Quench Temperature-Dependent Mechanical Properties During Nonisothermal Partitioning
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TRODUCTION
THE steels treated by an energy-efficient quenching and nonisothermal partitioning (Q&P) process[1–12] show stabilization of the austenite between the martensitic laths. In this process, the steel is austenitized, hot-rolled, quenched to a temperature between Ms and Mf and coiled. This involves an extremely slow rate of cooling from the quench temperature to room temperature. During this cooling step, carbon diffuses from martensite to the remaining austenite and the precipitation of carbides takes place.[1,4–12] Depending upon the extent of carbon enrichment of the remaining austenite, it may fully be retained at the room temperature or may partially transform to secondary martensite and/or bainite.[6–9,11,12] In some of the studies,[1–4] the effect of nonisothermal partitioning on the microstructure evolution and/or tensile properties of steel has been found to be comparable to that obtained by conventional isothermal
G.K. BANSAL, P.S.M. JENA, V.C. SRIVASTAVA, and S. GHOSH CHOWDHURY are with the Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India and also with the Materials Engineering Division, CSIR-National Metallurgical Laboratory, Jamshedpur 831007, India. Contact e-mail: [email protected]; [email protected] CHIRADEEP GHOSH and MONOJIT DUTTA are with the Research and Development Division, Tata Steel Limited, Jamshedpur 831001, India. V. RAJINIKANTH is with the Materials Engineering Division, CSIRNational Metallurgical Laboratory. Manuscript submitted June 3, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
partitioning process. Moreover, in a recent investigation,[7] the quenched and nonisothermally partitioned steel has been reported to have improved impact toughness, better work-hardening capacity and comparable wear resistance to that of a fully martensitic microstructure. However, in this investigation, the mechanical properties were evaluated only for the quench temperature that engendered maximum austenite retention. It is known that the mechanical properties of steel are determined not only by the volume fraction of different phases present but also by their composition, distribution and size. All these are somewhat dependent on the quench temperature. This is further substantiated by another recent finding of the present authors,[11] where they showed a slight variation in the amount of retained austenite and its carbon content with quench temperature. However, an increase in the retained austenite film thickness and enhanced self-tempering of martensite was reported with increasing quench temperature. The self-tempering of martensite is the carbide precipitation during slow cooling from quench temperature to room temperature. This differs from the conventional tempering process, which requires an additional heat treatment for carbide precipitation. These changes might alter the mechanical properties of the same steel for different quench temperatures, which were not a part of that report.[11] The available literature on quenching and nonisothermal partitioning process sh
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