Computer simulation of solid state transformation in Fe-C-Si ternary spheroidal graphite cast iron

A model was developed to simulate the post-solidification process of Fe-C-Si spheroidal graphite (SG) cast iron. The calculated results at the end of solidification, like the nodule count, austenite shell radius, graphite volume fraction, graphite spheroid radius and silicon distribution in the austenite shell were considered to be the initial conditions for the post-solidification model. After the end the solidification, the graphite spheroid continues to grow due to the decrease of carbon solubility in the austenite. This growth of graphite in austenite was controlled by the carbon flux diffusing through graphite/austenite interface. The eutectoid transformation was also simulated. The rate constants and indexes of ferrite and pearlite formation in the Kolmogrov-Johnson-Mehl-Avrami's equation were modified from those which had been used for steel or SG cast iron in the literature. The growth of graphite during the eutectoid transformation was also investigated. The calculated results showed that about 50% of the graphite growth occurred after the end of solidification. The graphite growth that occurrs between eutectic equilibrium temperature and eutectoid equilibrium temperature occupies 40% of the total fraction and the rest 10% occurs during the eutectoid transformation. The calculated fractions of ferrite and pearlite agreed well with experimental result. Also from both the calculated and experimental results, it could be understood that the fraction of ferrite increased with decreasing part thickness.