Application of first-order quasichemical theory to transformations in steels

Abstract

There are several theoretical and empirical thermodynamic models which adequately describe the equilibrium properties of carbon in ferrite and in austenite (e.g. Refs. 1, 2). These models usually contain unknown parameters which have to be deduced by ‘curve-fitting’ to experimental thermodynamic data, and this has made it difficult to judge the validity of particular theories, since the curve-fitting procedure always seems to ensure good agreement with the known Fe–C phase diagram. However, the analysis of reactions occurring under metastable equilibrium conditions (such as the bainite and martensite transformations) requires the extrapolation of the free energy surfaces of austenite and ferrite; it is clearly desirable to base such extrapolations on physically meaningful laws.