高性能超硬工具の開発

Abstract

WC-Co cemented carbide with grain size less than 1μm is a very important tool material because it has high hardness and strength, and easily yields mirror finish and a sharp edge. On manufacturing the alloy, grain growth control is necessary. This report introduces effects of duplex addition of VC and Cr3C2 as a grain growth inhibitor.For investigating basic characteristics, submicro-grained WC-12mass%Co alloys with various amount of VC and Cr3C2 and with medium carbon content were used as a specimen which were heated up in vacuum to certain temperatures and immediately cooled. Some of them were fully sintered at 1380°C for 1 hr. They were examined by density measurement, TG-DTA, SEM, HRTEM, etc. Coarse-grained alloys were also used for analyzing binder composition by EPMA. Mechanical properties were measured using two series of submicro-grained specimens as above. One has fixed Cr3C2 content and varied VC content. The other is vice versa. They were vacuum-sintered at 1380°C for 1hr and HIP treated.Major results are as follows. (1) Similar to the alloy mono-doped with VC, duplex addition of VC and Cr3C2 (a) lowered liquidus temperature, (b) retarded densification from solid stage of sintering and abruptly increased density later, (c) remarkably controlled grain growth from solid stage, (d) formed micro-facets on WC grains composed of WC(1010) and WC(0001), and segregation of V mainly on WC(0001), and (e) decreased W content in the binder phase.(2) In the alloy series with fixed Cr3C2 content, grain size decreased with increasing VC addition to 0.5mass% and then was saturated, resulting in the corresponding change in hardness and fracture toughness. Bending strength and compressive strength had a peak at 0.2mass% and 0.5mass% addition of VC, respectively.(3) In the alloy series with fixed VC content, change in grain size was similar to that of the other alloy series though the critical value was 0.6mass%Cr3C2, and hardness and fracture toughness showed the same tendency as above. Bending strength was almost constant, and compressive strength increased up to 0.6mass%Cr3C2, then was saturated in the scope of this experiment. These changes in strength could be understood by considering the change in grain size, the fracture mechanism and the (V,W)C phase which precipitates in the binder phase over 0.5mass% addition of VC.