURING COMPONENT HEATING
The major effects dur ing he at-up are initiat ed by three disto rtion-causi ng mechani sms acti ng
at the same time:
1. Shape chan ge due to relief of resi dual stress
2. Shape chan ge due to therm al stre sses causing plastic flow
3. Volume change due to phase chan ge on he ating
10.4. 2.1 Shape Chang e Due to Relief of Residua l Stress
As discus sed earli er, the presence of resi dual stre sses from prior ope rations will cause sh ape
changes if the stre sses are relieved by he ating the part to a point wher e the yiel d strength of the
mate rial decreas es below the resid ual stre ss level in the mate rial. The extent of the resulting
plastic de formati on wi ll theref ore de pend on the magnitud e and distribut ion of the stress
fields in the part.
10.4. 2.2 Shape Chang e Due to Therma l Stresses
If a pa rt co uld be heated at the same rate throughou t the sectio n, it would expand
unifor mly at a rate de termined by therm al expan sion coeff icient but maintain the same
shape. In actual practi ce, as the pa rt heats up, the surfa ce will heat first and expand or try
to occupy a great er volume than the c older inter nal material . Expa nsion of the outer layer s
is therefore con strained by the colder, strong er inner layer s of the mate rial. Compressi ve
stresses will be present in the outer layer s on heati ng, ba lanced by tensile stre sses in the
interior of the compon ent. Fur therm ore, shape chan ges will occur if these stre sses resul t in
the plastic deform ation when the yield stre ngth of the heated mate rial decreas es be low the
stress level in the mate rial. Ther efore, shape change depen ds on the geomet ry of the pa rt,
heati ng rate, the coefficie nt of therm al expansi on, mate rial pro perties, and fixturin g of
the part.
10.4.2.3 Volume Change Due to Phase Change on Heating
When a steel is heated from room temperature, thermal expansion occurs continuously up to
Ac1, and then steel contracts as pearlite (or pearliteferrite mixture) transforms to austenite
(i.e., the pea rlite-to-a ustenite pha se ch ange causes approxim ately 4% co ntraction; see Figure
10.3). The extent of decreas e in volume tric contrac tion is relat ed to the carbon content in the
steel composition. Further heating expands the newly formed austenite. The shape and
volume changes as transformation occurs depend on the heating rate, the part geometry,
and the phase volume change.
The major source of control of distortion during heat-up is the heating rate. Differences in
heat-up rate (due, for example, to position in load) will lead to inconsistent distortion. Rapid
heating or nonuniform heating causes severe shape changes. Slow heating and preheating
of parts prior to heating to the austenitizing temperature yield the most satisfactory result.
Unfortunately slow heating is in direct conflict with normal practice, since to increase produc-
tion rate, parts are usually heated as fast as possible to the treatment temperature.
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10.4.3 DISTORTION DURING HIGH-TEMPERATURE PROCESSING
Once the parts are at a constant temperature there are some minor factors that will cause
shape change, but the major changes will occur on further cooling. Carburized parts can be
directly quenched from carburizing temperature or just below the carburizing temperature,
or they can be slowly cooled, given an optional temper, reheated to austenitizing temperature,
and quenched. The latter treatment is used to give optimum case properties. The factors to be
considered during and after high-temperature processing are
. Volume expansion during diffusion treatments
. Distortion due to creep
10.4.3.1 Volume Expansion during Case Diffusion
The major heat treatment used for high-quality parts is a case hardening process designed to
form a hard surface layer on the gear surface. This layer not only gives the part a hard, wear-
resistant finish but also sets up a compressive stress system at the surface that helps to resist
fatigue failures. There is a measurable volume expansion during diffusion treatments depend-
ing on the diffused element (carbon, nitrogen, etc.), the depth of diffusion, the concentration
profile, the furnace temperature, and the atmosphere uniformity. The volume expansion in
the case causes a stretching of the core, which results in tensile stresses that are balanced by
compressive stresses in the case. Distortion due to the expansion will occur when these stresses
exceed the yield stress of the material.
Carburizing involves the diffusion of carbon from a gaseous atmosphere while the part is
heated in an atmosphere or vacuum furnace. Carbon is introduced to a level of 0.701.00% at
the surface. After carburizing, the part is quenched, usually in oil to produce a hard marten-
sitic layer on the surface. Diffusion times are usually in the range of 420 h depending on the
temperature of treatment and the case depth required. The case depth required by the
designer is related to the size of the part and is often greater for the larger part to produce
the correct residual stress pattern.
Nitriding involves the diffusion of nitrogen from a gaseous atmosphere in the temperature
range of 4955658C (92510508F). It may be performed in an atmosphere furnace or in
vacuum ion nitriding equipment. After nitriding, the parts are hard without quenching, and
the increase in volume in the case causes a stretching of the core, which results in tensile
stresses that are balanced by compressive stresses in the case. The magnitude of stresses in the
core and the case is affected by yield strength of the material, thickness of the case, and
amount and properties of nitrides formed.
Nitriding takes everywhere from one day to one week because of the slow diffusion rates.
As nitriding is performed at relatively low temperatures and quenching is unnecessary,
distortion is a minor problem. Another diffusion process sometimes used is carbonitriding,
the simultaneous diffusion of carbon and nitrogen, generally for lower cost parts. Carboni-
triding is a modified form of gas carburizing, rather than a form of nitriding.
10.4.3.2 Distortion Caused by Metal Creep
Distortion due to creep will depend on the geometry of the part, the support during process-
ing, the temperature and time of treatment, and the creep strength of the material. A part
subjected to elevated temperatures for extended times (as in carburizing) could creep under its
own weight unless it is properly fixtured and supported. Long slender parts are best sus-
pended vertically. If this is not practical, the support should have the same contour as the
component rests on it.
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10.4.4 DISTORTION DURING QUENCHING PROCESS
Among the various processes involved in heat treatment, quenching is one of the most
important processes related to distortion, cracking, and residual stress in quenched steel
parts. Although quench cracking can be eliminated, quench distortion cannot be. Instead,
the issue is distortion control, not elimination. Both quenching-related distortion control and
quench cracking will be discussed here.
One form of distortion that may occur upon quenching is defined as shape distortion such
as bending, warpage, and twisting. A second form of distortion is size distortion that includes
dimensional changes observable as elongation, shrinkage, thickening, and thinning. Size
distortion is due to volumetric variation that accompanies each of the transformational
phases formed upon quenching [4].
Distortion during quench hardening is related to following factors:
. Cooling characteristics in quenchingquenchant selection and agitation
. Quenching uniformity
. Parts shape and sizecomponent design
. Surface condition of parts
. Steel grade selection
10.4.4.1 Effect of Cooling Characteristics on Residual Stress and Distortion from
Quenching
Steel quenching requires a wide variation in cooling rates to achieve the required hardness
and strength, which is dependent on the hardenability of the steel and section size of the
workpiece. At the same time, distortion and crack formation must be minimized. However,
these are often contradictory objectives. For example, although increasing cooling rates
increases hardness, they often increase the potential for distortion, stress, and cracking.
