OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 3 2024 the composition of coatings and the method of its formation on the surface of the part is determined by the operating conditions, which leads to the need for an individual approach to the formation of multilayer coatings of different chemical composition [2–3]. The application of multilayer high-temperature coatings for piercing tools in the production of seamless hot-rolled steel pipes, which are used as structural pipes in construction, mechanical engineering, and oil industry, is promising [4]. The piercing mandrel is used in the production of hollow billets, from which a seamless pipe is produced in subsequent rolling, rolling, reducing, straightening and calibrating mills [5]. The mandrel is exposed to both high temperatures and abrasive wear during operation [6, 7]. To improve the strength and wear resistance, as well as to avoid sticking on the mandrel, its surface is subjected to hardening treatment, by applying a protective high-temperature coating on its toe and spherical surface, followed by an oxidized layer on the outer surface of the material. To form high-temperature coatings resistant to abrasive wear at high sliding speeds, various self-fluxing high and medium-carbon iron-based alloys alloyed with chromium, nickel, vanadium and manganese are used [8–10]. Iron powder is used to form an oxidized layer on the outer surface, which is applied in an oxidizing atmosphere. Applying an oxide outer layer has a number of advantages: firstly, the oxide layer prevents material from sticking to the mandrel during operation; secondly, at high operating temperatures it creates additional thermal resistance, increasing the thermal resistance of the coating itself; thirdly, at high operating temperatures, scale softens, and it begins to work as a lubricant with the contacted surface [11–15]. Thus, the purpose of the work is to study the influence of the chemical composition of sprayed coatings on the phase composition, structure, micromechanical and tribological characteristics under conditions of dry sliding friction of surface layers. Materials and research methods In this work, multilayer coatings of two different compositions consisting of three successively applied layers were investigated. The difference in the composition of the obtained coatings is the first layer, for the formation of which heat-resistant self-fluxing iron-based powders were used (Table 1). To obtain the first layer of coating of composition 1, Fe-Cr-Si-Mn-B-C powder with a particle size of 50–90 μm was used (Fig. 1 a). The first coating layer of composition 2 is Fe-Ni-Si-Mn-B-C powder with a particle size of 60–100 μm (Fig. 1 b). A mixture of Fe powder with self-fluxing powder in ratio (1:1) was used to form the second layer of both coatings and the third layer was obtained from Fe powder with particle size of 40–100 µm (Fig. 1 c). All layers of the studied coatings were obtained by plasma-powder spraying technology with contact excitation of arc discharge UPN-60KM TSP2017, manufacturer LLC “NPP TSP” (Ekaterinburg). The first (metal) layer, which chemical composition shown in Table 1, due to its high hardness and wear resistance protects the mandrel material from destruction in case of wear of the upper layers during operation. The second (transition) layer is obtained by spraying a mixture of self-fluxing powder with Fe powder. It is designed for a smooth change of properties as well as for better adhesion of the outer layer to Ta b l e 1 Composition of sprayed powders Powder Content of chemical elements, mass. % Ni Cr Si B C Mn Fe Composition 1 (powder Fe-Cr-Si-Mn-B-C) – 3.8 2.3 3.6 1.2 1.0 base Composition 2 (powder Fe-Ni-Si-Mn-B-C) 9.0 – 1.2 2.7 0.5 4.0 base
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