Carbides isolated electrolytically from molybdenum steels, heat-treated at 700° or 900°C, have been examined by X-ray, chemical, and thermo-magnetic analyses, and by electron-microscopic observations, with the object of elucidating the nature and occurrence of carbides in molybdenum steels, and to determine the carbide phase diagrams of molybdenum steel, below and above the eutectoid temperature. Five kinds of carbides, M₃C, M₂₃C₆, ξ, M₂C and M₆C have been found in molybdenum steels. These carbides have been classified into three groups according to their stability, (i) M₃C, ξ and M₆C are stable carbides, (ii) M₂₃C₆ is a metastable carbide, and (iii) M₂C is intermediate one. The chemical composition and the crystal structure of these carbides have been determined as follows;
M₃C : orthorhombic, 6.67%C, 0 to 2%Mo
M₂₃C₆ : face-centered cubic, 5.0 to 6.0%C, 6.0 to 13.0%Mo
ξ : probably orthorhombic, 5.5 to 6.0%C, 25 to 37%Mo
M₆C : face-centered cubic, 2.5 to 2.8%C, 45 to 62%C
M₂C : hexagonal, about 6.0%C, 70%Mo.