Induction Tempering vs Conventional Tempering of a Heat-Treatable Steel
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NOWADAYS induction hardening and tempering as a through-hardening technique becomes more and more important. Especially due to the shorter process times, the induction heat treatment is advantageous compared to a conventional heat treatment. Besides shorter process times also less decarburization and cost savings are additional benefits.[1] Nevertheless, variations in the mechanical properties of induction heat-treated steels have been reported,[2–4] where in some cases, the induction heat treatment is beneficial, but in other cases, it leads to a deterioration of the mechanical properties.[5]
STEPHANIE SACKL, Ph.D student, is with Christian Doppler Laboratory Early Stages of Precipitation, Montanuniversitaet Leoben, Franz Josef-Straße 18, 8700 Leoben, Austria, and also Department of Physical Metallurgy and Materials Testing, Montanuniversitaet Leoben. Contact e-mail: [email protected] MICHAEL ZUBER, Employee, is with Stahl Judenburg GmbH, Gußstahlwerkstraße 21, 8750 Judenburg, Austria. HELMUT CLEMENS, Professor, is with the Department of Physical Metallurgy and Materials Testing, Montanuniversitaet Leoben. SOPHIE PRIMIG, formerly Scientist with Christian Doppler Laboratory Early Stages of Precipitation, Montanuniversitaet Leoben, and also with the Department of Physical Metallurgy and Materials Testing, Montanuniversitaet Leoben, is now Lecturer with the School of Materials Science and Engineering, UNSW Australia, Sydney, NSW 2052, Australia. Manuscript submitted January 22, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
A previous study of the current authors, see Reference 6 on induction and conventional hardening of the commercial heat treatable steel 42CrMo4 demonstrated that due to the manufacturing process the alloying elements as well as carbon are segregated in form of lines. In case of the conventional hardening, carbon is equally distributed after quenching. The shorter austenitization time in case of induction hardening does not suffice to obtain a homogenous carbon distribution, which results in differences in the martensitic block size, dependent on the local carbon concentration. In consequence, this leads to a reduced tensile strength compared to the conventionally heat-treated steel.[6] According to these results, longer holding times or higher temperatures would lead to a more homogenous carbon distribution in the austenite and, consequently, would increase the hardness after quenching.[7] Furthermore, it was found that the critical steps for short-time austenitization are the dissolution of carbides and the carbon diffusion in the austenite.[8] Nevertheless, the kinetics of the dissolution of carbides also depends on the starting microstructure. In a fine lamellar pearlitic structure, the cementite dissolves faster than in a coarser microstructure, whereas a spheroidized pearlite shows the most sluggish transformation behavior.[7] After hardening heat treatable steels are usually tempered to obtain a good combination of strength and ductility. During tempering microstructural chan
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