Distortion due to heat treatment.pdf

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

2006 by Taylor & Francis Group, LLC.

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


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.


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.)


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.)


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.


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


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


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.


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.


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


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.)


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.)


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.)


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.)


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.)


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.)


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.)


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.)


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.)


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.)


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.


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.)


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.)


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


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

QQ

QQ

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

QQ

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

QQ

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.)


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.)


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.)


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.


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


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.)


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

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