Critical conditions for formation of spheroidal graphite

The influence of the interfacial energy between the Fe–C melt and basal plane of graphite, γGr/L, and the cooling rate on the formation of spheroidal graphite was investigated as a function of the sulfur content in cast iron using the sessile drop method in a He–3% H2 atmosphere. The result showed that the γGr/L decreased from 2.62 to 1.48 J/m2 with an increase in the sulfur. The corresponding surface energy of the graphite, γGr/V, also varied from 1.17 to 0.65 J/m2. It is found that the spheroidal graphite is formed when the γGr/L is greater than 2.5 J/m2, which is the critical interfacial energy of the spheroidal graphite formation. In addition, the cooling rate is also an important factor for producing the spheroidal graphite in the Fe–C alloy. The present study clearly shows that the critical cooling rate is a function of the sulfur content, namely γGr/L, in the Fe–C melt.