Evolution of M s Temperature as a Function of Composition and Grain Size
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INTRODUCTION
THE development of ‘‘3rd generation advanced high strength steels’’ is partly based on the enhancement of the transformation-induced plasticity effect.[1] This signifies that thermal and mechanical stability should be improved and controlled. One of the parameters that is commonly assumed as an indicator of thermal stability of austenite is the starting temperature of martensitic transformation (Ms). The Ms temperature represents the thermodynamic driving force required to initiate the shear transformation of austenite to martensite. Several factors can affect the Ms temperature; nonetheless, the chemical composition of the materials is considered to play the major role. Many studies were done in order to measure, to characterize, and to relate the Ms temperature with the chemical composition of the steel.[2–17] One of the most well-known works is the study by Andrews.[9] Recently, some review articles comparing different empirical equations were published.[18,19] The grain size of parent austenite (prior austenite grain size (PAGS)) also has an influence on the Ms temperature. The pioneer study about the austenite to martensite transformation and the effect of PAGS was done by Fisher et al. in 1949.[20] It was suggested that the amount of transformation depends on the number of nuclei formed randomly at the austenite parent phase A. ARLAZAROV, N. KABOU, and D. HUIN are with the ArcelorMittal Maizieres Research SA, Voie Romaine, BP30320, 57283 Maizie`res-le`s-Metz, France. Contact e-mail: artem.arlazarov@ arcelormittal.com E. SOARES BARRETO is with the ArcelorMittal Maizieres Research SA and also with the Leibniz-Institut fu¨r Werkstofforientierte Technologien-IWT Bremen, Badgasteiner Str. 3, 28359 Bremen, Germany. Manuscript submitted March 3, 2020. Accepted September 9, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
and the size of each transformed region. The latter is proportional to the austenite grain size during the early stages of transformation. Hence, the decrease in the grain size will decrease the volume of martensite transformed and, thereby, lower the Ms temperature. In later publications, two other explanations of PAGS’s influence on the Ms temperature were proposed. The first one considers the effect of PAGS on the Ms temperature related to the burst phenomenon[21] taking place during the austenite to martensite transformation.[22,23] The burst phenomenon is defined as the ability of martensite plates to activate the appearance and growth of other plates in an autocatalytic way and stimulated by elastic strain.[21] Rios and Guimaraes[23] showed that the burst phenomenon is more promoted in bigger austenite grains; consequently, the decrease of grain size decreases the amount of formed martensite at a certain temperature, thus delaying the detection of the Ms temperature. Another theory proposes Hall–Petch strengthening of austenite with the decrease of PAGS,[24] leading to a greater resistance at the nucleation stage and, thus, requiring a higher undercooling to start the martensitic transforma
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