Prediction of Martensite Start Temperature in Alloy Steels with Different Grain Sizes
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To achieve a high strength and hardness, martensite microstructure is necessary in carbon and alloy steels. The athermal martensite transformation is initiated at the martensite start (Ms) temperature during fast cooling from a high temperature where the austenite is stable. Most alloying elements, except for Co, lower the Ms temperature.[1] In particular, the addition of C and Mn as strong austenite stabilizers can suppress the martensite formation and result in a decreased Ms temperature. Several thermodynamic and empirical equations have been proposed to predict the Ms temperature as a function of chemical composition in carbon and alloy steels.[2–9] The other parameter which influences the Ms temperature is the grain size of austenite before the martensite transformation occurs. A small austenite grain size (AGS) increases the austenite stability and results in a lower Ms temperature. It has been reported that the AGS refinement effect is independent of the chemical composition.[8] The relationship between the AGS and the Ms temperature has been reported for several alloys.[10–15] The decrease of the Ms temperature with decreasing AGS is commonly observed experimentally, regardless of the type of alloy steel. Even though several equations have been proposed for the calculation of the Ms temperature as a function SEOK-JAE LEE, Assistant Professor, is with the Division of Advanced Materials Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea. Contact e-mail: [email protected] KYONG-SU PARK, Senior Researcher, is with the Next Generation Products Research Group, Technical Research Laboratories, POSCO, Pohang 790-785, Republic of Korea. Manuscript submitted February 7, 2013. Article published online May 24, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
of the chemical composition, the development of an equation to calculate the Ms temperature considering both the alloying element and AGS effects over a wide composition range has not been reported. Therefore, in this study, an empirical equation to calculate the Ms temperature considering the effects of the alloying element and the AGS is suggested based on experimental data obtained from the literature. The proposed empirical equation was compared with previously developed equations which only consider the alloying element effect. The experimental Ms temperatures of carbon and alloy steels were obtained from published time–temperature–transformation diagrams.[16] The data related to both the chemical composition and the AGS number were considered to derive an equation. The alloying elements selected in the present work were C, Mn, Si, Ni, Cr, Mo, and Cu. The literature data including alloying elements other than the seven elements mentioned above were not considered. The ranges of the chemical composition and AGS numbers of the adopted literature data are summarized in Table I. The total number of data points used in this study was 219. The empirical equations used to predict the Ms temperature considering only the alloying element effect have
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