OBRABOTKAMETALLOV Vol. 23 No. 3 2021 MATERIAL SCIENCE EQUIPMENT. INSTRUMENTS 7 2 5 The bulk area of powders was determined by measuring the volume of powder of known mass in a graduated cylindrical vessel. After sintering, the representative samples were ground on a diamond wheel and polished using standard metallographic technology. The density of the samples after sintering was measured by hydrostatic weighing. The average grain size of sintered samples was calculated by the secant method using optical photographs of the thermally etched surface. The strength was determined by the method of three-point bending of ground specimens of 4 ± 0,1 × 3 ± 0,1 × 40 mm using a compression testing machine (hydraulic press) manufactured by LLC “NIKTSIM Tochmashpribor” (Armavir). Evaluation of microhardness HV and crack resistance K1c was carried out on a universal hardness tester mod. Duramin-500 by Stuers A/S (Denmark) with an automatic force sensor by indenting the polished surface with a Vickers pyramid at a load of 98.07 N. The length of the indentation diagonals and the extent of cracks for the calculation of microhardness and fracture toughness were evaluated using an optical microscope. The ceramic inserts were tested during cutting on a MULTUS B300-W lathe made by Okuma (Japan) in the body cutting tool production shop of LLC “PK MION” (Tomsk, Russia) (Fig. 3). The workpiece was clamped in a hydraulic chuck by the tail section, which was coaxial with the machined surface and had a diameter of 70 mm and a length of 50 mm. a b Fig. 3. Views of the machine tool working area with a fixed workpiece (a) and the mandrel (b) with a ceramic insert Performance tests of ceramic cutting inserts were carried out in two stages. All tests were carried out during longitudinal turning without impacts in dry conditions. When the cutting edge chipped, the insert was rotated by ≈30° or flipped to the opposite side. Cutting modes were taken from sources [2, 6–8, 12, 14, 15, 26]. The cutting path at which chip formation is observed was taken as a criterion of tool performance. The onset of failure indicated brittle fracture of the cutting edge and cessation of chip formation. In the first test phase, ceramic inserts with cutting edges without additional processing were used, i.e. the microgeometry was generated in a flat grinding process. To establish the limits of permissible variation of modes during turning we used the following values: cutting speed V = 200; 300; 400 m/min; feed rate S = 0.05; 0.1; 0.2; 0.4; 1.0 mm/rev; depth of cut t = 0.1; 0.2; 0.4 mm. At the second stage, the performance was tested at constant modes: V = 200 m/min; S = 0.25 mm/rev; t = 0.1 mm. At the same time, an insert with a chamfer of 0.2×45° mm formed on the cutting edge was used. Results and discussion The chemical composition of AISI 5135 steel complies with GOST 4543-2016 “Structural alloy steel production. Specifications”. In the initial state, AISI 5135 steel has a ferrite-perlite structure, and hardness is HRC 21–26, and after quenching – martensite with residual austenite (5–8 %). Hardness measurements of the quenched steel showed that there are variations in the HRC 43–48 range along the cross-section depth.
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