On the mechanism of the formation of Widmanstatten graphite in flake graphite cast irons

The mechanism whereby Widmanstatten graphite develops during the solidification of flake graphite cast irons has been found to involve the preferential segregation and a complex interaction of specific elements at the surface of the graphite flake during solidification and the development of the plate like appendages in the solid austenite adjacent to the graphite flake. The literature has suggested that lead, calcium and hydrogen may be causal to the formation of Widmanstatten graphite, but has the interaction of these elements has not been effectively documented. While the formation of this degraded graphite is often attributed to the presence of a sufficient amount of lead alone, it has been observed that Widmanstatten graphite develops only in conjunction with a combination of factors operative at the graphite-austenite interface. Commercial flake graphite cast irons may exhibit Widmanstatten graphite as a function of lead and calcium content in the iron, moisture content in the molding media, solidification cooling rate and the rate of cooling immediately after solidification, etc. Lead contamination of cast irons was also observed to increase the chilling tendency of the iron. The detrimental effects of lead can be counteracted by the presence of rare earths in the iron, where rare earth elements react with lead to form stable, high melting point compounds.