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Professor Induction wel-comes comments, ques-tions, and suggestions forfuture columns. Since1993, Dr. Valery Rudnevhas been on the staff of In-ductoheat Group, where hecurrently serves as groupdirector science and tech-

nology. In the past, he was anassociate professor at severaluniversities, where he taughtgraduate and postgraduatecourses. His expertise is in ma-terials science, heat treating, ap-plied electromagnetics, com-

puter modeling, and processdevelopment. He has 28 yearsof experience in inductionheating. Credits include 15patents and 118 scientific andengineering publications. Healso is coauthor of the 800-pageHandbook of InductionHeating (published in 2003 byMarcel Dekker, www.dekker.com). Contact Dr. Rudnev atInductoheat Group, 32251North Avis Drive, MadisonHeights, MI 48071; tel: 248/585-9393; fax: 248/589-1062;

e-mail: rudnev@inductoheat.com; Web: www.inductoheat.com.

Residual stresses ininduction hardening:

simply complex

Heat treaters are often faced withthe necessity of making a rea-

sonable compromise between main-taining the required hardness and ob-taining a tough, ductile microstructurethat has the desired distribution ofresidual stresses.1 (Stresses are closelyrelated to cracking of induction hard-ened parts, as can be seen in the fish

bone diagram of cracking discussedin References 1 and 2.)

Stresses can be classified in severaldifferent ways, and although residualstresses are three-dimensional with

axial, circumferential, and radialcomponents this discussionwill be simplified by consid-ering them as one-dimen-

sional stresses.Depending upon the distance over

which they extend, residual stressescan be macroscopic or micro-

scopic. Macroscopic stresses typicallyappear at a distance that exceeds sev-eral grains of metal.3 In contrast, mi-crostresses take place within a grain,and include stresses that appear onthe atomic level. Studies of residualstresses in metal heat treating typicallyfocus on the distribution and magni-tude of macrostresses.

Stress groups: In general, stressesthat appear during induction heattreating can be divided into threegroups: initial, transitional, andresidual stresses.1

Initial stresses: Their distributionand value depend upon the opera-tions that preceded heat treatment(casting, forging, rolling, and/or

welding, for example). Transitional stresses: These stresses

appear during heating and cooling,and, depending upon the application,may partially or totally disappear afterheat treatment has been completed.

Residual stresses: They are theproduct of initial and transitionalstresses.

How residual stresses ariseLets examine how residual stresses

form during induction hardening.1

Note that the mechanism is differentfrom that associated with other heattreating processes, including gas car-

burizing and nitriding.In general, two types of stress are

encountered: thermal and phase trans-formation stresses. Thermal stressesare caused by different magnitudes oftemperature and temperature gradi-

Valery I. Rudnev Inductoheat Group

PROFESSORINDUCTION

HEAT TREATIN G PRO G RESS JAN UARY/ FEBRUARY 2004 27

Fig. 1 Formation of residual stresses after induction hardening.

Heating pattern Coil

OD of part afterinduction heating

Shear stresses

Tension Compression

Ra

dials

tresses

Compression

Tension

Tensile stressmaximum

Que

nching

Tensilestress

maximum

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ents, while transformation stressesprimarily occur due to volumetricchanges accompanying the formationof phases such as austenite, bainite,and martensite. The total stress is acombination of the two components.

At different stages of heat treating, theeffect of the components on total stressalso will differ.

Example: Figures 1 and 2 illustratethe dynamics of (macroscopic) stressappearance during induction hard-ening of a carbon steel cylinder.1,4,5Atthe first stage of the heating cycle, thesection of the cylinder located underthe coil will try to expand. The tem-perature of the workpiece at this pointis relatively low: less than 500C(930F). Carbon steel in this tempera-

ture range is in a nonplastic condi-tion and cannot easily expand. The re-sult: compressive stresses build up inthe surface of the workpiece.

Heating: As the temperature rises,surface compressive stresses form andincrease in magnitude (Fig. 2). In the520 to 750C (970 to 1380F) range,steels undergo plastic volumetric ex-pansion and stresses begin to decrease.Finally, when the temperature exceedsapproximately 850C (1560F), thesteels surface freely expands, and thediameter of the heated area becomes

greater than its initial diameter. Sincethe yield point of the surface layer isconsiderably lower at elevated tem-perature, the material will flow plas-tically and surface stresses will signif-icantly decrease.

