MODELING OF THE LIQUID SOLID AND THE EUTECTOID PHASE-TRANSFORMATIONS IN SPHEROIDAL GRAPHITE CAST-IRON

A model of phase transformations in spheroidal graphite (SG) cast iron has been developed to quantitatively describe the microstructural evolution during solidification and the subsequent solid-state phase transformations (eutectoid reaction) during continuous cooling and to predict some of the microstructural characteristics of final phases formed in SG iron castings. Such characteristics include phase fractions, phase spacings, and grain dimensions. In the model, the nucleation and growth of primary dendrites and eutectics were described based on existing theories, whereas the mathematical formulation for the eutectoid reaction, i.e., the formation of pearlite and ferrite from the as-cast austenite, was developed based on theories as well as physical evidence obtained from the experimental work. The Johnson-Mehl equation and the Avrami equation were used to calculate the fraction of transformed phases under continuous cooling conditions. The role of the grain impingement factor used in these two equations and the significance of the additivity principle in treating nonisothermal transformations were briefly discussed. The latent heat method was used for the numerical treatment of the release of latent heat during phase transformations. A two-dimensional finite element code which can be used in either Cartesian or cylindrical coordinates (ALCAST-2D) was used to solve the time-dependent temperature distribution throughout the metal/mold system. Numerical predictions were validated against experimental results, and good agreement was obtained.