Complex (Mn, X)S compounds - major sites for graphite nucleation in grey cast iron

Despite the cubic system, the ability of sulphides to nucleate graphite can be enhanced by inoculating elements which transform them in complex compounds with a better lattice matching to graphite, a low coagulation capacity, good stability and adequate interfacial energy. (Mn,X)S compounds, usually less than 5.0 mu m in size, with an average 0.4-2.0 mu m well defined core (nucleus), were found to be important sites for graphite nucleation in grey irons. A three-stage model for the nucleation of graphite in grey irons is proposed: (1) Very small micro-inclusions based on string deoxidizing elements (Mn, Si, Al, Ti, Zr) are formed in the melt; (2) Nucleation of complex (Mn,X)S compounds at these previously formed micro-inclusions; (3) Graphite nucleates on the sides of the (Mn,X)S compounds with lower crystallographic misfit. Al appears to have a key role in this process, as Al contributes to the formation of oxides in the first stage and favors the presence of Sr and Ca in the sulphides, in the second stage. The 0.005-0.010% Al range was found to be beneficial of lower undercooling solidification, type-A graphite formation and carbides avoidance.