A Thermodynamic-Based Model to Predict the Fraction of Martensite in Steels
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TRODUCTION
MARTENSITE, being one of the quench-hardening constituent in steels, is a microstructure with great practical importance.[1] The transformation from austenite to martensite can also increase the strain-hardening and the ductility through a transformation-induced plasticity (TRIP) effect. Many steels are partially martensitic, e.g., dual phase steels, quenching and partitioning steels, TRIP steels. In order to optimize the performance of these steels, the fraction of each constituent must be carefully considered. In low alloy steels, austenite transforms into martensite at a very high rate when the temperature is lower than the martensite start temperature, Ms, and no more martensite forms by prolonging the isothermal holding time but by decreasing the temperature. This type of martensite is denoted as athermal martensite, and the fraction of athermal martensite is thus a function of temperature but independent of time.[1–6] With the demand of improved high-performance steels, much attention is currently paid toward the design of new steels and the optimization of their production with the aid of computational tools. These tools are preferentially based on physical models and should have predictive capability and satisfactory accuracy. During the past several decades, many models, both theoretical and empirical have been developed to describe the fraction of martensite as a function of undercooling.[3–9] However, most theoretical models lack parameter determination and are not predictive, while the empirical models are only useful in a limited
FEI HUYAN, Ph.D. Student, PETER HEDSTRO¨M, Assistant Professor, LARS HO¨GLUND, Researcher, and ANNIKA BORGENSTAM, Professor, are with the Department of Materials Science and Engineering, KTH Royal Institute of Technology, 100 44, Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted March 15, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
alloying range and under certain conditions since they cannot account for the underlying physics. Hence, it seems that a semi-empirical phenomenological model is preferred at the present stage. All the previous models apply the temperature as the variable while using the chemical driving force, i.e., the difference of Gibbs energy between austenite and martensite, would link directly to thermodynamics. Furthermore, a model based on thermodynamics could be further developed and would be compatible with other physical models, as the semi-empirical model by Stormvinter et al.[10] (SBA˚ model), which predicts Ms based on the thermodynamic driving force for steels. Following the same path, a model is established based on the relationship between the driving force from thermodynamic calculations and the fraction of martensite from experimental results. Part of the model was presented in a short paper in a conference proceeding,[11] and this paper describes the details of the model and the predictions are validated vs experimental data from the literature.
II.
PREVIOUS MODELS
Early works on modeling of the fraction of a
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