When cast iron solidifies, graphite particles generally form on the material’s surface. In fact, the various types of cast iron can easily be differentiated by the shape of the graphite particles produced.

Precisely why graphite particles form on cast iron is uncertain and dominant views have varied over the years. At the end of the Nineteenth Century it was commonly believed that liquids were fully homogeneous mixtures and should maintain that property during solidification. Therefore, early explanations centered on the fact that graphite must segregate only after the solidification process was fully complete. Later opinions included the notions that graphite was a decomposition product from carbides and that liquid iron segregated into alloys of different composition and melting temperatures. More modern attempts at explaining the cast iron graphite formation center on chemical reaction, nucleation and crystal growth theories, but a consensus on the topic has yet to emerge.

Gray cast irons are so named because of the many flakes of graphite they contain, which convey a dull gray color to the material. Phosphorous is used to lower the freezing point and slow the solidification process of gray cast irons in order that they may be formed into intricate designs. Graphite formation, however, is increased during extended periods of solidification and the correspondingly high graphite content of gray cast irons reduces the metal’s strength and flexibility.