OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 2 2024 Fig. 3. Dependences of the specimen change in length Δ (after heating and cooling) on cobalt content As a result of processing the data shown in Fig. 2, a dependence of the specimen change in length Δ (after heating and cooling) on cobalt content was obtained (Fig. 3) by linear approximation (coefficient of determination R² = 0.897). Since the testing time is the same for all specimens, the residual elongation corresponds to the average oxidation rate. Thus, the average oxidation rate is inversely proportional to the cobalt content and increases with increasing content of tungsten carbides. This dependence is close to the linear law in the considered range of changes in the cobalt content in the alloy. Data on the effect of cobalt content on the oxidation rate for the tungsten carbides considered are consistent with the results of previous studies obtained for: WC-6Co and WC-12Co [1]; WC-6Co, WC-10Co and WC-18Co [12]; WC-15Co and WC-25Co [18] under isothermal conditions. Figs. 4 and 5 show curves of the first and second derivatives (for heating and cooling, respectively, see Fig. 2) of the dependences of expansion Δl on temperature T for WC-Co specimens with different cobalt content for the range of 550–850 °C, which is of greatest interest. The graphs generally confirm the inverse dependence of the average oxidation rate on cobalt content based on residual elongation. The second derivatives have two characteristic inflections: around 630 °C (T1) and 800 °C (T2). Before T1, the first derivative has a horizontal section, and the values of the second derivative are close to zero. Closer to T1, both derivatives begin to increase significantly, which indicates the emergence of new chemical processes (mainly, carbide oxidation with WO3 formation and cobalt oxidation) that affect the change in size. This point corresponds to the onset of oxidation of the cemented carbides. Around T2, an extremum at the maximum value of the second derivative of the heating line, as well as an extremum of the second derivative of the cooling line are observed. The obtained temperature values at T2 correlate with the temperature values of characteristic points on the curves of the heat flux changes obtained in previous studies [3, 5–7] using the differential scanning calorimetry (DSC) method. T2 corresponds to the transition to the active oxidation of the carbide, after which the ratio of CoWO4 in the total weight of the oxide layer probably begins to increase. The beginning of the graphs’ slowdown after T2 can be explained by the higher density of CoWO4 compared to WO3. Domination of CoWO4 at high temperatures is confirmed by spectral analysis of the oxide layer after heating WC-Co [3]. а b Fig. 4. Graphs of the first (a) and second (b) derivatives of the relationship of expansion Δl on temperature T for WC-Co specimens with different cobalt content obtained during heating
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