OBRABOTKAMETALLOV Vol. 23 No. 3 2021 MATERIAL SCIENCE EQUIPMENT. INSTRUMENTS 4 2 The results of the experiments showed that with the conventional method of turning steel 20Cr13Ni18, the maximum wear of the inserts is observed after machining during 10–15 minutes. And with plasmaassisted machining, the maximum wear of the inserts is observed after machining during 25 minutes. It has been established that in the process of plasma-ultrasonic machining, the maximum wear of inserts (h = 1.0 mm) is achieved after 90 minutes. This is due to the fact that when using ultrasound for plasmaassisted machining of high-temperature steels, the formation of microchips in the contact zone occurs under the infl uence of ultrasonic vibrations by the cutting edge of the inserts. The cutting edge of the cutter receives both in the longitudinal and in the radial ultrasonic vibrations with a frequency of 18 kHz and an amplitude of the order of A = 4 μm, leading to additional deformation of the chip during its descent, which actually eliminates the contact of the chip with the cutting edge. At the same time, the presence of ultrasonic vibrations improves the conditions for sliding and chip fl ow in the zone of its formation, which makes it possible to reduce signifi cantly the friction of chips on the contact surfaces of the insert. Fig. 3 shows the wear curves obtained during the machining of high-temperature steel grade 20Cr25Ni20Si2(cast), where 1, 1ʹ – wear during conventional cutting t = 3 mm, Sl = 0.31 mm/rev: 2, 2ʹ –wear during plasma-assistedmachining t = 6 mm, Sl = 0.31 mm/rev, I = 250 A, U = 150 V: 3, 3ʹ – wear during plasmaultrasonic-assisted machining t = 6 mm, Sl = 0.31 mm/rev, I = 250 A, U = 150 V, f = 18 kHz, A = 4 μm: 1, 2, 3 –whenmachining with T5K10 grade hard alloy inserts: 1ʹ, 2ʹ, 3ʹ – when machining with VK8 grade hard alloy inserts. Depending on the cutting time, the wear of the insert along the end fl ank changes similarly to Fig. 2. In other words, when machining the above material with conventional turning, the wear of the insert is much greater during plasma-assisted and plasma-ultrasonic-assisted machining. An analysis of graphs 1, 1ʹ in Fig. 2 and 3 shows that, as in the turning of high-temperature steels with T5K10 and VK8 carbide inserts, the greatest wear of the inserts is observed when turning steel 20Cr25Ni20Si2(cast). Studies have established that when machining 20Cr25Ni20Si2(cast) hightemperature steel in all processing modes, the linear wear of the tool and its intensity are much higher than when machining steel 20Cr13Ni18. The obtained results are explained by the fact that high-temperature steel 20Cr25Ni20Si2(cast), compared with steel 20Cr13Ni18, contains more alloying elements such as chromium (2 % more), nickel (2 % more), and silicon, which leads to the formation of a large amount of carbides. A large amount of carbides in steels causes an increase in the intensity of wear of the cutting tool during machining, including plasma-assisted and plasma-ultrasonic-assisted machining. From the given curves presented in Fig. 2 and 3, it was possible to fi nd out that during plasma-assisted machining of high-temperature steels, the wear rate of tool material is reduced compared to the conventional cutting method. At the same time, tool life increases by about 1.8–2.5 times compared to the conventional machining method. Studies have shown that with the conventional method of high-temperature steels turning, high specifi c loads and temperatures are observed on the contact surfaces of the cutting insert, which are continuously formed during the cutting process, which creates unfavorable conditions for the operation of the cutting tool. At the same time, high-temperature steels tend to adhere to the tool material and have high strength, Fig. 3. Wear on the end fl ank of the cutter under various processing conditions when turning steel 20Cr25Ni20Si2(cast) slag
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