Kinetics of continuous cooling transformations of austenite in high-chromium iron alloys

The work develops the models of two principal diffusional transformations in high-chromium iron alloys on continuous cooling, viz. precipitation of secondary carbides (Cr, Fe)7C3 from austenite and pearlitic decomposition of the latter. Literature experimental data for alloys of various composition were used for analysis. Chemical composition of austenite by the end of austenitizing holding was determined using previously developed models, and then the exact expression describing isothermal precipitation of carbides from supercooled austenite was found based on the general equation following from the kinetic theory of phase transformations. It was found out that the time exponent n in that expression equals to 3/2, which corresponds to the case of preferred precipitation of very fine carbide particles. For transition to the continuous cooling condition the C-curve of isothermal carbide precipitation was approximated by a square parabola. This permitted to find the relation between cooling rate and the amount of precipitated carbide phase by means of the Scheil-Steinberg integral. Further comparison with the experimentally observed increase of martensitic point allowed to determine the numerical parameters of the model and their dependence on carbide composition of a given alloy. For the pearlitic transformation, a previously developed model describing the parameters of temperature dependence of the Avrami equation coefficients on austenite composition was employed. That model was generalized for the continuous cooling case in the same was as above. As a result, CCT diagrams of diffusional austenite transformations in high-chromium alloys were calculated, that in general corresponded well the experimental ones.