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The influence of Mo and W addition on the high temperature hardness of M 7 C 3 carbide was investigated using unidirectionally solidified hypereutectic cast iron with 25 mass%Cr. Concentrations of alloying elements in primary M 7 C 3 carbide were measured by EDS. As Mo or W content of the cast iron increases, the concentration of Mo and W in the carbide increased and that of Fe decreased. However, the Cr content was almost the same as about 60 mass% in spite of increasing the Mo and W contents. It was found from the XRD results that the lattice constant of M 7 C 3 carbide changed and its attendant volume of a unit cell increased as the Mo or W content was increased. From this point of view, it can be considered that Fe atoms in the M 7 C 3 carbide were substituted by Mo or W which has larger atomic radius than Fe. In all the specimens, the hardness of M 7 C 3 carbides are about 1600HV0.3 at the room temperature, and it decreases gradually with a rise of the test temperature. The decreasing ratio of carbide hardness becomes smaller at high temperature as the Mo or W concentration in M 7 C 3 carbide increases. Thus, the dissolution of Mo or W atom into M 7 C 3 carbide is very effective to maintaining the high temperature hardness of carbide. However, the increment of the hardness became smaller at higher Mo and W contents, so that an excess addition of both elements gave less effect than expected. The fracture toughness of M 7 C 3 carbide at the 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 7 C 3 carbide.

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Two series of hypoeutectic high chromium cast iron specimens containing 16 mass% Cr and 26 mass% Cr with and without Mo were prepared to study the variation of hardness and volume fraction of retained austenite (V) during repeated tempering. In the as-hardened state, the hardness did not change in the 16 mass% Cr cast irons, but it increased gradually in the 26 mass% Cr cast irons as their Mo content increased. The V rose gradually with an increase in the Mo content of both series of cast iron specimens. In the tempered state, the hardness decreased and then increased with a successive number of tempering cycles. In other words, a secondary hardening was evident due to the precipitation of secondary carbides in the austenite and the transformation of destabilized austenite into martensite during cooling. The hardness increased with rising tempering temperature. The V decreased gradually with repeated tempering and increasing tempering temperature. The maximum tempered hardness (H Tmax ) was obtained at 723-798 K after triple tempering. The H Tmax increased proportionally with a rise in Mo content in both the 16 and 26 mass% Cr cast irons. The highest values of H Tmax were 900 HV30 in 16 mass% Cr and 950 HV30 in 26 mass% Cr cast irons, respectively.