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

If a binary Fe-C alloy with a single-phase microstructure of the austenitic gamma phase is isothermally annealed in an appropriate decarburization atmosphere, a layer of the ferritic alpha phase is formed on the surface of the gamma phase and gradually grows into the gamma phase. The kinetics for the growth of the alpha 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 alpha layer is proportional to the annealing time t as described by the relationship l(2) = Kt. This relationship is called the parabolic relationship. As the initial concentration x(gamma 0) of C in the gamma phase increases from the minimum value to the maximum value for the gamma single-phase region at each annealing temperature T, the parabolic coefficient K monotonically decreases from the maximum value K-max(d) to the minimum value K-min(d). As T decreases, K-max(d) decreases, but K-min(d) increases. However, both K-max(d) and K-min(d) vary depending on T in a complicated manner. Thus, an Arrhenius equation is not applicable even to the temperature dependence of K-max(d) in the whole annealing temperature range. At a constant value of x(gamma 0), K monotonically decreases with increasing value of T. This means that the growth of the alpha layer takes place faster at lower annealing temperatures than at higher annealing temperatures. Such temperature dependence of the kinetics coincides well with experimental observations. [doi:10.2320/matertrans.M2012161]