Papers by Author: Tsutomu Yamamoto

Abstract: WC-βt-Co alloys have been widely applied to cutting tools because they are superior to WC-Co alloys in oxidation, plastic deformation and adhesion resistances. In this study, the effects of HIP treatment, WC grain size and content of βt phase on bending fatigue characteristics were investigated for fine and coarse-grained WC-6.7mass%βt-10.4mass%Co alloys （6.7βt(F) and 6.7βt(C) alloys） and fine-grained WC-20mass%βt-11.3mass%Co alloy (20βt(F) alloy) by comparing with the case of fine-grained WC-10mass%Co alloy（10Co(F) alloy） without βt phase. The results obtained were as follows: 1) The bending fatigue life of HIP-ed 6.7βt(F) alloy was longer than that of normally sintered 6.7βt(F) alloy. The S-N curve of HIP-ed alloy showed a small gradient in a high stress range and a large gradient in a low stress range. 2) The fatigue life of HIP-ed 6.7βt(F) alloy remarkably decreased in the high stress range, but slightly decreased in the low stress range, compared with that of 10Co(F) alloy. 3) The fatigue life of HIP-ed 6.7βt(C) alloy was superior in all the stress range, compared with that of HIP-ed 6.7βt(F) alloy. 4) The fatigue life of HIP-ed 20βt(F) alloy was shorter than that of 6.7βt(F) alloy in all the stress range．The above-mentioned results were mainly discussed relating to the fatigue fracture origins such as pore, Co pool, agglomerate of βt grains, etc.

Abstract: Three cemented carbide specimens, i.e., WC-Co (Sample A), WC-Cr3C2-Co (Sample B) and WC-βt-Co (Sample C) in which βt indicates (Ti,W,Ta,Nb)C, are prepared by liquid phase sintering, and their corrosion behavior has been investigated by the electrochemical methods. The similar activation, passive state and over passive state regions as SUS304 stainless steel during the polarization test, and the preferential dissolution of Co were observed for all of the three specimens mentioned above. Both Cr3C2 and βt improved the corrosion resistance of WC-Co alloy, in which the effect of the former was better than that of the latter. Sample B also showed the best corrosion resistance in all the samples for the electrochemical impedance spectroscopy (EIS) test.