Similarly, distortion and stress during quenching are affected by many factors, such as
quenchant, bath temperature, and agitation. The dimensions, shape, and material of the
workpiece also influence the distortion, stress, and cracking.
10.4.4.1.1 Effect of Quenchant SelectionThe selection of quenchant is the most basic factor affecting the cooling characteristics
of workpieces. Therefore, it is the basic factor to be considered for stress and distortion
control during quenching. The selection of a particular quenchant depends on the quench
severity desired. For example, water, brine, or lower concentrations of aqueous polymer
solutions are used for plain carbon steels. Accelerated oils are used for low-alloy steels.
Conventional oils or higher concentrations of polymers are used for high-alloy steels.
Molten salts or liquid metals are often used for martempering (marquenching) and
austempering processes.
Dimensions and shape of the workpiece that is quenched should also be considered in
selecting a quenchant. In general, the thicker the workpiece, the more severe the quenchant.
However, severe quenching often increases stress and distortion of the quenched workpiece.
For steel parts with thick and thin cross sections, the selection of a quenchant is more
difficult. Many such shapes increase the nonuniformity of cooling, and therefore increase
the potential for stress, cracking, and distortion.
Wetting behavior during quenching in a volatile quenchant, such as water, oil, or aqueous
polymer solutions, results in nonuniform (uneven) cooling of the workpieces producing high
surface thermal gradients and often increasing distortion and stress. Many aqueous polymer
2006 by Taylor & Francis Group, LLC.
que nchants will provide more unifor m wettin g propert ies, whi ch will result in substa ntial
reducti ons in crack ing and dist ortion [5].
Figure 10.5 an d Figure 10.6 sho w the distorti on and resid ual stre ss of 30 mm diameter
and 10-mm thick carbon steel disk specimens quenched in various quenchants without
agitation. Different stress distributions and distortions were obtained for each quenchant.
This is a result of the difference of the cooling power of each quenchant, which dominates the
cooling path on the continuous cooling transformation (CCT) curve and therefore the
internal distribution of martensite and ferritepearlite. The wetting process on the surface
of the specimen during water, polymer, and oil quenching also affects the stress and distor-
tion. After vapor blanket cooling, a collapse of the vapor blanket (i.e., wetting) occurs
progressively during water quenching (WQ). This results in nonuniform cooling of a steel
specimen and increases stress and distortion. However, if the vapor blanket collapses simul-
taneously or explosively, as in polymer quenching, the simultaneous collapse provides uni-
form quenching that is effective for reducing stress and distortion. The results shown in
Figure 10.5 and Figure 10.6 illustrate the effectiveness of uniform quenching by using a
polymer quenchant.
15
(a)
(b) 2r = 30 mm diameter0.1
5
2.5
0.05 0
0
2.5
5
Dis
tanc
e fro
m th
e ce
nter
of t
hick
ness
ds,
m
m
0.05 0.1Change of radius (Dr ), mm
t =
10 m
m
ls
ls
ds
Chan
ge o
f thi
ckne
ss(D
t/2), m
m
0
0.025
0.05
0.05
0.025
10(Left side)
Oil (808C, still)
Water (308C, still)(Right side)
10% PAG polymer solution (308C, still)
Radius horizontal (ls), mm5
o
o
0 5 10 15
FIGURE 10.5 Effect of quenchants on quench distortion of JIS S45C carbon steel disk quenched in stillquenchants. Specimen dimensions were 30 mm in diameter by 10 mm thick. (a) Distribution of axial
distortion. (b) Distribution of radial distortion. (From M. Narazaki, M. Kogawara, A. Shirayoria, and
S. Fuchizawa, Proceedings of the Third International Conference on Quenching and Control of Distortion,
2426 March, 1999, Prague, Czech Republic, pp. 112120; M. Narazaki, G.E. Totten, and G.M.
Webster, in Handbook of Residual Stress and Deformation of Steel, G.E. Totten, M.A.H. Howes, and
T. Inoue, Eds., ASM International, Materials Park, OH, 2002, pp. 248295.)
2006 by Taylor & Francis Group, LLC.
Molten salt and liquid metal quenching also provide uniform quenching, decreased cool-
ing rate, and nonuniformity of the temperature of a steel part because of the high quenchant
temperature. These characteristics are effective for the reduction of stress, distortion, and
cracking.
2 23 35 5 5
30
OilWater
151000
800600400200
10 5 0
0
0
200400600800
1000
Res
idua
l stre
ss, s
q (M
P a)
1000800600400200
200400600800
1000
Res
idua
l stre
ss, s
r (M
P a)
(a)
(b)
5 10
Radial stress on end surface
15Position on flat end surface (mm)
15 10 5 0 5 10 15Position on flat end surface (mm)
Circumferential stress on end surface
Aqueous polymer solution
OilWater Aqueous polymer solution
10
5
sq
sr
f
FIGURE 10.6 Effect of quenchants on residual stresses on side surface of JIS S45C carbon steel diskquenched in still quenchants. Specimen dimensions were 30 mm in diameter by 10 mm thick.
(a) Circumferential stress on end surface. (b) Radial stress on end surface. (From M. Narazaki, G.E.
Totten, and G.M. Webster, in Handbook of Residual Stress and Deformation of Steel, G.E. Totten,
M.A.H. Howes, and T. Inoue, Eds., ASM International, Materials Park, OH, 2002, pp. 248295.)
2006 by Taylor & Francis Group, LLC.
10.4. 4.1.2 Effect of Agit ationQuench nonuni formity may aris e from nonuni form flow fields around the part surfa ce during
the quench or nonuniform wetting of the surface . In ad dition, poor ag itation design is a major
source of que nch nonuni formity . The purp ose of the ag itation is not only to increa se cooling
power of que nchant, but also to pro vide unifor m cooling to supp ress excess ive dist ortion and
stre ss of quench ed steel pa rts.
Figure 10.7 and Figure 10.8 show the effe ct of agit ation of que nchants on the profi le of the
flat surface of the steel disk after que nching [5,6 ]. Figure 10.7 shows that nonuniform surfa ce
coo ling in still-w ater quen ching on a c oncave surfac e of the steel disk test specimen an d that
agit ation of the water signific antly decreas es quench dist ortio n in WQ. This occurs because
agit ation reduces the nonuni formity of the surface cooling of the steel disk becau se agitati on
accele rates the propagat ion of the va por blanket colla pse on the surface . How ever, agitati on
of a polyme r que nchant doe s not decreas e quench distorti on (see Figure 10.8) because the
inst antaneous an d explosi ve co llapse of the vapor blanket on the surfa ce occurs wi th or
withou t the agitati on.
