OBRABOTKAMETALLOV Vol. 23 No. 3 2021 MATERIAL SCIENCE EQUIPMENT. INSTRUMENTS 6 4 4 Fig. 10. Results of elemental analysis of the synthesized sample of pure tungsten metal powder During the tooth pressing stage, a number of issues were addressed to eliminate the tendency of pressing cracks, resulting from the increased dispersion of the pressing powder [20]. Characteristic cracks were formed perpendicular to the pressing vector, due to the delamination of the pressing powder, as a result of the so-called “spring eff ect”, associated with the occurrence of a pressure gradient. Fig. 11 demonstrates delamination cracks occurring during the pressing of teeth, made of highly dispersed pressing powders. When loading the upper working surface of the ballistic tooth with the upper punch, a horizontal annular layer of compacta was found on the lateral cylindrical surface, located near the upper spherical surface. The tooth surface, formed in contact with the upper punch, was found to be characterised by the formation of a porous structure with depressions, that is, with the formation of the “reptile skin”. In order to prevent the occurence of delamination cracks in the area of the upper working part of the tooth, where the greatest strength and wear resistance are required, and to exclude the formation of the “reptile skin”, the tooth shape was changed from ballistic to semi-ballistic [21, 22]. As a result of changing the shape, previously subjected to the specifi ed press defects, the upper surface of the tooth came into contact with the lower punch. In this case, the upper punch with the modifi ed surface shape formed the lower part of the tooth. Fig. 11 shows the samples of stamped semi-fi nished products of ballistic and semi-ballistic teeth. Subsequent operational tests confi rmed the correctness of changing the tooth shape to semi-ballistic. Figs. 12 and 13 present the condition of tooth face surfaces when manufacturing in ballistic and semiballistic shapes. The physical and chemical parameters of carbide tooth samples, sintered in a hydrogen furnace under various modes, were tested [23, 24]. The test results showed the compliance of the obtained tooth samples with the normative requirements for the VK10-KC alloy (Table 1). Compared to Atlas Copco (Sweden) bits, the hardness and the grain size diff er by no more than 2.5 %. The hardness of the teeth on the Atlas Copco tool is 88.3 HRA. This is almost identical to the hardness of the teeth, obtained by the developed technology. The average grain size is also almost the same; for Atlas Copco teeth it is 4.1 μm. This also does not diff er signifi cantly from the grain size of the teeth obtained using the developed technology.
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