The WC-(10, 20, 30)%Co alloys were minutely vacuum-sintered, so that the defect acting as a fracture source in each alloy might almost consist of coarse WC grains smaller than about 20μ. Two sorts of cooling rates after sintering, about 15°and 0.5°C/min, were adopted in the temperature range from 1360°to 1300°C for 10%Co alloy, 1350°to 1300°C for 20%Co alloy and 1340°to 1300°C for 30%Co alloy, respectively. Two sorts of specimens different in cooling rates in each alloy were ground with a diamond wheel, and transverse-ruptured according to JIS. After the test, the dimension (2a) and location of coarse WC grains as a fracture source were measured on the ruptured surface.
The results obtained were as follows: 1) The average transverse-rupture strength (σm), and also the external stress (σd) corresponding to σm were always higher in rapidly cooled specimen (R-specimen) than in slowly cooled specimen (S-specimen). 2) The 2a of R-specimen was generally smaller than that of S-specimen, showing that of an anomalous grain growth of WC also occurred during the time of cooling specimens, when the cooling rate was decreased. 3) The distance from the tension surface to the defect (Δt) of R-specimen was larger in 10%Co alloy than that of S-specimen. However, the difference in Δt was hardly observed in (20, 30)%Co alloys. It was found in a slowly cooled 30% Co alloy that the location of the defect was always on the domain boundary of binder phase. 4) The increase in Δm and σd due to increasing cooling rate was considered to be affected mainly by the increase in the strength of WC-Co matrix including that of the domain boundary, and partially by the changes in 2a and Δt.