Figure 10.9 and Figu re 10.10 show the effect of agit ation methods of a que nchant on
que nch dist ortion of a 20 mm diame ter and 60 mm long 0 .45% carb on steel bars quen ched in
water and a polymer quenchant [6]. Figure 10.9a shows that nonuniform surface cooling in
still-water quenching results in an uneven diameter of the steel bar. The increases of diameter
near the ends of bars were observed, which are attributable to heat extraction from the edges
(a) (b) (c)
100 mm100 mm100 mm
FIGURE 10.7 Effect of agitation of water on quench distortion of JIS S45C carbon steel disk. Specimendimensions were 30 mm in diameter by 10 mm thick. Quenchant was 308C city water. Flow velocity:
(a) still water, (b) 0.3 m/s, and (c) 0.7 m/s. (From Ref. M. Narazaki, M. Kogawara, A. Shirayoria, and
S. Fuchizawa, Proceedings of the Third International Conference on Quenching and Control of Distortion,
2426 March, 1999, Prague, Czech Republic, pp. 112120; M. Narazaki, G.E. Totten, and G.M.
Webster, in Handbook of Residual Stress and Deformation of Steel, G.E. Totten, M.A.H. Howes, and
T. Inoue, Eds., ASM International, Materials Park, OH, 2002, pp. 248295.)
(a) (c)
100mm100mm100mm
(b)FIGURE 10.8 Effect of agitation of polymer quenchant on quench distortion of JIS S45C carbon steeldisk. Specimen dimensions were 30 mm in diameter by 10 mm thick. Quenchant was 308C 10% polymer
(PAG) quenchant. Flow velocity: (a) still water, (b) 0.3 m/s, and (c) 0.7 m/s. (From M. Narazaki,
M. Kogawara, A. Shirayoria, and S. Fuchizawa, Proceedings of the Third International Conference on
Quenching and Control of Distortion, 2426 March, 1999, Prague, Czech Republic, pp. 112120;
M. Narazaki, G.E. Totten, and G.M. Webster, in Handbook of Residual Stress and Deformation of
Steel, G.E. Totten, M.A.H. Howes, and T. Inoue, Eds., ASM International, Materials Park, OH, 2002,
pp. 248295.)
2006 by Taylor & Francis Group, LLC.
of the bar by an edge-ef fect. Upward flow of water decreas es the edge-ef fect, beca use
agitati on reduc es the nonuni form ity of surfac e cooling of the steel bar. How ever, the diame ter
near the bottom end is large r than that near the top end beca use upwar d agitati on pro duces
great er heat loss at the bottom end than at the top end.
Lateral submer ged a nd ope n spray decreas e the diame ter and increa se the lengt h of the
steel bars, because the lateral flow c auses fast c ooling of the side-surf ace and therm al
shrinka ge of the diame ter which also results in elongat ion of the lengt h of the steel ba r
(Figur e 10.9b) . On the other ha nd, agita tion of the pol ymer quench ant hardly affects quen ch
disto rtion (see Fi gure 10.10) because the instan taneous and explosi ve colla pse of the va por
blanket on the su rface of specimen will oc cur with or withou t agit ation.
Figure 10.11 an d Figure 10.12 sho w the effect of agitati on of a que nchan t on the resi dual
stresses on the side surfa ce of 20-mm in diame ter and 60 mm long carbon steel ba rs que nched in
water an d a polyme r quench ant [6]. Figu re 10.11 shows that nonuni form surface cooling in
still-w ater quench ing results in nonuni form resi dual stress dist ribution on the surface of the
steel bar. Agitat ion of water results in ununifor m stress distribut ions except near the bot h
ends. In add ition, sub merge d an d ope n spray coo ling resul t in high compres sion stresses .
Figure 10.12 shows the effect of agitati on on stre ss dist ribution afte r polyme r quen ching.
Agitat ion of the polyme r quenc hant resul ts in uniform stress dist ribution and high compres sion
stresses except near both t he ends. However, still-polymer quenching may r esult i n uni form
and high compression stress because uniform cooling occurs with or w ithout the agitation.
Table 10.1 shows the effe ct of agit ation on the frequency of quen ch cracki ng in water
and polyme r quenching of steel disks with respect to geo metry and dimens ional varia tion is
0
0
Chan
ge o
f dia
met
er, m
m
0.05
0.1
0.2
0.4
0.05 0.2
(a)
(b)0
Still
Open spray(lateral)
Submergedspray (lateral)
Upward
Upward(0.7m/s)(0.3m/s)
0.05 0.10 0.15 0.20 0.25 0.30 0.35
Rat
e of
dia
met
er c
hang
e, %
Change of length, mm
20Distance from lower end, mm
40
Still
Open spray
Upward (0.7m/s)Submerged spray
Upward (0.3m/s)Water, 308C
60
0
FIGURE 10.9 Effect of agitation methods on distortion of JIS S45C steel rod (20-mm diameter by60 mm long). Quenchant was 308C city water. Agitation methods were still, 0.3 m/s upward flow, 0.7m/s
upward flow, and lateral submerge in immersion quenching, and lateral open spray quenching in air. (a)
Change of diameter, (b) change of length. (From M. Narazaki, G.E. Totten, and G.M. Webster, in
Handbook of Residual Stress and Deformation of Steel, G.E. Totten, M.A.H. Howes, and T. Inoue, Eds.,
ASM International, Materials Park, OH, 2002, pp. 248295.)
2006 by Taylor & Francis Group, LLC.
shown in Figure 10.13 [6]. The test specim en (30 mm diame ter by 1 0-mm thick) co ntains an
eccentr ically locat ed 10-mm-di ameter hole. This specimen , was used in the work of Owak u [4]
and was adopted by the Quench Crack ing Working Grou p of the J apan Heat Trea tment
Soc iety. The steel material s that wer e used wer e Japanese standar d S4 5C, SK4, and SCM43 5.
Thes e results show that agit ation of wat er largely suppress es the occurrence of que nch
cracki ng. On the other hand , que nch-cra cking suscept ibilit y to agitati on of the polyme r
que nchant is not clear be cause there is no crack on polyme r-quenched specim en with or
withou t agit ation.
10.4. 4.1.3 Workpiece Size Effe ctsThe c ooling rate of a quen ched workpie ce cau sed by a shows a n invers e relationshi p with
increa sing thickne ss caused by a mass effect. In addition , cooling rates of the core are lim ited
by therma l diffusion in the workpi ece. Therefor e, stre ss and distorti on dur ing quen ching are
affe cted by the dimens ions, shape, and mate rial of the workpi ece that is quen ched.
Figure 1 0.14 [7] shows the axial stress dev elopment during WQ of AISI 1045 solid steel
cyli nders. The 10-m m (0.4 in.) diameter cyli nder starts to trans form to mart ensite at the
surfa ce and the trans form ation front mo ves gradu ally inward, resulting in a typical tensile
stre ss at the surfa ce. Larg e diame ter cyli nders first trans form to ferrite pearl ite at inter medi-
ate radii and then to martensite at the surface. This cau ses tw o stress mini ma as seen in the
dashed cu rves in Figure 10.14a through Figure 11.14c . The final resi dual stre ss is co mpressive
at the surfa ce and tensi le in the co re. The relat ionship of stre ss to specimen diame ter and
que nching medium is su mmarized in Figure 10.15 [7,8 ]. The difference betw een oil an d water
quenching decreases with increasing diameter.
