Induction Hardening vs Conventional Hardening of a Heat Treatable Steel
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INTRODUCTION
DURING the last decades, induction heat treatments for quenching and tempering as a through-hardening technique have become more and more important. The advantages of induction heat treatments are numerous. The shorter heat treatment cycles lead to a high potential for cost savings. Furthermore, the process provides less deviation of the mechanical properties and less decarburization.[1,2] However, the mechanical properties after induction heat treatments are not always equal to conventionally heat-treated steels. Dengel[3] studied the short time tempering of different heat treatable steels. Even though the tempering mechanisms proceed during the heating as by conventional heat treatments, the temperature regions of the four wellknown tempering stages are shifted to higher temperatures with higher heating rates. Furthermore, the dislocation densities in induction tempered steels are higher because the dwell time at elevated temperatures is shorter in case of rapid heating. Due to the higher STEPHANIE SACKL, Ph.D. Student, is with the Christian Doppler Laboratory Early Stages of Precipitation, Montanuniversitaet Leoben, Franz Josef-Straße 18, 8700 Leoben, Austria, and also with the Department of Physical Metallurgy and Materials Testing, Montanuniversitaet Leoben. Contact e-mail: [email protected] HARALD LEITNER, Scientist, is with the Bo¨hler Edelstahl GmbH & Co KG, Mariazellerstraße 25, 8605 Kapfenberg, Austria. MICHAEL ZUBER, Employee, is with the Stahl Judenburg GmbH, Gußstahlwerkstraße 21, 8750 Judenburg, Austria. HELMUT CLEMENS, Professor, and SOPHIE PRIMIG, Scientist, are with the Department of Physical Metallurgy and Materials Testing, Montanuniversitaet Leoben. Manuscript submitted March 3, 2014. Article published online August 26, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
heating rates, recovery processes are retarded and more defects, such as dislocations and vacancies, stay in the material when compared to a conventional tempered one. Consequently, this leads to a finer dispersion of cementite during tempering, which in consequence again retards recovery.[3,4] The frequently applied tempering parameter links tempering temperature and time. For a given tempering parameter, a certain hardness should be reached for a given steel.[5] Even if the tempering parameter is taken into account, the hardness values of a heat treatable steel are lower after the induction heat treatment, while for a given tensile strength the reduction of area increases.[6] Other researchers investigated the hardness-toughness relationship of three different heat treatable steels. Slightly lower toughness values were found after a short time heat treatment compared to a conventional heat treatment for a certain hardness level.[7] Also an effect of the induction heat treatment on the mechanical properties of high speed steels is present. Leitner et al.[8] observed a shift of the maximum of the secondary hardening curve of a high speed steel. Furthermore, a drop in the maximum hardness was found, which is r
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