抄録
Studies done on the solidification characteristics of synthetic cast-irons of various purity levels melted and cast under vacuum varying from 13.3 Pa to 1.33×10−4 Pa bring out certain significant results. Properly degassed clean cast-irons under conductive heat transfer conditions are prone to inverse chill formation. This happens when growth of eutectic graphite terminates at certain undercoolings and further solidification proceeds by cementite nucleation. The shapes of graphites, as revealed by scanning electron microscopy, at the transition zones between white and gray iron, across the inverse chill zone show how the eutectic graphite coarsens into different forms before its growth is stopped. Nodular graphite forms at high cooling rates and coral graphite forms at low cooling rates in high purity cast-irons only. The conditions like purity level, cooling rates and superheat etc., leading to various nodular graphite densities in such cast irons have been identified. Based on this a theory is proposed explaining the mechanism of formation of nodular graphite in such cast irons involving carbides as nucleating agents. Implications of the above studies in the formation of nodular graphite and inverse chill in commercial cast irons inoculated with magnesium are discussed.
