Continuous Heating Transformation of Spheroidal Graphite Cast Iron

Abstract

This study is aimed to make clear the effect of heating rate on A₁ transformation of spheroidal graphite cast iron by dilatometric and metallographic methods.
The dilatation-temperature curve is affected markedly by the heating rate and the initial microstructure. In the case of ferritic structure, dilatometric curves show only expansion during the transformation instead of contraction in steel and in pearlitic structure. A₁ transformation temperature (A_c1s and A_c1f) is shifted to higher temperature with increase of the rate of heating in ferritic structure than in pearlitic structure. In ferritic iron, the volume fraction of γ phase is proportional to transformation time at a constant rate of heating, and γ phase forms mainly around the graphite nodules at higher rate of heating but it forms along the grain boundaries of ferrite at lower one. The continuous heating transformation curve (CHT) appears higher in temperature and longer in time than isothermal austenitizing curve (TTA).