Relations between Impact Strength and Structural Defects in WC-Co Cemented Carbide

The impact strength of WC-Co cemented carbide was studied in relation to the microstructural defects acting as a fracture source. The WC-10%Co and WC-20%Co two-phase alloys having mean grain size of about 1.7μ, were vacuum-sintered and ground with diamond wheel for obtaining the specimen without notch (the dimension; about 4×4×48 mm). The impact strength test was done at room temperature using a Charpytype impact machine (capacity; 0.4 kg⋅m, span; 40 mm). The impact strength, impact-transverse-rupture strength, stress-strain curve in impact test, and the dimension and location of the defect appearing on the ruptured surface were measured. The correlations between these values were examined, comparing with the result (in static-transverse-rupture test) previously reported.The results obtained were as follows: 1) Such defects as residual micropores or coarse WC grains were found to act as a source of impact fracture, in the same way as they were in static-rupture test. However, the true impact strength (It) didn't directly correlate to the dimension of defects (2a). 2) Then, a close correlation was found between It and σIm (impact-transverse-rupture strength), and also between σIm⋅f and √a: therefore, It can be expressed by the following equation.It-1/2⋅f-1=(l/18E)-1σIo-1+2{(l/18E)-1σIo-1ρI-1/2}×√a(where f; a function of the location of the defect, l; span, E; Young modulus of the alloy; σI0, the impact-trans-verse-rupture strength of the matrix around the defects, ρI; the effective crack radius of the defects). Thus, it was made clear that It was directly controlled by the dimension and location of the defects, the same as in the static-transverse-rupture strength. 3) By decreasing the 2a value down to about mean grain size, the It value of cemented carbide would sharply increase in particular in the alloy with comparatively low Co contents, because the defect size is apt to increase as the cobalt content decreases.