In this study, the influence of Mo and W addition on the high temperature hardness of M
7C
3 carbide was investigated using unidirectionally solidified hypereutectic cast iron with 25mass%Cr. Concentrations of alloying elements in primary M
7C
3 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 M
7C
3 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 M
7C
3 carbide were substituted by Mo or W which has a larger atomic radius than Fe. In all the specimens, the hardness of the M
7C
3 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 M
7C
3 carbide. Therefore, it can be said that the dissolution of Mo or W atom in the M
7C
3 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 M
7C
3 carbide caused the increasing rate of hardness to decrease. The fracture toughness of the M
7C
3 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 M
7C
3 carbide.
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