Mechanisms of graphite formation in ductile cast iron containing rare earth metals

Abstract

In the present study, the separate and combined effects of cerium and lanthanum on 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. As other sites become activated at larger undercoolings, the nodule count passes through a maximum when the cerium or the lanthanum content of the iron is progressively increased. The drop in the nodule count at high cerium and lanthanum concentrations is accompanied by extensive iron carbide formation.