Tempering of Martensite and Subsequent Redistribution of Cr, Mn, Ni, Mo, and Si Between Cementite and Martensite Studied
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INTRODUCTION
THERMAL agitation of martensite leads to a number of aging and tempering reactions. Aging reactions, collectively termed the zeroth stage of tempering, include pre-precipitation phenomena such as the redistribution of solute carbon atoms to martensite imperfections such as dislocations, boundaries, sub-boundaries, and interfaces with retained austenite. The occurrence of these processes has been confirmed by atom probe tomography (APT) investigations.[1–3] Another structural change which can occur near room temperature (RT) is the formation of a modulated
JAVAD MOLA, GUOQING LUAN, DAVID BROCHNOW, and OLENA VOLKOVA are with the Institute of Iron and Steel Technology, Technische Universita¨t Bergakademie Freiberg, Leipziger St. 34, 09599 Freiberg, Germany. Contact e-mail: [email protected] JUN WU is with The State Key Laboratory of Refractories and Metallurgy, Faculty of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan, 430081, P.R. China. Manuscript submitted March 6, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
structure consisting of alternating carbon-rich and essentially carbon-free regions.[4] The tempering stage I involves the formation of transition carbides with hexagonal (e-carbides)[5] or orthorhombic (g-carbides)[6] crystal symmetries, the former being reported more often. In the tempering stage II, retained austenite in carbon and low-alloy steels decomposes by transformation to bainite.[7] The tempering stage III is characterized by the gradual dissolution of transition carbides and the precipitation of orthorhombic cementite (h) or monoclinic Ha¨gg (v) carbides.[8] The tempering stage IV, specific to steels containing carbide-forming alloying elements, involves the formation of alloy carbides.[9] Dilatometry analysis of length changes associated with tempering reactions is a quick method of studying tempering reactions.[10–15] Due to the difficulty of obtaining martensitic steels without retained austenite under practical conditions, the overlap of the expansion due to the austenite decomposition (stage II) and the contractions associated with the tempering stages I and III might interfere with the interpretation of dilatometry results for carbon and low-alloy steels containing retained austenite.[16] In the present study, tempering
reactions in ternary low-alloy steels are studied by correlative dilatometry and magnetic measurements at RT. The latter technique is particularly sensitive to retained austenite decomposition and partitioning of alloying elements between carbides and martensite. Compared to in situ thermomagnetic measurements in which magnetization is measured during continuous heating,[17,18] the present method of tempering at progressively higher temperatures and magnetic measurements at RT offers the following advantages. Measurements at RT as the reference temperature eliminate the need to know the temperature dependence of magnetization for constituents. Furthermore, whereas magnetic and thermal systems in thermomagnetic measurement
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