Keiichi Kobori

Micro-grained cemented carbide with high strength

Abstract The present work is concerned with further improvement of the strength of ultra-fine grained cemented carbide. It has been reported already that the strength of the alloy is strongly affected by its structural defects, e.g. pores, Co pools, extraordinary coarse carbide grains or impurity compounds. In this work, the effects of carbide grain growth inhibitors. Co contents and impurities of starting powders on the strength of the alloy have been investigated in relation to the structural defects appearing on the fracture surface of transverse-ruptured specimens. The results obtained showed that the amount of impurities of starting powders most closely affected the strength of the alloy, and an extremely high-strength cemented carbide having transverse-rupture strength of over 500 kgf/mm2 was obtained by using highly purified WC powder.

Studies on the Development of Al2O3-SiC Ceramics

At first, oxidation resistance of Al2O3 based ceramics having a single addition of different carbides was investigated, with the result that the ceramics containing SiC in powder(SiC) or whisker(SiC(w)) form showed the highest resistance to oxidation. Attending to this result, the following studies were made. Some properties of Al2O3-SiC ceramics was first studied. Then the improvement of sinterability of this ceramics due to adding SiO2 was studied. Next, cutting performance of Al2O3-SiC ceramics was examined in comparison with the other ceramics tools, and it was made clear that flank wear of Al2O3-SiC tool became larger and smaller than that of Al2O3-TiC tool and Al2O3- SiC(W) one, respectively. Finally, interfacial reaction between Ni and the three sorts of ceramics as above was studied to describe the phenomenon. The outlines of these results were shown.

Improvement of Sinterability of Al2O3-SiC Ceramics

The Al2O3-SiC mixed powder with additional SiO2 powder and pre-oxidized Al2O3-SiC mixed powder containing SiO2 were hot-pressed in Ar. The sinterability of these starting powders were examined comparing with the Al2O3-SiC mixed powder free from SiO2. The structures and transverse-rupture strength (T.R.S.) of hot-pressed ceramics were also examined.It was found that the sinterability of Al2O3-SiC mixed powder was markedly improved with containing over 2-3% SiO2. This was considered to be due to mullite formation and appearance of mullite-SiO2 eutectic liquid during the period of rising temperature to hot-pressing one. With increasing SiO2 contents, the microstructure became locally coagulated and T.R.S. decreased slightly. As the fracture source, an even area of φ70-80 μm, probably due to the coagulated structure, was observed. These results were in detail discussed.

Structural Defects in P30 Cemented Carbide

The microstructural defects acting as fracture source in P30 alloy, prepared by the use of recycled starting powder, were studied. Special defects, that is, pores accompanied by the βt layer (A1 defect) and binder pools to which βt layer attached (A2 defect) were often observed in sintered alloy and HIP-treated alloy, respectively. The strength of HIP-treated alloy was sharply affected by the appearance of the A2 defect. It was confirmed that the formation of A1 defect was caused by the segregation of extremely fine particles of impurity oxides, etc., where the segregation took place inevitably in the mixed powder. The result of model experiments showed that impurity atoms dissolved in the binder phase at the periphery of pores and then decreased the solid-solubility of W. It was suggested that the concentration gradient of W, Ti and Ta solutes in the liquid binder was produced near the pore, leading to the formation of the βt layer as above mentioned.