Influence of sulfur on heterogeneous nucleus of spheroidal graphite

Abstract

The nuclei of spheroidal graphite (abbreviated SG) were studied using an FE-SEM and an EDS. Many kinds of SG irons with different sulfur contents were prepared for the observation of the nucleus-like core materials. The Mg-treated SG iron was cast into a thin CO2 sand mold and a chilled test piece that is usually used for chemical analysis. The core materials were observed using the chilled test piece. The number of SGs is maximum in the range from 0.010 to 0.025 mass%S of the base melt and the nucleus material is MgS. We propose that this is the most desirable S content for the production of the SG iron castings. This S level is nearly identical to that of the base melt chemical composition for commercial SG iron production. If the S content of the base melt is greater than 0.005 mass%S, the SG nucleus is a spherical MgS in the Mg-treated SG iron. On the other hand, when the S content is less than 0.0022 mass%, the nucleus is a hexagonal (Mg, Si, Al)N as reported by Skaland and Solberg et al.