A Model for the Graphite Formation in Ductile Cast Iron: Part I. Inoculation Mechanisms

Abstract

Part I of this investigation deals with the inoculation mechanisms in ductile cast iron, with particular emphasis on the theoretical aspects of heterogeneous nucleation of graphite at inclusions. It is shown that the majority of the inclusions in ductile cast iron are primary or secondary products of the magnesium treatment(e.g., MgS, CaS, MgOSiO{ni2}, and 2MgO-SiO2). After inoculation with (X,Al)-containing ferrosilicon (X denotes Ca, Sr, or Ba), hexagonal silicate phases of the XO-SiO2 or the XO-Al2O3-2SiO2 type form at the surface of the oxide inclusions, probably through an exchange reaction with MgO. The presence of these phases, will enhance the nucleation potency of the inclusions with respect to graphite. In particular, the (001) basal planes of the crystals are favorable sites for graphite nucleation, since these facets allow for the development of coherent/semicoherent low-energy interfaces between the substrate and the nucleus. In contrast, the fading of inoculation can be explained by a general coarsening of the inclusion population with time, which reduces the total number of catalyst particles for graphite in the melt. A theoretical analysis of the reaction kinetics gives results which are in close agreement with experimental observations.