On the Nucleation of Graphite in Lamellar Graphite Cast Iron

Extensive research on graphite nucleation in lamellar graphite irons was conducted over the years, with many theories, more or less rooted in experimental facts, being formulated. This work is a pursuit of the same goal-understanding nucleation of graphite. To this purpose, quenching experiments at successive stages during solidification of gray cast iron with various sulfur and titanium levels have been carried out. The objective is to elucidate the effect of these elements on the nucleation of graphite on its lamellar and superfine morphology. Scanning electron microscopy was carried on to analyze the evolution of graphite as a function of fraction solid. The correlation between the percentage of sulfur and the main mechanisms of nucleation was established. It is confirmed that at S levels as low as 0.02wt% graphite nucle-ates mostly at the austenite liquid interface on carbon-rich regions in the liquid. At sulfur levels higher than 0.02wt%, graphite nucleates mostly on MnS inclusions. At 0.02wt% S both mechanisms are possible. In the case of superfine graphite, titanium carbides were also identified as possible nucleation sites.