Influence of mo and w on high temperature hardness of M7C 3 carbide in high chromium white cast iron

Abstract

The influence of Mo and Waddition on the high temperature hardness of M7C3carbide was investigated using unidirectionally solidified hypereutectic cast iron with 25 mass%Cr. Concentrations of alloying elements in primary M7C3 carbide were measured by EDS. As Mo or W content of the cast iron increases, the concentration of Mo and Win 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 M7C 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 M7C3 carbide were substituted by Mo or W which has larger atomic radius than Fe. In all the specimens, the hardness of M7C3 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 M7C3 carbide increases. Thus, the dissolution of Mo or W atom into M 7C3 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 M7C 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 M7C3 carbide. © 2014 The Japan Institute of Metals and Materials.