0(a)
(b)
0
Chan
ge o
f dia
met
er, m
m
0.05
0.1
0.05 0.2
(0.3 m/s)
20
StillUpward (0.3 m/s)Upward (0.7 m/s)
10% polymer quenchant, 30 8C
40
Open spray (lateral)
Distance from lower end, mm60
Rat
e of
dia
met
er c
hang
e, %
0
0 0.05 0.10 0.15 0.20Change of length, mm
0.25 0.30
Still
0.2
Submergedspray (lateral)
Upward
Upward
(0.7 m/s)
0.4
FIGURE 10.10 Effect of agitation methods on distortion in polymer quenching of JIS S45C steel rod(20-mm diameter by 60 mm long). Quenchant was 308C 10% polymer (PAG) quenchant. Agitation
methods were still, 0.3 m/s upward flow, and 0.7 m/s upward flow in immersion quenching. (a) Change
of diameter, (b) change of length. (From M. Narazaki, G.E. Totten, and G.M. Webster, in Handbook of
Residual Stress and Deformation of Steel, G.E. Totten, M.A.H. Howes, and T. Inoue, Eds., ASM
International, Materials Park, OH, 2002, pp. 248295.)
2006 by Taylor & Francis Group, LLC.
60
20
600
400
200
0
0(a)
(b)
10
Still waterSpray (open) lateralSpray (submerged) lateral
Spray (open) lateralSpray (submerged) lateral
0.3m/s upward0.7m/s upward
0.3m/s upward0.7m/s upward
20 30Distance from lower end, mm
40 50 60
0 10
Still water
20 30
Distance from lower end, mm40 50 60
Res
idua
l stre
ss, M
P a
200
400
600
800
1000
600
400
200
0
Res
idua
l stre
ss, M
P a
200
400
600
800
1000
A
B
sz
sq
101064
FIGURE 10.11 Effect of agitation methods on residual stress after water quenching of JIS S45C steelrod (20-mm diameter by 60 mm long). Quenchant was 308C city water. Agitation methods were still,
0.3 m/s upward flow, 0.7 m/s upward flow, and lateral submerge in immersion quenching, and lateral
open spray quenching in air. (a) Axial stress on surface, (b) tangential stress on surface. (From
M. Narazaki, G.E. Totten, and G.M. Webster, in Handbook of Residual Stress and Deformation of
Steel, G.E. Totten, M.A.H. Howes, and T. Inoue, Eds., ASM International, Materials Park, OH, 2002,
pp. 248295.)
2006 by Taylor & Francis Group, LLC.
10.4.4.2 Effect of Surface Condition of Components
10.4.4.2.1 Effect of Surface RoughnessSurface texture and roughness are very important factors for quench cracking because the
microscopic geometry and roughness of the surface affect the tendency for cracking. Narazaki
et al. have shown an example of such a case [6,9]. The results were as follows:
. Surface roughness increases the tendency for quench cracking of steel if surface rough-
ness (maximum height of irregularities Ry, or ten points height of irregularities Rz) is
larger than approximately 1 mm.. Surface texture made by lapping tends to cause a higher occurrence of quench cracking
than by grinding or emery-polishing when the surface roughness is approximately the
same.
This phenomenon is caused mainly by the stress concentration at the surface of the steel
workpieces. The geometric shapes on the surface such as polishing marks, lapping marks,
grinding marks, cutting tool marks, and micronotches act as stress riser, providing a trigger
for inducing quench cracking.
10.4.4.2.2 Effect of Oxide or Coating LayerThe presence of a thin layer such as oxide scale and or a coating may cause a cooling
acceleration effect by suppression of vapor blanket formation or by acceleration of the
60
20
600400200
0
0(a)
(b)
10
Still polymer,10% Spray (open),10%0.7m/s upward,10%0.3 m/s upward,10%
Still water,10% Spray (open),10%0.3 m/s upward,10%
20 30Distance from lower end, mm
40 50 60
0 10 20 30Distance from lower end, mm
40 50 60
Res
idua
l stre
ss, M
P a200400600800
1000
600400200
0
Res
idua
l stre
ss, M
P a
200400600800
1000
A
B
szsq
101064
0.7m/s upward,10%
FIGURE 10.12 Effect of agitationmethodson residual inpolymerquenchingof JISS45Csteel rod (20-mmdiameter by 60 mm long). Quenchant was 308C 10% polymer (PAG) quenchant. Agitation methods were
still, 0.3 m/s upward flow, and 0.7 m/s upward flow in immersion quenching. (a) Axial stress on surface,
(b) tangential stress on surface. (From M. Narazaki, G.E. Totten, and G.M. Webster, in Handbook of
Residual Stress and Deformation of Steel, G.E. Totten, M.A.H. Howes, and T. Inoue, Eds., ASM
International, Materials Park, OH, 2002, pp. 248295.)
2006 by Taylor & Francis Group, LLC.
collapse [6]. In addition, uniform cooling is caused by the existence of such a thin layer.
Therefore, the existence of an oxide scale or clay coating largely suppresses the occurrence of
quench cracking. However, a heavy oxide scale of the steel workpiece often causes unstable
cooling and decarburization [6].
10.4.4.3 Minimizing Quench Distortion
10.4.4.3.1 Component DesignOne of the causes of unacceptable distortion and cracking of steel parts is component design.
Poor component design promotes distortion and cracking by accentuating nonuniform and
nonsymmetrical heat transfer during quenching. The basic principle of successful design is to
TABLE 10.1Effect of Agitation on Quench Cracking in Water and Polymer Quenching of Steel Disks
Shown in Figure 10.13. Steel Materials Are Japanese Standard S45C, SK45 and SCM435
Quenchants and their Agitation
Frequency of Occurrence of Quench Cracking
S45C
0.45%C0.67%Mn
SK4
0.98%C0.77%Mn
SCM435
0.35%C0.76%Mn
1.06%Cr0.20%Mo
City water (308C)
Still (non-agitated) 100% 100% (flat surface) 100%
0.3m/s upward 70% 30% (flat surface) 100%
70% (hole surface)
0.7m/s upward 0% 0% (flat surface) 60%
100% (hole surface)
5m/s open spray 0% 10% (flat surface) 0%
90% (hole surface)
10% polymer quenchant (308C, PAG)
Still (non-agitated) 0% 0% 0%
0.3m/s upward 0%
0.7m/s upward 0% 0% 0%
Source: From M. Narazaki, G.E. Totten, and G.M. Webster, in Handbook of Residual Stress and Deformation of
Steel, G.E. Totten, M.A.H. Howes, and T. Inoue, Eds., ASM International, Materials Park, OH, 2002, pp. 248295.
2
7f10
10
f30
FIGURE 10.13 Disk specimen for quench-cracking test. Specimen dimensions were 30-mm diameter by10 mm thick; specimen contains an eccentrically located 10-mm-diameter hole. (From M. Narazaki,
G.E. Totten, and G.M. Webster, in Handbook of Residual Stress and Deformation of Steel, G.E. Totten,
M.A.H. Howes, and T. Inoue, Eds., ASM International, Materials Park, OH, 2002, pp. 248295.)
