OBRABOTKAMETALLOV Vol. 23 No. 3 2021 MATERIAL SCIENCE EQUIPMENT. INSTRUMENTS 6 4 4 Results and Discussion Determination of the causes of destruction of the teeth of bits manufactured using standard technology The experimental testing of the fi rst samples of KNSh 40×25 mm roller cones was characterised by low wear resistance during penetration, compared to that of the imported samples produced by Atlas Сорсо (Sweden). Fig. 1 shows a sample of KNSh 40×25 mm bit after testing for penetrating granite to a drilling depth of 18 cm (a), and an enlarged image of the porous surface of the tooth with depressions forming the so-called “reptile skin” surface (b). a b Fig. 1. Seven-cone drill bit type KNSh40×25 manufactured by NPO, after operational tests during drilling of granite rock; drilling depth 18 cm (a); the condition of the surfaces of the teeth of the crown, characterized by porosity with depressions known as the formation of the “reptile skin” surface (b) Fig. 2 presents an enlarged image of the surface area at the boundary of the tooth wear area with the tooth surface before wear, which shows that along the boundary there is a separation of whole clusters of grains of the hard VK (WC-Co) alloy as a result of wear. The “reptile skin” type surface is the result of maximum tensile stresses occurring at individual points of contact with the asperity of the rock. Fig. 3 provides a schematic example, explaining the mechanism of crack formation on the tooth surface. According to the source, a protruding part of the rock is pressed into the tooth surface, creating localised stress on its surface. When this procedure is repeated several times, small cracks aggregate, eventually forming a “reptile skin” structure. Fig. 2. Cross-section of the boundary of the surface of the tooth wear area with the tooth surface before wear: left – original surface; right – surface as a result of wear
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