EFFECT OF CR CONTENT ON PROPERTIES OF (MO , NI) BORIDE BASE HARD ALLOYS

Effects of Cr on the mechanical properties, corrosion and wear resista nces, and phase formation of Mo2NiB2 base hard alloys were studied usi ng the alloys whose compositions were Ni-6 mass%B-58.6 mass%Mo-(0-15) mass%Cr. The alloys consist of a Mo2NiB2 type ternary boride as a hard phase and a nickel base binder alloyed with Cr and Mo, both of which are formed by reaction boronizing sintering. The transverse-rupture st rength (TRS) of the alloys depended strongly on the microstructure, wh ich varied significantly with Cr content. The maximum TRS of 2.3 GPa a nd a hardness of 87 R(A) was obtained at 10 mass%Cr content. The struc ture of the boride phase changes from the orthorhombic M3B2 (M: Metal) to the tetragonal M3B2 with increasing Cr content. The highest TRS va lue obtained at 10 mass%Cr was attributed to the formation of fine M3B 2 with tetragonal structure. The corrosion resistance of the alloys in a10 mass%HF and a 10 mass%HCl solutions was superior to the corrosion resistance of a WC-10 mass%Co cemented carbide, PM high speed steel, stainless steels such as JIS SUS405, 440C, 316L and 304 and a tool ste el JIS SKD11, but close to that of Hastelloy C. Moreover, in a 10 mass %HNO3 solution, Cr-containing alloys also showed good corrosion resist ance with increasing Cr content. Abrasive and sliding wear resistances of the alloys were better than those of PM high speed steel, stainles s steels and SKD11, and comparable to that of cemented carbide.