2006 by Taylor & Francis Group, LLC.
select shapes that will minimize the temperature gradient through the part during quenching.
Component designs that minimize distortion and cracking are as follows:
. Design symmetry: It is important to provide greater symmetry. One of techniques for
design symmetry is to add dummy holes, key grooves, or other shapes to steel parts.. Balance of cross-sectional area: The difference between large cross-sectional area and
thin one should be decreased by using several techniques as follows:. Avoiding abrupt cross-sectional size changes by using large radii. Adding dummy holes to large cross-sectional areas. Changing from blind holes to through-type holes. Changing from thick solid shapes to thin hollow shapes. Dividing a complicated shape to sectional components
. Avoiding sharp corners and edges: Distortion and cracking encountered when quenching
a part with sharp corners and edges that increase cooling nonuniformity and act as stress
risers. Therefore, it is effective to round corners and edges or to employ a tapered shape.
10.4.4.3.2 Steel Grade SelectionAlthough quench distortion and cracking are most often due to nonuniform cooling, material
selection canbe an important factor. Someattention shouldbepaid to select amaterial as follows:
. The compositional tolerances should be checked to assure that the alloy is within the
specification.. It is often better to choose a low-carbon content, because the high-carbon content often
causes the higher susceptibility for distortion and cracking.. If possible, it is better to choose a combination of a high-alloy steel and a very slow
cooling. As a matter of course, the selection of high-alloy steels markedly rises the
material cost.
600
240
65
80
3626 15
2
2 130
60
85
05
114.4
10.418
3
40
97
81 3
400
200
0
0
D = 100 mm D = 50 mm D = 30 mm D = 10 mm
0 0 0 50
Axia
l res
idua
l stre
ss, k
si
5050
Position in cylinder from center (0%) to surface (100%)
50(a) (b) (c) (d)
100 100 100 100
Axia
l res
idua
l stre
ss, M
P a
200
400
115
85
60
30
600
800
FIGURE 10.14 Axial stress distribution during water quenching for various AISI 1045 steel cylinderswith diameter D, at selected times (in seconds) after the start of quenching from 8508C (15608F) in 208C
(70 8F) water. The final microstructure of the 10-mm (0.4 in.) diameter cylinder is completely martensite,
while the others have a ferriticpearlitic core. (From T. Ericsson, ASM Handbook, Vol. 4, Heat Treating,
ASM International, Materials Park, OH, 1991, p. 16; H.J. Yu, U. Wolfstieg, and E. Macherauch, Arch.
Eisenhuttenwes., 51, 1980, 195.)
2006 by Taylor & Francis Group, LLC.
Crack ing propen sity increa ses as the Ms tempe rature a nd the carb on equ ivalent (C E)
increa se. Quench cracks were preval ent at carbon equ ivalent values abo ve 0.525, as illu strated
in Figure 10 .16 [2].
10.4. 4.3.3 Selec tion of Quenchant a nd AgitationQuenchan ts must be selected to provide cooling rates cap able of prod ucing an accepta ble
micro structure in the section thickne ss of inter est. However, it is not desirable to use
quen chants with exce ssively high he at-remova l rates . Typical ly, the greater the quenc h
severity, the great er the propen sity for increa sed disto rtion or crackin g. Alt hough a reductio n
of que nch severi ty leads to reduced dist ortion, it may also be accompani ed by undesir able
micro structures . Ther efore, it is difficul t to selec t an opt imal quench ant an d agitati on. Cool-
ing power (quen ch severity) of quen chant should be as low as possibl e while maintaini ng a
suffici ently high cooling rate to en sure the requir ed microstr ucture, hardn ess, and strength in
critical secti ons of the steel parts .
Quench severi ty is define d as the ab ility of a quenc hing medium to extra ct heat from a hot
steel workpi eces express ed in terms of the Gross mann num ber (H ) [10]. A typical range of
Grossmann H-values (numbers) for commonly used quench media are provided in Table 10.2,
and Figure 10.17 provides a correlati on between the H -valu e an d the ab ility to harden steel , as
indica ted by the Jominy dist ance ( J-dis tance) [11]. Altho ugh Tabl e 10.2 is useful to obta in a
relative measur e of the quench severity offered by different quen ch media , it is diff icult to
apply in practice, becau se the actual flow rates for moderat e, good , strong , a nd viole nt
agitati on are unknown.
Alternativel y, the measur ement of a ctual cooling rates or heat fluxe s pro vided by a
specific quen ching med ium does provide a quantita tive meani ng to the quench severi ty
provided. Some illustra tive values are provided in Table 10.3 [12] .
Cylinder diameter, mm10 30 50
0.4 1.2Water quench, core
Oil quench, core
Oil quench, surface
Water quench, surface
2.0Cylinder diameter, in.
4.0
0
30
60
85
115
100
600
800
400
400
200
0
Axia
l res
idua
l stre
ss, M
P a
Axia
l res
idua
l stre
ss, k
si
200
600
800
145
115
85
60
30
1000
FIGURE 10.15 Dependence of axial residual stresses on cylinder diameter. Same steel as in Figure10.14. The core is martensite for 10-mm (0.4 in.) diameter, but is ferritepearlite for larger
diameters. (From T. Ericsson, ASM Handbook, Vol. 4, Heat Treating, ASM International, Mater-
ials Park, OH, 1991, p. 16; H.J. Yu, U. Wolfstieg, and E. Macherauch, Arch. Eisenhuttenwes., 51,
1980, 195.)
2006 by Taylor & Francis Group, LLC.
Typically, the greater the quench severity, the greater the propensity of a given quenching
medium to cause distortion or cracking. This usually is the result of increased thermal stress,
not transformational stresses. Specific recommendations for quench media selection for use
with various steel alloys is provided by standards such as Aerospace Material Specification
(AMS) 2759.
300
17.3(%Ni)17.7(%Cr)25.8(%Mo)], 8CMs[521353(%C)22.0(%Si)24.3(%Mn)
0
Frac
tion
of q
uenc
h cr
ackin
g, %
Frac
tion
of q
uenc
h cr
ackin
g, %
20
40
60
80
00.4 0.5
CE C +
Ni, Cr, and Mo steels
Not fully martensiticCMn steels
0.6
+ + + ,
0.7
%
0.8
20
40
60
80
100
100
340 380 420
Mn Mo Cr Ni5 5 10 50( (
FIGURE 10.16 Effect of MS temperature and carbon equivalent on the quench cracking of selectedsteel. (From C.E. Bates, G.E. Totten, and R.L. Brennan, in ASM Handbook, Vol. 4, ASM International,
Materials Park, OH, 1991, pp. 67120.)
