Modified Methodology for the Quench Temperature Selection in Quenching and Partitioning (Q&P) Processing of Steels
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quenching and partitioning (Q&P) processing was proposed to obtain a steel microstructure consisting of a martensitic matrix with a considerable volume fraction of C-enriched retained austenite.[1,2] Figure 1 shows a schematic of the thermal cycle used for Q&P processing. The Q&P processing consists of four stages: a full or partial austenitization stage, an initial quenching stage, a partitioning stage, and a final quenching stage. When the fully austenitized or intercritically annealed steel is initially quenched to a quench temperature (TQ) between the martensite-start (Ms) and the martensite-finish (Mf) temperatures, austenite is partially transformed to primary martensite (a¢primary). The quenched steel is then reheated to a partitioning temperature (TP). During the partitioning stage, C diffuses from the supersaturated a¢primary into the untransformed austenite (cprimary). As a result, the Ms
EUN JUNG SEO, Graduate Student, LAWRENCE CHO, Postdoctoral Research Associate, and BRUNO C. DE COOMAN, Professor and Director, are with the Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea. Contact e-mail: [email protected] Manuscript submitted March 19, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
temperature of the C-enriched austenite is lowered and this results in the stabilization of the untransformed austenite at room temperature. Some austenite may transform to martensite during the final quenching stage due to insufficient C partitioning. This martensite is referred to as secondary martensite (a¢secondary). The microstructure obtained after Q&P processing consists of C-enriched austenite in a lath martensite matrix. The martensite matrix gives the material a high strength and the C-enriched austenite enhances the elongation and the toughness. In earlier studies,[3–7] the Q&P processing of advanced high strength steel has been shown to improve their ductility due to the activation of the transformation-induced plasticity (TRIP) effect. In Q&P processing, the selection of TQ is of primary importance in obtaining the maximum volume fraction of retained austenite. If TQ is too high, a large volume fraction of untransformed austenite remains present in the microstructure after the initial quenching stage. The C content in the austenite after the partitioning stage is low, resulting in a less stable retained austenite. On the other hand, if TQ is too low, a small volume fraction of austenite remains after the initial quenching stage. In this case, the C content in the austenite after the partitioning stage is high. This results in a very stable retained austenite which may not undergo strain-induced martensitic transformation. The optimum TQ is usually taken as the temperature for which the volume fraction of retained austenite is maximum. Speer et al.[1] developed a TQ selection methodology to predict the retained austenite phase fraction as a function of TQ, and defined the optimum quench temperature (TQ,ma
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