Influence of Thermal Cycling Modes on VK8 Hard Alloy Mechanical and Tribological Properties

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Hard alloys are popular materials widely used in the toolmaking industry. The refractory carbides included in their composition make carbide tools very hard (80 to 92 HRA) and heat-resistant (from 800 to 1000°С), so they can be used at cutting speeds several times higher than those used for high-speed steels. However, hard alloys differ from the latter in their lower strength (from 1000 to 1500 MPa) and the absence of impact strength; this is an urgent problem that needs to be solved. We study the influence of thermal cycling modes on the mechanical and tribological properties of VK8 (WC–8Co) hard alloy used in the manufacture of cutters and cutting inserts for metal working on metal-cutting machines. As the object of study, we select 5 × 5 × 35 mm rods made of VK8 (WC–8Co) alloy manufactured by powder metallurgy at the Dimitrovgrad Tool Plant. The following criteria are selected for evaluating the heat-treatment mode: Vickers hardness, flexural strength, and mass wear resistance (compared to the wear of as-received samples that were not heat treated). Plates in the initial state and after heat treatment (HT) are subjected to abrasion tests. Wear results are evaluated by the change in the mass of plates. Regularities of the influence of various time and temperature conditions of HT on the tribological properties of products made of VK-group tungsten hard alloys are determined. An increase in the number of thermal cycling cycles improves mechanical properties of the VK8 hard alloy such as strength and hardness. When repeating the cycle five times, an increase in abrasive wear resistance over the initial non-heat-treated sample is obtained. The elemental composition of the VK8 hard alloy changes insignificantly after thermal cycling; only a slight increase in oxygen is observed on the surface of the plates. The grain size after thermal cycling increases in comparison with the initial VK8 hard alloy. It is found that VK8 hard alloy thermal cyclic treatment leads to a change in the phase composition. X-ray phase analysis shows the presence of a large amount of α-Co with an hcp-type lattice on the surface of a hard alloy and a solid solution of WC in α-Co. A change in the cobalt modification ratio causes a decrease in microstresses. An analysis of the carbide phase structure state shows that the size of crystallites and microstresses changes after thermal cycling. The lattice constant of the cobalt cubic solid solution decreases, which may indicate a decrease in the amount of tungsten carbide and carbon dissolved in it. Statistical processing of the experimental results includes a calculation of the average value of the mechanical property, its dispersion, and standard deviation in the selected confidence interval.