TABLE 10.2Grossmann H-Values for Typical Quenching Conditions
Quenching Medium Grossmann H-Value
Poor (slow) oil quenchno agitation 0.20
Good oil quenchmoderate agitation 0.35
Very good oil quenchgood agitation 0.50
Strong oil quenchviolent agitation 0.70
Poor water quenchno agitation 1.00
Very good water quenchstrong agitation 1.50
Brine quenchno agitation 2.00
Brine quench violent agitation 5.00
Ideal quench
It is possible with high-pressure impingement to achieve H-values greater than 5.00.
Source: From R. Kern, Heat Treat., 1985, 4145.
2006 by Taylor & Francis Group, LLC.
10.4.4.4 Quench Uniformity
Quench nonuniformity is perhaps the greatest contributor to quench distortion and cracking.
Nonuniform cooling can arise from nonuniform flow fields around the part surface during
quenching or nonuniform wetting of the surface [4,11,13,14]. Both lead to nonuniform heat
transfer during quenching. Nonuniform quenching creates large thermal gradients between
the core and the surface of a steel part, or among the surfaces of the parts.
Poor agitation design is a major source of quench nonuniformity. The purpose of the
agitation system is not only to take hot fluid away from the surface to the heat exchanger but
also to provide uniform heat removal over the entire cooling surface of all of the parts
throughout the load that is being quenched.
In the batch quench system where vertical quenchant flow occurs throughout a load, the
bottom surfaces of the parts experience greater agitation than the top surfaces. Agitation
produces greater heat loss at the bottom, creating a large thermal gradient between the top
and the bottom surfaces.
TABLE 10.3Comparison of Typical Heat Transfer Rates for Various Quenching Media
Quench Medium Heat Transfer Rate, W/m2 K
Still air 5080
Nitrogen (1 bar) 100150
Salt bath or fluidized bed 350500
Nitrogen (10 bar) 400500
Helium (10 bar) 550600
Helium (20 bar) 9001000
Still oil 10001500
Hydrogen (20 bar) 12501350
Circulated oil 18002200
Hydrogen (40 bar) 21002300
Circulated water 30003500
Source: From P.F. Stratton, N. Saxena, and R. Jain, Heat Treat. Met., 24(3), 1997, 6063.
Quench severity in terms of H-valueX
5.02.01.51.0
0.700.500.350.20
6 9 12 15 18 21 24J-distance (1/16 in.)
H-va
lue
FIGURE 10.17 Quench severity in terms of Grossman (H ) values. Jominy distance (J-distance). (FromR. Kern, Heat Treat., 1985, 4145.)
2006 by Taylor & Francis Group, LLC.
If a submerged spray manifold is used to facilitate more uniform heat removal, the
following design guidelines are recommended:
. The total surface of the part should experience uniform quenchant impingement.
. The sufficiently large holes and proper spacing between holes should be used.
. The manifold face should be at least 13 mm (0.5 in.) from the surface of the part that is
quenched.. The repeated removal of hot quenchant and vapor should be possible.
Excessive distortion was also obtained with an agitation system illustrated in Figure
10.18 when the quenchant flow was either in the same direction relative to the direction
of part immersion or in the opposite direction [14]. The solution to this problem was to
minimize the quenchant flow to that required for adequate heat transfer during the
quench and to provide agitation by mechanically moving the part up and down in the
quenchant. Identifying sources of nonuniform fluid flow during quenching continues to
be an important design goal for optimizing distortion control and minimizing quench
cracking.
Nonuniform thermal gradients during quenching are also related to interfacial wetting
kinematics which is of particular interest with vaporizable liquid quenchants including: water,
oil, and aqueous polymer solutions [15]. Most liquid vaporizable quenchants exhibit boiling
temperatures between 100 and 3008C (210 and 5708F) at atmospheric pressure. When parts
are quenched in these fluids, surface wetting is usually time-dependent which influences the
cooling process and the achievable hardness.
Another major source of nonuniform quenching is foaming and contamination. Contam-
inants include sludge, carbon, and other insoluble materials. It includes water in oil, oil in
water, and aqueous polymer quenchants. Foaming and contamination lead to soft spotting,
increased distortion, and cracking.
10.4.4.5 Quenching Methods
Part design, material and quenchant selection, agitation, etc. are the most important factors
to suppress quench distortion and cracking of steel parts. In addition, several methods for
minimizing distortion and eliminating cracking are employed; for example, interrupted
quenching, time quenching, marquenching, austempering, press quenching, and plug quench-
ing. These quenching methods are based on the improvement of cooling uniformity by
controlling of cooling, or restraint of distortion by using restraint fixtures. For the detailed
description of these methods, the reader is referred to Ref. [2,6].
Quenchantflow
Quenchantflow
FIGURE 10.18 Effect of quenchant flow direction on distortion. (From R.T. Von Bergen, inQuenching and Distortion Control, G.E. Totten, Ed., ASM International, Materials Park, OH, 1992,
pp. 275282.)
2006 by Taylor & Francis Group, LLC.
10.5 DISTORTION DURING POST QUENCH PROCESSING
There are many possibl e treatmen ts that can be carri ed out afte r quen ching. The typic al
operati ons are:
. Straigh tening
. Tempe ring
. Stabilizat ion with tempering and subzero treat ment
. Metal remova l by grindin g, etc.
10.5.1 STRAIGHTENING
If it is necessa ry to reduce the disto rtion of que nch-hardene d parts , straight ening is done by
flexing or selec tive peening the pa rts. Flexing or pe ening resul ts in the change of the stress
distribut ion and poses a risk of cracki ng. Therefor e, it is the usu al pra ctice to stra ighten after
tempe ring. Straighteni ng whi le parts are still hot from the tempe ring furnace is often per-
form ed to avoid cracki ng.
10.5.2 TEMPER ING
Steel parts are often tempe red by reheat ing after quench-harde ning to obtain sp ecific values
of mechani cal prop erties. Temperi ng of steel increa ses ductilit y and toughness of que nch-
hardene d steel, relieves que nch stresses , and en sures dimens ional stabi lity. The tempe ring
process is divide d into four stage s:
1. Temperi ng of mart ensite struc ture
2. Transfor mation of retained au stenite to mart ensite
3. Temperi ng of the decomp osition produc ts of mart ensite
4. Decompos ition of retained au stenite to marte nsite
Micros tructural varia tion during tempe ring results in vo lume chan ges dur ing the tempe ring
of hardened steel [16]. In addition to dimensional change by microstructural variation,
tempering may also lead to dimensional variation due to relaxation of residual stresses and
plastic distortion which is due to the temperature dependence of yield strength.
Figure 10 .19 shows the dist ortion of round steel bars (200 mm diame ter and 500 mm
in length) by quenching and by stress relieving during tempering [17]. A medium-carbon steel
bar (upper diagrams) and a hardenable steel bar (lower diagrams) were used in this experi-
ment. Figure 10.19a and d shows the results of quenching from 6508C without phase
transformation. The distortion in each case is almost the same regardless of the different
quenchants and the different steel chemical composition. These convex distortions are caused
by nonuniform thermal contraction and resultant thermal stress during cooling. Figure 10.19b
and Figure 10.19e shows the results of quenching from 8508C with phase transformation. The
distortion in Figure 10.19e (hardenable steel) shows a convex configuration, but the distortion
in Figure 10.19b (medium-carbon steel of poor hardenability) shows a configuration that
combines convex and concave distortions. In addition, WQ has a greater effect on distortion
than oil quenching (OQ). Figure 10.19c and Figure 10.19f shows the configurations after
tempering. These results show that tempering after quenching results in not only volumetric
changes but also convex distortions. Such distortions seem to be related to relieving of
residual stresses by tempering.
