OBRABOTKAMETALLOV MATERIAL SCIENCE Том 23 № 3 2021 EQUIPMEN . INSTRUM TS Vol. 4 No. 3 2022 Introduction In mechanical engineering, various hard-to-process materials are widely used for the manufacture of parts and structural elements of equipment for electrochemical, chemical and other industries. The use of hard-to-process and heat-resistant steels for the manufacture of thermal equipment is hampered by the fact that these materials are poorly subjected to machining with edge tools. In this regard, in the production of electrothermal equipment, attempts are being made to increase the effi ciency of processing heat-resistant and hard-to-process materials by using various combined methods of chip removal, one of which is machining with plasma heating of the workpiece before processing. Plasma-heated hightemperature steel machining is a combined process in which mechanical energy, together with lowtemperature plasma energy, is used to increase performance and reduce cutting tool consumption when machining these materials. There are various methods for plasma heating of a workpiece during machining [1–6]. These and other works provide data on the performance of plasma-mechanical turning, milling, etc. It has been established that plasma heating improves the machinability of materials in cutting cases when the increase in tool life due to a decrease in the specifi c cutting work is greater than the negative effect of elevated temperatures on the increase in the intensity of adhesion and tool wear phenomena. As is known, the wear of a cutting tool is an integrated process accompanied with complex and mutually infl uencing phenomena at the points of contact between the tool with the chip and the workpiece, occurring under conditions of high temperatures and pressures. Therefore, it is recommended to use cutting tools with internal cooling during plasmaassisted machining. The analysis of research works [7–20] showed that insuffi cient attention has been paid to the issue of determining the relationship between the wear of the cutting tool and the parameters of plasma-machining of hard-to-process materials. Also, among the available research papers, there are no works devoted to the use of ultrasonic vibrations in combinations of plasma-assisted machining of hard-to-process materials. Therefore, the task was set to investigate the process of plasma-ultrasonic-assisted machining of hard-toprocess materials and the wear of the cutting tool that accompanies it. Hard-to-process materials have a number of such specifi c physical, chemical and mechanical properties as high strength, high temperature strength, heat resistance, toughness, corrosion resistance, refractoriness, etc. Hard-to-process materials have a complex carbide-forming structure. High-temperature steels and alloys belong to hard-to-process materials, which, according to their basic composition, are divided into high-temperature steels based on iron, nickel, cobalt and titanium. These steels and alloys are often used in the manufacture of parts for electrothermal equipment. High-temperature steels based on iron, nickel, cobalt and titanium are diffi cult to machine with an edge tool, that is, turning and milling due to a number of specifi c features, in particular: dependence of the increase in hardening of high-temperature steels in the process of deformation during cutting on the structure of the crystal lattice of these materials, which determines the number of possible sliding directions during plastic strain in the process of machining. For example, crystals of steels of the ferritic-pearlitic group have a lattice of a body-centered cube with eight possible slip directions; crystals of steels of the austenitic class have the shape of a face-centered cube with nineteen possible slip directions [1]; high ductility of high-temperature steels, which leads to an increase in microhardness in the chip formation zone during turning, which, in turn, complicates the process of separating materials along the front surface of the cutting edge; low thermal conductivity of high-temperature steels, which leads to an increase in temperature on the contact surfaces during machining, causing an increase in the intensity adhesion and diffusion phenomena and, as a result, the destruction of the cutting part of the tool; the ability of these materials to maintain its original strength and hardness at elevated temperatures that occur in the zone of deformation and chip fl ow during cutting, which leads to a very high specifi c pressure at the point of contact of the material with the tool during the machining;
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