OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 2 2024 Results and discussion Table 1 presents the results of heating the specimens of WC-Co cemented tungsten carbides with different cobalt content by weight in a laboratory furnace to 900 °C. The top row of the table shows the initial appearance of the specimens, and the bottom row – its appearance after heating and cooling. As we can see from the photographs, all WC-Co cemented tungsten carbides are highly oxidized. The oxide layers are of considerable thickness and in some cases peel off from the base. The cross sections of these layers correspond to the shape of the surfaces on which they are formed. All specimens changed color. None of the specimens had traces of interaction with the corundum substrate. The experimental results confirm that WC-Co cemented tungsten carbides highly oxidize with the material structure being destroyed when heated to temperatures above 800 °C. These results are consistent with the data obtained by other researchers for other cobalt contents [3, 5–8]. Ta b l e 1 Results of heating WC-Co specimens with different cobalt content in air in a laboratory furnace Со, % 3 6 8 10 15 20 20 °С 900 °С Fig. 1 shows the results of heating a specimen of a WC-Co cemented tungsten carbides with a cobalt content of Co = 8 % in a dilatometer to a temperature of 1,150 °C. The appearance of the specimen before heating is shown in Fig. 1, a and after completion of the experiment in Fig. 1, b. The last photo demonstrates that the specimen has significantly deteriorated and lost its shape. The surfaces of the specimen holder and push rod were covered with a blue coating. Fig. 1, c shows the relationship between the specimen expansion Δl and temperature T. The curve shows three characteristic temperature ranges: slight expansion up to about 650 °C, a sharp increase in size from 650 °C to approximately 950 °C, and destruction of the specimen at a higher temperature. In all these ranges there are sections of the curve close to the linear law. In the first range, changes are caused almost exclusively by the thermal expansion of the specimen with a slightly increasing TCLE [35]. In the second range, thermal deformation is accompanied by the growth of the oxide layer, which begins to dominate. If in the range of 20–650 °C the specimen increased by only 0.096 mm, in the range of 650–1050 °C the specimen increased by 1.534 mm. In the third temperature range, the increase in specimen size slows down and then the specimen shortens, which can be explained by loss of shape and destruction of the specimen. Fig. 2 shows relationships between the expansion and temperature for WC-Co specimens with different cobalt content, obtained by heating to 850 °C and subsequent cooling to room temperature. All the curves are similar and have a characteristic hysteresis, indicating permanent elongation due to the presence of oxide layers. The section of the curves corresponding to cooling looks like a mirror image of the heating section. The first two characteristic temperature ranges of slight expansion and a sharp increase in specimen size are observed.
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