Teruyoshi Tanase

Computational study on grain growth in cemented carbides

Micro-grained cemented carbides are widely applied to the tools such as solid endmills and micro-drills. The knowledge on grain growth of carbides is very important for manufacturing micro-grained cemented carbides. In the present study, continuous and discontinuous grain growth in such cemented carbides is investigated using the Monte Carlo computer simulation technique. The Ostwald ripening process (solution/re-precipitation) and the grain boundary migration process are assumed in the simulation as grain growth mechanism. The effects of liquid phase fraction, grain boundary energy and an implanted coarse grain are examined. The results of these simulations qualitatively agree with experimental ones and suggest that distribution of liquid phase and WC/WC grain boundary energy as well as contamination by coarse grain are important factors controlling discontinuous grain growth in cemented carbides.

Some properties and cutting performance of polycrystalline cubic boron nitride with no additives

Abstract Hexagonal BN disks made by chemical vapor deposition were treated under ultra high pressure of 6.8 GPa and at temperatures from 1800 to 2500 °C for 1.8 ks. When treated at temperatures above 2100 °C, only a c-BN phase was detected. The sample treated at 2100 °C had a homogeneous microstructure having fine grain size less than 1 μm, whereas that treated at 2300 °C had a heterogeneous one with some large grains. The sample treated at 2500 °C showed remarkable grain growth. Some physical properties and cutting performance of these compacts were investigated. Then hardness clearly decreased at 2500 °C and the thermal conductivity was higher when synthesized at 2500 °C than at 2100 °C. The fine grained compact of 2100 °C exhibited better wear resistance than the coarse grained one of 2500 °C in cutting of Co base super alloy and cemented carbide.

Observations an Peculiar Structures Appearing around Fracture Source in WC-Co Base Cemented Carbides

Peculiar fracture sources and related surface structures formed around fracture sources in WC-Co, WC-TiC(TaC)-Co and WC-VC-Co alloys were examined by scanning electron micrography.It was found that: (1) In a WC-Co alloy, a flat region or a stepped region appearing around the carbide defects with the size less than about 8, um, would closely be related to the domain (or grain) bounda-ry of binder phase. The stepped region formed in high cobalt alloys seems to lower the strength of the alloy. (2) In a WC-TiC(TaC)-Co alloy, a stepped defect formed by the addition of a small amount of TiC(TaC) is found to be a sort of pore with irregular shape. (3) In an ultra fine grained WC-VC-Co alloy, a peculiar source would be a region having densely distributed micropores.

Effect of Specimen Volume on Transverse-Rupture Strength of WC-10%Co Cemented Carbide

The transverse-rupture strength of conventional WC-10%Co high carbon two-phase alloys (mean grain size is about 1.4μ) was measured as a function of width (w) or thickness (t) of the specimen. Then, the observed strength was examined in relation to the structural defects which appeared on the fracture surfaces as a fracture source.Results obtained were as follows. (1) The increase of w or t resulted in the decrease of strength, showing the so-called volume effect. (2) The strength decrease was accompanied by the increase of the average defect (or source) dimension, and by that of the average distances from the tension surface and also from the span center to the defect. It was also accompanied by the change in the sort of defects. Thus, the volume effect as above was considered not to be simply related to usually cited risky volume, but to be related to the volume containing a defect which actually acts as a fracture source. (3) The effect of specimen-volume on the strength of conventional cemented carbide could not be satisfactorily explained by Weibulls equation.