Quench Temperature-Dependent Phase Transformations During Nonisothermal Partitioning
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TRODUCTION
IN recent years, the quenching and nonisothermal partitioning (Q&P) process[1–10] on steel has been demonstrated to achieve significant austenite retention between the martensitic laths. The Q&P process chain involves austenitization, hot rolling, and quenching to a temperature between Ms and Mf, followed by coiling and slow cooling of the coil to room temperature (RT). Though the intended austenite stabilization is achieved through carbon diffusion from martensite to untransformed austenite during coil cooling, other simultaneously occurring transformations such as carbide precipitation[1,4–10] and decomposition of untransformed austenite to secondary martensite and/or bainite[6–9] are also reported. The distinction between martensite and bainite, using microscopic techniques, poses difficulty due to similarities in their morphology,
G.K. BANSAL, S. TRIPATHY, 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] L.P. JUNIOR and V. RAJINIKANTH are with the Materials Engineering Division, CSIR-National Metallurgical Laboratory. CHIRADEEP GHOSH and A.N. BHAGAT are with the Research and Development Division, Tata Steel Limited, Jamshedpur 831001, India. Manuscript submitted December 30, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
carbon supersaturation, and crystallographic features.[11–17] The X-ray diffraction (XRD) measurements also do not assist in distinguishing and quantifying their content, particularly at low-carbon contents, due to similarities in their crystal structures. On the contrary, dilation measurement is helpful to detect these phase transformations and quantify them at each stage of the heat treatment cycle. In some of the studies on quenching and isothermal partitioning process, the occurrence of bainitic transformation and carbide precipitation, along with the carbon enrichment of austenite, has been reported.[18–22] However, during the nonisothermal partitioning, a continuous drop in temperature, along with the concurrent phase transformations, complicates the analysis of dilation data. In a recent study by Li et al.,[8] the dilation curve indicated only the signatures of concurrent carbon partitioning and phase transformations during nonisothermal partitioning. However, the authors did not quantify individual phase fractions and their composition, which has a direct effect on mechanical properties. Therefore, in the present investigation, an attempt has been made to analyze the dilation curves of Al-free low-Si steel processed through quenching and nonisothermal partitioning and methodically extract the amount and composition of different phases formed, using combinations of established empirical relations. Based on the dilation results, experiments were also performed on bulk samples to investigate the changes, if any, in phase transformation
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