A MODEL FOR THE GRAPHITE FORMATION IN DUCTILE CAST-IRON .1. INOCULATION MECHANISMS

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 th at the majority of the inclusions in ductile cast iron are primary or secondary products of the magnesium treatment (e.g., MgS, CaS, MgO.SiO 2, and 2MgO.SiO2). After inoculation with (X,Al)-containing ferrosilic on (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 inclusion s, probably through an exchange reaction with MgO. The presence of the se phases will enhance the nucleation potency of the inclusions with r espect to graphite. In particular, the (001) basal planes of the cryst als are favorable sites for graphite nucleation, since these facets al low for the development of coherent/semicoherent low-energy interfaces between the substrate and the nucleus. In contrast, the fading of ino culation can be explained by a general coarsening of the inclusion pop ulation with time, which reduces the total number of catalyst particle s for graphite in the melt. A theoretical analysis of the reaction kin etics gives results which are in close agreement with experimental obs ervations.