Thermal Treatment Near Eutectic Temperature Effect on Graphite in Ductile Iron

Abstract

Recent studies of the formation of spheroiclal graphite during solidification have indicated that spheroidal growth of graphite was permitted in the eutectic freezing range, due to the presence of a protective austenite shell which separated the graphite from the liquid. The importance of this austenite shell has been studied by heating specimens of ductile iron to temperatures of interest in the solidification temperature range, and noting the effect of these treatments on the graphite shape. The temperature to which the sample was heated, time at temperature and thermal treatments subsequent to heating were varied, and the resulting structures related to flake graphite cast iron treatments and known graphite formation theory. The results of treatments on cOmmercial irons support previous graphite formation hypotheses, and the necessity of the protective austenite shell during solidification of spheroidal graphite cast irons. The authors have also demonstrated possible mechanisms by which defective graphite shapes may develop in ductile iron, including exploded graphite, vermicular graphite and flake-like shapes.