Grain Size of WC in Sintered WC-VC-Co Alloys

Abstract

The restriction effects of the additional VC on the grain growth of WC in WC-Co alloy were in detail investigated in relation to the grain size of starting WC powder, and Co and carbon contents of the alloy. The WC powder was prepared by heating the mixture of W powder (-0.5μ) and carbon black powder at 1000-1300°C for 1.5 hr in hydrogen atmosphere. The grain size of this WC powder was reduced to various sizes by ball-milling. WC-(0-2)%VC-(5-20)%Co alloys with various grain sizes and 'carbon contents were vacuum-sintered mainly at 1400°C for various times.The results obtained were as follows: (1) The restriction effect was found to be controlled almost by the ratio of VC/Co, and not by the ratio of VC/WC. Even under the same ratio of VC/Co, the effect was affected by the carbon content of the alloy. (2) When the mean grain size of starting WC powder exceeded a certain critical value, the addition of VC more than a definit value resulted in the normal grain growth of WC in VC contained alloys. However, in the case of the mean grain size smaller than the critical value, it resulted in the abnormal grain growth of WC (with fine grain size of WC in the matrix). For instance, in the usual WC powder having grain size larger than -0.3μ, addition of VC more than -5% in the binder checked the abnormal grain growth (mean grain size of WC in the matrix decreased with increasing amount of VC). However, in the fine grained WC powder having mean grain size smaller than -0.2μ, such a restriction method of adding VC was by this time invalid because of the occurrence of the abnormal grain growth, leading to the impossibility of obtaining micro-grained alloys. The above critical grain sizes (0.2-0.3μ) and the critical amount of VC were naturally considered to vary according to size distributions of the starting WC powder, sintering temperatures, Co contents, etc. (3) The three phase region and the compositions of the binder phase in WC-VC-Co alloys were also examined.