Figure 10.20 and Figure 10.21 [18] show the examples of st ress-relief by temper ing.
A solid cylinder with 40 mm diameter and 100 mm length was examined for analyses
2006 by Taylor & Francis Group, LLC.
and e xper iments of tempering performed after W Q. Calculated residual s tress distributions
after W Q are illustrated in Figure 10.20. Open and s olid circles i n the figure correspo nd to
measured stresses on the s urface of the c yli nder by x -ray diffraction. Residual stress
distributions afte r t em pering at 4008C is s hown in Figure 10.21a and Figure 10.21b for
typical elapsed times of 2 and 50 h with measured values on the s urface. These r es ults show
that the s tresses i n all directions decrease with elapsed t empering time.
10.5.3 S TABILIZATION WITH T EMPERING AND S UBZERO T REATMENT
To achieve dimens ional stabi lity ov er long periods , the amo unt of retain ed austeni te in
que nched parts sho uld be reduced. Dim ensional stability is a vital requir ement for gauges
and test block s.
Stabilizat ion can be obtaine d by multiple tempering and subzero treatment (cold treat-
ment) . It is the usu al practice to con duct a single or repeat ed subzero treatment afte r the
initial tempe ring. Subz ero treatment is normal ly accompl ished in a refr igerator at tempera-
ture of 60 to 90 8C ( 75 to 130 8F). Subz ero treatment may cau se a size ch ange by furtherausteni te-to-m artensit e trans formati on resul ting in further expan sion. If the size change is
rest rained, then ad ditional stre sses will be locked in. This effe ct de pends on the Ms M f
WQ0.100.05
00.050.10
0.10
0.100 100 200 300 400 500
Bottomend
Topend
0.050
0.05
WQ
lC = 0.170 mm
lC = 0.209 mm
lC = 0.291 mm
lC = 0.148 mm
(a)Ch
ange
of d
iam
eter
, mm
(d)
0.20.1
00.10.2
1.00.5
00.51.0
0 100 200 300 400 500Bottomend
Topend
QQlC = 0.81 mm
WQlC = 2.45 mm
lC = 0.022 mm
WQlC = 0.174 mm
(e)
(b)
0.050
0.050.100.20
0.05
0.050.100.150.20
0
0 100 200 300 400 500Bottomend
Topend
QQlC = 0.12 mm
WQlC = 2.371 mm
WQ
lC = 0.427 mm
lC = 0.240 mm
(f)
(c)
Position along length, mm
FIGURE 10.19 Deformation of medium-carbon and hardenable steel bars by quenching from belowand above transformation temperature and by stress-relieving. lC, change of length. (a) and (d) quenched
from 6508C. (b) and (e) quenched from 8508C. (c) and (f) tempered at 6808C. (a) to (c) JIS S38C steel
(0.38%C). (d) to (f) JIS SNCM 439 steel (0.39C1.80Ni0.80Cr0.20Mo). (From Y. Toshioka, Mater.
Sci. Technol., 1, 1985, 883892.)
sZsesr
150
100
50
0
50
100
1500 5 10
Radius (r ), mm
Stre
ss (s
), kg/m
m2
15 20
FIGURE 10.20 Stress distribution in steel cylinder after quenching. (From T. Inoue, K. Haraguchi, andS. Kimura, Trans. ISIJ, 18, 1978, 1115.)
2006 by Taylor & Francis Group, LLC.
tempe rature range, the tempe ratur e and time of sub zero treatment , and the creep strength of
the mate rial. Tools must be retem pered immed iately after returning to room tempe ratur e
followi ng subze ro treatment to reduce inter nal stress an d increa se toughn ess of the fres h
marten site [19].
10.5.4 METAL R EMOVAL AFTER HEAT TREATMENT
A finishing proc ess such as grindi ng is often required to correct dimens ional changes caused by
heat treatment . The tendency is to try to use parts as he at-treated wi thout touch ing the surfa ce
again bec ause in this cond ition the part may exhibi t a much great er fatigue stre ngth [20] . This
is particu larly true for parts loaded under concentra ted co ntact such as bearing s or gears.
For parts with close tolera nces, howeve r, the comp onent size must be brought unde r
control in the fini sh grinding . This leads to a dilemma: if excess material is left on the part
prior to heat treatment, there will be enou gh stock to ena ble the size to be brough t unde r
control . Ho wever, if too much is taken off, the most effective regions of the carburized (or
nitrided ) case are remove d.
Figure 10.22 shows a g ear with excess ive material remove d from a tooth afte r case
hardening treatmen t [21]. In the exampl e shown in Figure 10.22, if the tooth has dist orted
to the right, more mate rial has to be groun d from the right side of the tooth. Thi s has severa l
serio us con sequences. First , the lack of uniformit y in case dep th leads to uneven resi dual
stress dist ribution. Seco nd, its mechanical strength will be less than optimum performance.
Third, a considerable thickness of material has to be removed during grinding, increasing the
probability of grinding burns and cracking.
10.6 MEASUREMENT OF RESIDUAL STRESSES
In the previous discussion, it was shown that propensity for distortion and cracking is dependent
on thermally-inducedand transformation-inducedstresses.X-raydiffractionmethodsareusually
used for these stresses. There are, however, applicable measurement methods. Detailed descrip-
tion on various measurement methods of residual stress are provided in Refs. [2224].
0
(a)
Stre
ss (s
), kg/m
m2
Stre
ss (s
), kg/m
m2
(b)
0
50
0
50
100
150
100
150T = 4008Ct = 2h
T = 4008Ct =
150 150
100 100
50 50
5 10Radius (r ), mm Radius (r ), mm
15 20 0 5 10 15 20
szsesr
szsesr
FIGURE 10.21 Stress distribution in steel cylinder during tempering. (From T. Inoue, K. Haraguchi,and S. Kimura, Trans. ISIJ, 18, 1978, 1115.)
2006 by Taylor & Francis Group, LLC.
10.6.1 X-R AY DIFFRACTION METHOD
X-ray diffract ion is the most co mmon method for measur ement of stre sses [25] . This proced-
ure involv es irrad iating a sampl e with x-rays. When steel is irra diated with x-ray, a charac-
teristi c diffract ion pa ttern that is de pendent on the c rystal struc ture of the iron and alloy ing
elem ents is present . The spacing betw een the lattice points , or d-spaci ng, can be calcul ated
from the diffract ion patte rn.
