China Foundry (Nov 2009)
Complex (Mn, X)S compounds - major sites for graphite nucleation in grey cast iron
Abstract
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 μm in size, with an average 0.4-2.0 μm well defi ned 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 microinclusions based on strong 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 misfi t. Al appears to have a key role in this process, as Al contributes to the formation of oxides in the fi rst 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 benefi cial for lower undercooling solidifi cation, type-A graphite formation and carbides avoidance.
