Patent ID: 8128867

Claim:
A method for producing a cemented carbide material comprising a surface layer and a core that has a grain size gradient structure where the mean hard grain size of the surface layer is larger than that of the core, and a concentration gradient of binder metal wherein the concentration of the binder metal in the surface is smaller than that of the core, comprising the steps of: subjecting a WC—Co system compact to a treatment for producing M 3 C type double carbide (wherein M comprises M1 and M2; M1 represents one or more elements selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W; and M2 represents one or more elements selected from the group consisting of Fe, Co and Ni) as a main component of the surface portion wherein the step of subjecting a WC—Co system compact to a treatment for containing M 3 C type double carbide is carried out by dipping the WC—Co system compact in a solution for supplying both of W and an oxidizing agent and reducing heat treating the compact at a vacuum atmosphere, subjecting the resulting WC—Co compact containing M 3 C type double carbide to a carburization treatment at a temperature of 800 to 1100° C. to decompose said M 3 C type double carbide; subjecting the carburized compact to liquid phase sintering at a temperature of more than 1350° C. to form a sintered body; and coating a surface layer of the sintered body with a compound containing boron and/or silicon and subjecting the coated sintered body to a diffusion heat treatment at a temperature within a range from 1200 to 1350° C.