한국주조공학회

In this study, the phase fraction and hardness of spheroidal graphite cast iron heat-treated at various intercritical temperatures and cooling rates (water cooling and furnace cooling) were measured. Based on the phase fraction measurement data including the experimental results under air cooling conditions reported in the author’s previous study, a mathematical model was developed to predict the phase fraction of stable phases in spheroidal graphite cast iron heat-treated at arbitrary heat treatment temperatures and cooling rates. The phase transformation behav- ior during the isothermal heat treatment of spheroidal graphite cast iron at the intercritical temperature was simulated through thermodynamic calculations, and the eutectoid transformation behavior during subsequent cooling was predicted by using the spheroidal graphite cast iron phase transformation model reported in the literature. In addition, a model that converts the volume fraction of stable phase obtained through the developed model into the area fraction was proposed to improve the accuracy of phase fraction prediction. The calculation results from the present phase fraction prediction model well-reproduced the experimentally measured phase fraction changes at different heat treatment tem- perature and cooling rate conditions. The hardness values of the heat-treated spheroidal graphite cast iron were predicted using the calculation results of the present phase fraction prediction model and the hardness calculation formula reported in the literature. As a result, the hardness change according to the heat treatment temperature measured under air-cooling and furnace-cooling conditions was similarly simulated.

Spheroidal graphite cast iron, Heat treatment, Intercritical temperature, Microstructure modeling, and Brinell hardness.