Mechanisms of graphite formation in ductile cast iron containing cerium and lanthanum

In the present investigation the separate and combined effects of cerium and lanthanum on the graphite formation in ductile cast iron have been investigated, using directional solidification. It is shown that both elements are carbide formers in that they increase the undercooling for graphite formation. The observed behaviour is related to a change in the chemical composition of the type A inclusions which are the primary nucleation sites for graphite nodules in ductile iron. Because other sites become activated at larger undercoolings, the nodule count passes through a maximum when the cerium or lanthanum content of the iron is progressively increased. The drop in the nodule count at high cerium and lanthanum concentrations is accompanied by an extensive iron carbide formation.