Influence of Niobium on the Microstructure of Gray Cast Iron

The final properties of grey cast iron depend on graphite morphology. Additions of certain trace elements can improve or degrade these properties for constant carbon level. The goal of this research was to investigate the effect of different levels of niobium on the microstructure and nucleation propoensity for hypereutectic gray cast irons. Cooling curves were recorded, and metallographic analysis was conducted on thermal analysis inoculated cups. It was found that niobium additions alter significantly the solidification process, modifying the morphology, type and size of graphite, the number of eutectic cells and the perlite spacing. These effects are more evident as the content of niobium increases. Early precipitation and growth of kish graphite (type C) seems to be reduced. Mn sulfides nucleating on complex oxides were detected as the main nuclei through SEM study. The formation of NbTi particles is directly related to the Nb content.