抄録
In this study, the influence of Mo and W addition on the high temperature hardness of M7C3 carbide was investigated using unidirectionally solidified hypereutectic cast iron with 25mass%Cr. Concentrations of alloying elements in primary M7C3 carbide were measured by EDS. As Mo or W content of the cast iron increased, the concentration of Mo and W in the carbide increased and that of Fe decreased. However, the Cr content was more or less the same as about 60mass% in spite of the increasing Mo and W contents. The XRD results showed that the lattice constant of the M7C3 carbide changed and the volume of the unit cell increased with the increase in the Mo or W content. It is thought that the Fe atoms in the M7C3 carbide were substituted by Mo or W which has a larger atomic radius than Fe. In all the specimens, the hardness of the M7C3 carbide was about 1600HV0.3 at room temperature, and it decreased gradually with increasing test temperature. The decreasing rate of the carbide hardness in the cast iron with Mo or W was small at high temperature ranges in comparison with that of alloy-free cast iron. In addition, this decreasing rate at high temperatures decreases with increasing Mo or W concentration in the M7C3 carbide. Therefore, it can be said that the dissolution of Mo or W atom in the M7C3 carbide is very effective for maintaining the high temperature hardness of carbide. It was also found that an excess concentration of both elements in the M7C3 carbide caused the increasing rate of hardness to decrease. The fracture toughness of the M7C3 carbide at room temperature was measured using an indentation fracture method, and the values were very similar among the carbides regardless of Mo and W concentration in the M7C3 carbide.
