Effect of Cr Content on Properties of (Mo, Ni) Boride Base Hard Alloys

Abstract

Effects of Cr on the mechanical properties, corrosion and wear resistances, and phase formation of Mo₂NiB₂ base hard alloys were studied using 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 strength (TRS) of the alloys depended strongly on the microstructure, which varied significantly with Cr content. The maximum TRS of 2.3 GPa and a hardness of 87 R_A was obtained at 10 mass%Cr content. The structure of the boride phase changes from the orthorhombic M₃B₂ (M: Metal) to the tetragonal M₃B₂ with increasing Cr content. The highest TRS value obtained at 10 mass%Cr was attributed to the formation of fine M₃B₂ with tetragonal structure.
The corrosion resistance of the alloys in a 10 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 steel JIS SKD11, but close to that of Hastelloy C. Moreover, in a 10 mass%HNO₃ solution, Cr-containing alloys also showed good corrosion resistance with increasing Cr content.
Abrasive and sliding wear resistances of the alloys were better than those of PM high speed steel, stainless steels and SKD11, and comparable to that of cemented carbide.