Interfacial Energies and the Graphite Crystallization in Cast Iron

Abstract

The conditions necessary for nodular graphite crystallization in cast iron have been studied. It was found that, apart from the presence of sufficient foreign nucleating substrate particles, a high rate of heterogeneous nucleation on the nucleating substrates, such that basal planes are parallel to the surface of the nucleating substrate, and favourable surface-tension conditions are necessary for nodular graphite crystallization. From thermodynamic considerations it is deduced that the growth of a graphite flake directly after nucleation is energetically more favourable and that the growth of nodules becomes possible only after a critical volume of graphite has formed. It is shown that additions to the melt which decrease the ratio σ_ss/σ_s1 (specific interfacial tension between graphite crystallites in a nodule σ_ss divided by the interfacial tension between solid graphite and the melt σ_s1) decrease the critical volume at which nodules can form and therefore assist in the formation of nodules.