Formation of spheroidal carbide in vanadium white cast iron by rare earth modification

Abstract

In this paper spheroidal VC carbide was produced by rare earth (RE) + N complex modification of white cast iron and the mechanism of modification was investigated. Primary crystallisation of VC in the melt, required for its independent growth into spheroids, was prompted by high V content of the alloy and heterogeneous nucleation. Nuclei in spheroidal VC were found to contain the compositions of the modifier. The formation of effective duplex nuclei from RE + N complex modifier was thermodynamically favourable. Cubic VC tended to dendritic growth in the alloy melt owing to S segregation in the melt close to cubic corners. Thus, purification of the melt by RE preventing dendritic growth favoured the spherical growth of VC. The effects of impurity and modifier elements on the spheroidisation of VC were proved further by the alteration of carbide morphology after adding these elements either separately or simultaneously to pure Fe–C–Valloy. High cooling rates did not favour the spherical growth of VC.MST/1151