Application of the Avrami Equation to the Dilatometric Analysis of ADI Austempering Kinetics

The method and results of evaluating the kinetics of austenite isothermal decomposition in austempered ductile iron (ADI) samples are presented based on the dimensional changes in austenitized and isothermally hardened cast iron samples. Experimental measurements were carried out on samples intended for the production of ADI castings under industrial conditions of ODLEWNIE POLSKIE S.A. A partial solution of the Kolmogorov-Johnson-Mehl-Avrami statistical theory of phase transformations as proposed by Avrami was applied to analyze the experimental results of dilatometric measurements. It is shown that Avrami diagrams can be used to evaluate changes in the kinetics of phase transformations occurring in ADI samples during the first stage of isothermal austenite decomposition. The application of the proposed method has made it possible to identify three steps of ausferrite growth during the first stage, with two statistically significant slowdowns. Using quantitative metallography methods, it is demonstrated that the slowdown in the rate of austenite decomposition during the transition from the first to the second step is related to the development of the microstructure of the metallic matrix of cast iron.