HIGH-TEMPERATURE TUNGSTEN CARBIDE WITH HEAT TREATMENT

characteristics are at the level of the normal grades. As a result of this the life of alloys of the series S, KS, and K with cyclic loading is increased by a factor of ten, and for articles made from them (dies, upsetting tools) by a factor of three to five. In order to achieve these indices in the operation of articles made of hard alloys of the normal grades they are given a strengthening heat treatment the technology for which was developed In the Institute of Superhard Materials of the Ukrainian Academy of Sciences [3]. The essence of the heat treatment involves quenching hard alloy articles. The heat temperature for hardening and the cooling rate depend on the material structure and composition and is geometric dimensions. After quenching there is a marked increase in alloy mechanical properties. The strengthening mechanism for hard alloys with heat treatment continues to be a subject of discussion [4, 5]. At the same time it is possible to assume an established fact that in quenching hard alloys three parameters change simultaneously: the degree of tungsten solubility in cobalt increases; the juxtaposition and bonding of carbide grains decrease; residual tensile stresses in the surface layers are reduced and converted into compressive stress. Due to the first factor there is an increase in the strength of the bonding phase, and due to the second there is an increase in material ductility characteristics. The third factor (residual microstresses) will have a marked effect with the presence of large stress gradients, that is hard alloy articles of considerable size and complex configuration. It was of considerable interest to determine to what extent features of the change in properties established for normal grades with heat treatment may be used in order to improve the mechanical properties of hard alloys based on high-temperature tungsten carbide which in the original condition (after sintering) have high deformation parameters and life indices with cyclic loading. Three series of specimens were prepared based on high-temperature tungsten carbide differing in the duration of grinding. In alloys of series S the average grain size dwc varied within the limits from 2.0-2.2 pm, in alloys of the KS series from 3.6-4.6/~m, and in alloys of series K from 8.6-9.5/~m. The cobalt content differed within a series a series of alloys. The standardized characteristics for hard alloys are presented in Table 1. Sintered specimens had quite a dense structure (the porosity did not exceed 0.1%), and they did not contain free carbon or ~l-phase. Exceptions were alloys VK15K and VK20K in which up to 0.2% graphite was detected. Half of the specimens of each grade were heat treated, the schedule for which was established taking account of the cobalt content in them and the WC grain size. After heat treatment specimens were ground with a diamond wheel to a size of 4 x 4 x 35 mm simultaneously with specimens in the original condition. Grinding conditions were the optimum for this given class of materials [6].