Retained Austenite Decomposition and Carbide Formation During Tempering a Hot-Work Tool Steel X38CrMoV5-1 Studied by Dil
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CHROMIUM hot-work tool steels gain their microstructure and mechanical properties from a well-defined composition and heat treatment. The latter commonly consists of a hardening treatment followed by a subsequent multi-step tempering procedure in the case of X38CrMoV5-1. The microstructure after hardening would be a fully martensitic microstructure which could, in principle, be produced by extremely rapid quenching but technically this is very difficult to achieve. The real microstructure additionally consists of cooling-rate dependent amounts of carbon enriched retained austenite and a few primary carbides which are not dissolved during austenitisation.[1] The retained austenite is primarily located as thin films between martensitic laths[2] and has been found to negatively influence the materials properties during tempering.[3] Therefore, the development of these retained austenite films during tempering is investigated, especially during heating to the final temperature. CHRISTOPH LERCHBACHER, Ph.D. Student, is with Christian Doppler Laboratory of Early Stages of Precipitation, University of Leoben, Franz-Josef-Straße 18, 8700 Leoben, Austria. Contact e-mail: [email protected] SILVIA ZINNER, Product Engineer, is with Bo¨hler Edelstahl GmbH & Co. AG, Mariazellerstraße 25, 8605 Kapfenberg, Austria. HARALD LEITNER, Scientist, is with Department of Physical Metallurgy and Materials Testing, University of Leoben, Franz-Josef-Straße 18, 8700 Leoben, Austria. Manuscript submitted May 10, 2012. Article published online August 8, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
Tempering of iron-carbon martensites including the decomposition of retained austenite has been subject of scientific and technical interest for a long time[4] and has been intensively investigated employing different experimental methods. However, the tempering process is generally classified into the following stages[5]: Stage 0 indicates the carbon redistribution to dislocations and carbon cluster formation. Stage I follows with the precipitation of transition carbides. Retained austenite decomposition into ferrite and cementite occurs with Stage II. Stage III describes the formation of cementite from the transition carbides and from carbon clusters. At elevated temperatures Stage IV occurs with the formation of alloy carbides. Details on temperature ranges of the different stages can be found elsewhere.[6] Dilatometric experiments on several model alloys[7–9] as well as on technical materials[10,11] have been widely used to identify tempering reactions. Caballero et al.[12] proposed that the relative length change caused by the retained austenite decomposition depends on the carbon content of the austenite. High carbon content leads to a volume decrease whereas a low carbon content in the retained austenite leads to an expansion. The decomposition of retained austenite is generally proposed to be a transformation into ferrite and cementite[13] or the formation of bainite.[5] Van Genderen et al.[14] divided the retained austen
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