Silicon microsegregation and first stage graphitization in white cast irons

Abstract

The distribution of silicon in a series of as-cast white irons containing up to 3.25 pct Si and from 2.39 to 3.94 pct C was studied using electron probe microanalysis. The silicon content of the proeutectic austenite dendrites is a linear function of the silicon content of the alloy. The periphery of these dendrites is richer in silicon than the core due to the rejection of the element from the cementite formed during eutectic solidification and the enrichment is most pronounced at slow rates of solidification. It was also firmly established that silicon is present in the eutectic cementite, the level increasing with solidification rate and with the silicon content of the alloy. The nonuniform distribution of silicon persisted for a large part of a graphitization anneal. The effects of the silicon in the cementite and at the austenite-cementite interface on the nucleation of graphite during first stage graphitization are discussed in terms of the thermodynamics of graphitization.