The role of interphase boundary adsorption in the formation of spheroidal graphite in cast iron

Abstract

The interphase boundaries in gray and ductile cast iron were studied with a scanning Auger microprobe (SAM). Sulfur and oxygen were found to be adsorbed at the flake/ metal interfaces in the gray iron, while the nodule/metal and intercrystalline graphite interfaces in the ductile iron were free of foreign elements. The only magnesium detected in the magnesium modified ductile iron was combined with phosphorus and sulfur as a compound. A model is presented which proposes that Fe-C-Si eutectic alloys in the absence of surface active impurities (such as in vacuum casting of high purity materials) produce nodular graphite due to the inherent instability of the graphite/melt interface. The sulfur and oxygen always present in commercial alloys adsorb at the graphite/melt interface, effectively "stabilizing" the active sites on the graphite basal planes, and preventing spherulitic growth. The purpose of modifiers is to getter these impurities. © 1977 The Metallurgical Society of American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc., and American Society for Metals.