Effects of Cr on the mechanical properties and phase formation of WCoB base alloys were studied using the model alloys whose compositions were Co-1.93mass%B-33.3mass%W-(0-20)mass%Cr. The new alloys consist of a WCoB type ternary boride as a hard phase and a cobalt base binder alloyed with Cr, W and other elements, 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 highest TRS of 2.85GPa obtained at 15mass%Cr was attributed to the refinement and good distribution of WCoB boride. Moreover, the TRS of the alloy with 15mass%Cr still kept a value of about 1.8GPa at 1173K. The weight gain of the alloy oxidized in air at 1173K was less than that of SKD11 and close to that of SUS304.
Corrosion resistance of the alloys in a non-oxidizing 10mass% HCl solution was better than those of a PM high speed steel, stainless steels such as JIS SUS 405, 440C, 316L and 304 and a tool steel JIS SKD11, and close to that of Hastelloy C. On the other hand, in an oxidizing 10mass% HNO₃ solution the alloys showed better corrosion resistance than a cemented carbide of WC-10mass%Co, a PM high speed steel and SKD11, and were almost same as stainless steels and Hastelloy C.