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FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | F E B R U A R Y / M A R C H 2 0 1 8 6 1 els in the induction surface-hardened case compared with hardness levels typically expected based on Jominy harden- ability curves. The surface hardness of an induction surface hardened part could be 2-4 HRC higher (1-3 HRC being more typical) than that expected for a given carbon content for parts with identical chemical compositions. The superhardness phenomenon is not clearly under- stood, and its basis has not been established nor widely accepted by metallurgists worldwide. However, it has been obtained experimentally on numerous occasions and sever- al interpretations have been offered [1,3] . INDUCTION HARDENING STAINLESS AND SPECIALTY BEARING STEELS Martensitic stainless steels (MSS), precipitation-hard- enable (PH) stainless steels, and specialty bearing steels specified in certain aerospace applications can be induc- tion hardened, forming a martensitic structure in the as- quenched condition. Electromagnetic and thermal proper- ties of these alloys are noticeably different compared with those of plain carbon steels, which influences process recipe selection. Electrical resistivity ( ρ ) and magnetic permeability ( μ r ) of MSS are greater and lower, respectively, compared with those of plain carbon steels with similar carbon content. For example, at room temperature, ρ of MSS is typically two to three times higher than those of corresponding plain car- bon steels. Both parameters produce a greater depth of heat generation ( δ ) for a given frequency. In addition, depending on the grade of MSS, the Curie temperature can be 40-60°C lower comparedwithplain carbon steels, which shortens the magnetic stage of the heating cycle. Whilemartensitic stainless steels have the highest ther- mal conductivity among other stainless steels, their values canbe 30%to 45% lower thanplain carbon steels. Therefore, there is a noticeably lower heat transfer effect, potentially resulting in greater surface-to-core thermal gradients during rapid induction heating, as well as the occurrence of a high- er magnitude of transient stresses and greater probability of crack development, suggesting the need to apply lower heat intensities. The initialmicrostructureofMSSbefore inductionhard- ening is often annealed or martempered. Somewhat longer austenitizing times and higher temperatures are common- ly specified due to slow dissolution of chromium carbides during austenitization. Some complex carbides may still re- main upon completion of the austenitization stage. The as- quenched hardness of many MSS can be represented by a bell-shaped curve due to several factors; initially, hardness increases with a rise in austenitizing temperatures, reaching a maximum and then starting to decline [1] . Excessively high austenitizing temperatures should be avoided, because they produce coarse grains and a greater amount of retained austenite (RA) upon quenching to room temperature. This may necessitate short soaking/holding at austenite phase temperatures. The as-quenched microstructure of MSS, PH stainless steels, and specialty bearing steels can contain a substan- tial amount of RA. Thus, a subzero cryogenic treatment fol- lowed by single or double tempering is commonly specified. The presence of greater than expected amounts of RA might also be associated with a stabilization phenomenon, which can be caused by an interrupted quench. Stabilization often occurs in hardening alloys with sufficiently low M s and M f temperatures. Lower amounts of RA are formed during unin- terrupted quenching. An interrupted quench with a consid- erable quench delay at both room and low-range elevated temperatures can stabilize the austenite, exhibiting a greater amount of RA. This phenomenon introduces certain restric- tions for a time delay between quenching and cryogenic treatment. ~HTPro Note: Statitron is a registered trademark and IFP is a trade- mark of Inductoheat Inc. Some of the information presented here was first published in the Handbook of Induction Heat- ing, 2nd Edition, by V. Rudnev, D. Loveless, and R. Cook, CRC Press, 2017. CRC Press has granted permission to publish these materials. For more information: Valery Rudnev is director, sci- ence and technology, Inductoheat Inc., an Inductotherm Group Co., 248.629.5055, 248.310.2741 (mobile), rudnev@ inductoheat.com, www .inductoheat.com. References 1. V. Rudnev, D. Loveless, and R. Cook, Handbook of In- duction Heating, 2nd ed., CRC Press, 2017. 2. J. Orlich, A. Rose, and P. Wiest, Atlas zur Warmebe- handling der Stahle, Vol 4, Zeit-Temperatur-Austenitis- ierung-Schaubilder, Verlag Stahleisen M.B.H., Düsseldorf, Germany, 1976. 3. D. Matlock, Metallurgy of Induction Hardening of Steel, Induction Heating and Heat Treating, Vol 4C, ASM Hand- book, (V. Rudnev and G. Totten, Eds.), ASM International, p 45-57, 2014. 11