Analysis for Kinetics of Ferrite Growth due to Isothermal Decarburization of Austenite in the Binary Fe–C System

Abstract

If a binary Fe–C alloy with a single-phase microstructure of the austenitic γ phase is isothermally annealed in an appropriate decarburization atmosphere, a layer of the ferritic α phase is formed on the surface of the γ phase and gradually grows into the γ phase. The kinetics for the growth of the α layer during the decarburization was quantitatively analyzed using a diffusion model at annealing temperatures between 1011 and 1185 K. In the analysis, the diffusion coefficient of C in each phase is considered independent of the chemical composition. According to the model, the square of the thickness l of the α layer is proportional to the annealing time t as described by the relationship l² = Kt. This relationship is called the parabolic relationship. As the initial concentration x^γ0 of C in the γ phase increases from the minimum value to the maximum value for the γ single-phase region at each annealing temperature T, the parabolic coefficient K monotonically decreases from the maximum value K^d_max to the minimum value K^d_min. As T decreases, K^d_max decreases, but K^d_min increases. However, both K^d_max and K^d_min vary depending on T in a complicated manner. Thus, an Arrhenius equation is not applicable even to the temperature dependence of K^d_max in the whole annealing temperature range. At a constant value of x^γ0, K monotonically decreases with increasing value of T. This means that the growth of the α layer takes place faster at lower annealing temperatures than at higher annealing temperatures. Such temperature dependence of the kinetics coincides well with experimental observations.