Influence of Mo and W on Solidification Microstructure Formation for Ni-hard Type Cast Iron

The solidification microstructure of Ni-hard type cast iron was investigated to evaluate the influence of Mo and W additions on the formation of carbides and graphite. The specimen was prepared based on the composition of Fe-3.3%C-0.8%Si-0.8%Mn-4.4%Ni in mass%, and Mo or W was added to the reference specimen to a maximum of 6.7%Mo or 2.9%W, respectively. Each alloy was cast into a permanent mold for the microstructural analysis. The solidification process of each alloy was also investigated by thermal analysis and quenching experiments. According to EDS and XRD analyses, it is revealed that the solidification microstructure of standard Ni-hard type cast iron consists of primary gamma, gamma+M3C eutectic structure and graphite, and the addition of Mo and W provides gamma+M2C eutectic structure. The further addition of Mo and W increases the amount of gamma+M2C eutectic structure and decreases the amount of gamma+M3C eutectic structure, whereas it has little effect on the amount of primary gamma. The quenching experiment reveals the graphite formation as eutectic structure between the formations of gamma+M2C eutectic structure and gamma+M3C eutectic structures. The addition of Mo reduces the amount of graphite, while addiction of W increases the amount of graphite. The influence of each alloying element on graphite formation could be estimated by carbon solubility and the composition of the residual liquid.