The inter planar d-spacing for any set of parall el planes is calcul ated from the x-ray
diffract ion data using Bra ggs equati on:
nl 2d sin u;
wher e n is an integ er, l is the wave lengt h of the x-ray beam, d is the spacing between reflec ting
planes, an d u is the angle of incide nce of the beam with sampl e. This relationshi p is ill ustrated
in Figure 10.23.
When a load is ap plied to the sample, there wi ll be a perturb ation in the d-spacing. Thus,
chan ges in the measur ed diffract ion patterns ( Dd) are related to the lattice strain ( Dd/d). The
stra in ( Dd/d ) is calculated from the diffract ion data:
Dd
d cot
D2u
2
;
Ther e are a numbe r of experimenta l proced ures for c alculati ng stress from the diffract ion
data. The most common method is the sin2 mehtod, in which the sample is irradiated and
Case
Toothdistortion
Size beforeheat treatment
Size aftergrinding
FIGURE 10.22 Schematic of material ground from a distorted gear tooth after case hardening treat-ment. (From M.A.H. Howes, in Quenching and Control Distortion, G.E. Totten, Ed., ASM Inter-
national, Materials Park, OH, 1992, pp. 251258.)
2006 by Taylor & Francis Group, LLC.
changes in the diffraction angle are related to the interplanar spacing d and to strain Dd/d.
The change in interplanar spacing is determined by measuring d with different applied
stresses. The stress s is calculated from
s d d0
d0
E
1 n
1
sin2 c
;
where n is Poissons ratio. The d-spacing is determined from the Bragg equation. If a Dd/d
versus sin2 plot is constructed, the stress can be calculated from the slope of the straight line
as follows:
Slope s(1 n)
E;
which can be rearranged to solve for s:
s slopeE
(1 n);
Because Poissons ratio n is known and the (or preferably measured) modulus E is also
known, the stress s can be readily calculated.
Possible sources of x-ray measurement errors include [26]:
. Error in peak position
. Stress-relief by aging
. Sample anisotropy
. Grain size
. Round surfaces (flat surfaces are preferable)
10.6.2 HOLE-DRILLING METHODS
Residual stress may be measured by a method in which a hole is drilled into the material
tested and the change in strain is measured, usually with strain gauges. Residual stress is then
calculated from the magnitude and direction of this strain, hole size, and material properties.
There are many hole-drilling methods. However, one of the most commonly used methods is
the classical Sachs bore-out method [27]. Changes in residual stress with depth can be
determined by incrementally drilling the hole and measuring the changes in stress with depth.
q
q
q
q
P
l
FIGURE 10.23 Illustration of the Bragg relation.
2006 by Taylor & Francis Group, LLC.
The Sachs bore-out method involves following assumptions [28]:
. The metal is effectively isotropic and has a constant Youngs modulus and Poissons
ratio.. The residual stresses are distributed with rotational symmetry about the axis of the bar.. The tube formed by boring the bar is circular in cross section, and its inner and outer
walls are concentric.. The specimen is sufficiently long (or thick) to prevent bending.
The Sachs bore-out procedure, while one of the best-known methods for the determination of
residual stresses, has a number of disadvantages [28,29]:
. It is slow and relatively expensive.
. Care must be taken to ensure that plastic deformation does not occur during hole-
drilling process.. Strain gauge corrections for drift measurement must be made.. It can only measure final stress thus cannot be readily applied to stress during cooling.. It results in damage to test specimen.
10.6.3 BENDING AND DEFLECTION METHODS
Bending and deflection methods involve the measurement of a change in the diameter of a slit
tube or the curvature of a flat plate [30]. The use of such methods requires knowledge of the
interrelationship of stress and the amount of deflection observed. Although these methods are
not applicable to the determination of radial stresses, with appropriate procedural adaptation
they may be low-cost options for the determination of a systematic distribution of residual
stress and uniform biaxial stresses in bars, tubes, sheets, and plates [28].
For best results, to properly account for material variation, the elastic modulus should be
determined experimentally instead of using reference book values. The modulus is determined
by attaching a strain gauge to the test specimen and then measuring the corresponding
deflection with the application of different loads.
10.6.4 OTHER RESIDUAL STRESS MEASUREMENT METHODS
There are a number of other less commonly used but valuable experimental methods that
have been used for residual stress measurement. These include magnetic method [31,32],
ultrasonic method [3335], and neutron diffraction method [36].
The magnetic method is based on the stress dependence of the Barkhausen noise amplitude.
As Barkhausen noise depends on composition, texture, and work hardening, it is necessary to
do calibration for each application. In addition, the use of this method is limited to only
ferromagnetic materials. Ultrasonic method has the potential for greater capability and use-
fulness in the future, but has the disadvantage that it requires transducers shaped to match the
inspected surface. Neutron diffraction method has a much deeper penetration than x-ray, but
has the disadvantages of safety and cost of the apparatus.
10.7 TESTS FOR PROPENSITY FOR DISTORTION AND CRACKING
Numerous tests have been applied to evaluate the potential of a steel to undergo distortion or
cracking upon heat treatment. In most cases, the test specimens are manufactured specifically
for these test procedures. One of the most difficult challenges is to devise a testing procedure
2006 by Taylor & Francis Group, LLC.
that accounts for the statistical nature of the occurrence of cracking, because it is seldom that
100% of all parts actually undergo cracking in the heat treatment process.
10.7.1 NAVY C-RING AND SLOTTED DISK TEST
One of the oldest standard tests for evaluating quench distortion is the so-called Navy C-ring
test (see Figure 10.24) [37]. A modified Navy C-ring test specimen (see Figure 10.25) has also
been reported [38]. This notched test specimen has greater crack sensitivity to evaluate
propensity for cracking in addition to distortion.
10.7.2 CYLINDRICAL SPECIMENS
Many workers have simply quenched cylindrical specimens and observed them for cracking
and volumetric changes. For example, Moreaux [39] used round bar test specimens of 0.60%
C, 1.6% Si, and 0.5% Cr steel whose length was three times their diameter. These studies
showed that the transition temperature from film to nucleate boiling contributes primarily to
thermal stress. The nucleate boiling to convective cooling transition will primarily affect the
0.5 in. 1.0 in.
E
C
1.9
in.
D
B
A
5.0 in.
2.9 in.
FIGURE 10.24 Example of C-ring test specimen used for quench distortion studies. (From H. Websterand W.J. Laird, ASM Handbook, Vol. 4, ASM International, Materials Park, OH, 1991, p. 144.)
0.25 in. 0.5 in.
0.25 in.
2.50 in.
0.42
5 in
.
0.12
5 in
.0.
125
in.
1.45 in.
FIGURE 10.25 Modified Navy C-ring distortion test specimen. (From C.E. Bates, G.E. Totten, andR.L. Brennan, ASM Handbook, Vol. 4, ASM International, Materials Park, OH, 1991, p. 100.)
2006 by Taylor & Francis Group, LLC.
form ation of transform ationa l stre sses. Bec k [40] studied the effect of the inter relationshi p
betw een quen ch severity and average co oling rates on the severi ty of crack form atio