Carbide Precipitation During Tempering of a Tool Steel Subjected to Deep Cryogenic Treatment
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THIS article describes precipitates in the tempered tool steel X153CrMoV12 submitted to quenching at RT and subsequent deep cryogenic treatment, DCT. Under deep cryogenic treatment one denotes a long-time holding at temperatures below 173 K (100 C) immediately after quenching at room temperature, RT. The cooling and holding at temperatures above 173 K (100 C) is denoted as conventional cryogenic treatment, CCT. According to a number of studies, in comparison with CCT, DCT leads to an increase in the toughness and abrasive wear resistance, whereas the hardness is slightly changed and can be even decreased. V.G. GAVRILJUK, Professor, Head of Department, V.A. SIROSH, Postgraduate Student, and A.I. TYSHCHENKO, Ph.D, Senior Scientific Researcher, are with the G.V. Kurdyumov Institute for Metal Physics, 03680 Kiev, Ukraine. Contact e-mail: [email protected] YU.N. PETROV, Doctor of Science, Head of Laboratory, formerly with the G.V. Kurdyumov Institute for Metal Physics, 03680 Kiev, Ukraine, is now deceased. W. THEISEN, Professor, Doctor of Engineering, Head of the Chair, is with the Ruhr University Bochum, 44780 Bochum, Germany. A. KORTMANN, Ph.D, CEO (in Deutsch: Gescha¨ftsfu¨hrer) is with the Ingpuls GmbH, 44894 Bochum, Germany. Manuscript submitted August 21, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
Since the first statements in Reference 1, it is generally accepted that martensitic transformation has no relation to the DCT because it is completed at temperatures above 173 K (100 C) (see also References 2, 3). The precipitation of the so-called g-carbide instead of the ecarbide during subsequent low-temperature tempering[4] or even in the course of holding at cryogenic temperatures with subsequent heating to RT[5] was considered to be the main effect of DCT on the microstructure of tool steels. This idea was used in many subsequent studies, where DCT was claimed to activate carbon clustering and transition carbide precipitation,[6–8] resulting in improving the toughness and wear resistance; and reducing fatigue cracking. The following experimental data are not consistent with the current approach to the nature of DCT. First, the g-carbide, as discovered by Hirotsu and Nagakura in a plain high-carbon steel,[9] is precipitated during tempering of the as-quenched martensite without any DCT. Taylor et al.[10] have shown that it is formed due to the ordering in carbon atoms distribution, which transforms the e hcp lattice into the orthorhombic g one. Following Reference 10, we denote it as the e¢-carbide. Second, like the e-carbide, the e¢-carbide is a transient one and, starting from 573 K (300 C), it is transformed
to cementite. For this reason, they both cannot be responsible for properties of tool steels subjected to tempering at significantly higher temperatures. The other point is that the denial of any role of the low-temperature martensitic transformation in DCT ignores the existence of the isothermal martensite formed at temperatures where, in spite of the increased driving force, the burst a
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