Cooling: After quenchant issprayed onto the heated surface, theoutermost layer quickly loses its plas-ticity and a pronounced tensile stressmaximum appears at the surface ofthe workpiece (Fig. 2). This maximumvalue typically occurs just aboveMs

(martensite start) temperature. The ap-pearance of martensite reduces sur-face tensile stresses and leads to theformation of surface compressivestresses. Upon completion of cooling,a complex combination of compres-sive and tensile stresses exists withinthe part (Fig. 1).

It is important to remember that theresidual stress system is self-equili-

brating; that is, there is always a bal-ance of stresses in the workpiece. Ifcertain regions have compressiveresidual stresses, then somewhere elsethere must be offsetting tensilestresses. If the stresses werent bal-

anced, movement would then result.Benefits: Surface compressive

residual stresses are considered usefulin most applications. They providesome protection against crack initia-tion and propagation caused by stressrisers; for example, microscopicscratches, notches, and microstructuralheterogeneities. Compressive residualstresses are particularly beneficial toparts that experience bending and/or

torsion in service.Tensile residual stresses, on the other

hand, can be dangerous. Note the ten-sile stress maximum located just be-neath the hardened case in Fig. 1. Theseare the stresses primarily responsiblefor subsurface crack initiation.

Relieving, measuring stressThe overall residual stress condition

of the induction hardened part usu-ally increases its brittleness and notchsensitivity, which reduces toughness

and reliability. Therefore, stress reliefis required. Goals are to reduce sub-surface tensile stresses and move thetensile stress maximum farther fromregions of applied stress, while re-taining the useful surface compressivestresses. Stress relieving during in-duction tempering is discussed in Ref.1. In addition, a final grinding opera-tion also can have a pronounced effecton residual stress distribution andcrack sensitivity.6 Grinding should beconsidered when developing the re-quired residual stress distribution.

Measurements:Residual stress dis-tributions are not nearly as simple in

induction hardened parts of complexshape as they are in a plain cylinder.As a result, measurement of residualstresses is often not an easy task, andspecial equipment and a great deal oftime may be needed. Techniques forquantifying residual stresses includethe sectioning, hole drilling, layer re-moval, bending deflection, X-ray dif-fraction, magnetic, and ultrasonicmethods.3 Important process selection

factors include the specifics of the heattreated part; for example, kind ofmetal, grain size, and required in-spection depth and accuracy. HTP

References1. Handbook of Induction Heating, by V.Rudnev, D. Loveless, R. Cook, and M.Black: Marcel Dekker Inc., New York, 2003,800 p.2. Troubleshooting Cracking in Induc-tion Hardening, by V. Rudnev: HeatTreating Progress, Vol. 3, No. 5, August2003, p. 2728.3. Handbook of Measurement of ResidualStresses, Society for Experimental Me-chanics, Jian Lu (Ed.): Fairmont Press Inc.,Lilburn, Ga., 1996, 238 p.4. High-Frequency Induction Heat Treating,by G. Golovin and M. Zamjatin: Mashino-stroenie, Saint Petersburg, Russia, 1990,230 p. (in Russian).5. Industrial Applications of InductionHeating,by M. Lozinskii: Pergamon Press,London, 1969, 420 p.6. A Review of the Influence of GrindingConditions on Resulting Residual StressesAfter Induction Surface Hardening and

Grinding, by Janez Grum:Journal of Ma-

terials Processing Technology, Vol. 114, Issue3, 7 Aug. 2001, p. 212226.

PROFESSOR INDUCTION,continued

28 HEAT TREATIN G PRO G RESS JAN UA RY/ FEBRUARY 2004

Fig. 2 Stresses at the surface of a carbon steel cylinder during heating-quenching cycle.

200 (392) 400 (752) 600 (1112) 800 (1472) 1000 (1832)

Temperature, C (F)

Quenching stage

Heating stage

Stresses

Compression

Tension

0