Thermodynamically Based Prediction of the Martensite Start Temperature for Commercial Steels
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I.
INTRODUCTION
MARTENSITIC steels have become very important engineering materials in modern society. Crucial parts of everyday products are made of martensitic steels, from surgical needles and razor blades to car components and large-scale excavators. The demands for superior material performance have increased over the past few decades; similarly, the global competition between steel producers has also increased. As a result, steel producers have increased their interest in product optimization through material modeling. For example, models can be used to predict characteristics of phase transformations, such as transformation temperatures, transformation rates, and effects of alloying elements on the phase transformation. To increase the accuracy and the possibility for extrapolations, the physics behind the phase transformations must be considered, i.e., to say that the models should not be derived based on pure empirical relations. However, the martensitic transformation in steels is rather complex and the collected knowledge on the subject is not mature enough to derive a fully physical model. Therefore, a semi empirical approach may be the best option; this approach which would combine well-established thermodynamics with experimental data on the martensite start temperature ðMs Þ corresponding to binary and ternary systems. Hence, in the present work, a thermodynamic method is developed for the prediction of Ms in steels, by taking into account the effect of important alloying elements on the driving force for starting the martensitic transformation. Moreover, as this method is based on the available experimental data on Ms, obtained from rapid quenching experiments on binary and ternary alloys that are important constituents of commercial steels, this ALBIN STORMVINTER, Postdoctoral Student, ANNIKA BORGENSTAM, Associate Professor, and JOHN A˚GREN, Professor, are with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, 10044 Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted April 26, 2011. Article published online June 27, 2012 3870—VOLUME 43A, OCTOBER 2012
study will explore the possibility to predict Ms of commercial steels.
II.
PREVIOUS MODELS
An early attempt to rationalize information on the martensite start temperature, Ms, in commercial steels was made by Payson and Savage in 1944; they proposed an empirical expression that could be used for predicting the temperature of new steels: Ms ¼ 772 317C 33Mn 28Cr 17Ni 11Si 11Mo 11W
ðKelvinÞ
½1
with alloying content given in mass percent (mass pct). The original equation gave Ms in Farenheit but has here been recast to give Ms in Kelvin. A large number of similar empirical expressions were later proposed.[2–5] They have been reviewed a number of times, for instance, by Wang et al.,[6] by Skrotzki and Hornbogen,[7] and by Sourmail and Garcia-Mateo.[8] Wang et al.[6] tried a different approach by examining the relation between the experimental Ms temperature and the T0 temperature,
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