OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 25 No. 2 2023 Boronizing is one of the promising methods for increasing the surface hardness and wear resistance, oxidation and corrosion resistance of machine building parts. There are several boronizing methods, such as powder, chemical, and electrolytic ones [4–7]. It has been noted that the process of diffusion boronizing is characterized by a long duration (8–10 hours) and a small hardening depth (less than 200 μm) [8–10]. In addition, saturation of the steel surface with boron usually results in the formation of FeB and Fe2B, which has an acicular microstructure. This microstructure makes the boride layer very brittle. This does not allow efficient use of boronized parts when subjected to impact and high local loads during operation. The destruction of the acicular structure on the surface leads to the formation of a globular structure, which can significantly increase the strength and plasticity of the surface [11]. Saturation of the steel surface with boron using a laser, an electron beam, or a plasma arc [12–15] makes it possible to reduce the boronizing process to 0.1–1 min and obtain a hardened layer depth in the range of 1–5 mm. In [16], CrB powder was used to alloy the surface of carbon steel using a laser. The results showed that at low scanning speed (10 mm/s) the microstructure and properties of the alloyed layer are uniform. The authors of [17] used a laser to modify the structure of borated steel without disturbing the microstructure and properties of the base metal. It was found that laser surface modification with a power of 250 W reduces the hardness gradient of the alloyed layer to the base metal and leads to a significant increase in the ductility and toughness of the steel. The authors of [18] investigated the boronizing process and noted that laser boronizing of mild steel can be performed faster and without any pre-treatment. It has been found that the most desirable microstructure for laser boronizing of AISI 1018 steel is Fe2B, which has a high hardness in the range of 1,300–1,700 HV and a compressive stress on the machined surface. Powdered boron carbide was used for surface hardening using an electron beam [19]. The authors noted that the hardened layer after treatment has a dendritic structure and a surface hardness that is more than 6 times higher than the hardness of the base metal. The authors of [20] studied the structure and properties of boride coatings formed on AISI 1018 steel using a plasma heating source. According to the results of the study, it was noted that the thickness of the coatings ranged from 1 to 1.5 mm, the hardness was from 400 to 1,600 HV. The wear rate of boronized coatings is approximately four orders of magnitude lower than that of the steel base. Based on the results of literature analyzes, it was noted that boride coatings on a steel base can be obtained using laser, electron beam, and plasma arc heating sources. In addition, in particular, there are very few works using plasma surface heating for steels boronizing. The purpose of the work is the formation of boride coatings on low-carbon steel using plasma plasmajet hard-facing technology. To achieve this purpose, microscopic investigation, phase composition analysis and microhardness testing of deposited coatings were carried out. Methodology of research Steel 20 (0.17–0.24% C, 0.17–0.37% Si, 0.35–0.65% Mn, ≤ 0.25 % Ni , ≤ 0.04 % S, ≤ 0.04 % P, ≤ 0.25 % Cr, ~98 % Fe) was used as the base material. Steel plates with a size of 75×15×15 mm were cut and polished with sandpaper (with an increase in grain size up to 1,200 grit). The suspension was prepared by mixing amorphous boron powder with BF-6 adhesive with a weight ratio of 1:1 and preliminarily applied to the surface of each plate. The thickness of the coating is fixed at 1 mm. After that, the coated plates were dried in a furnace at a temperature of 60 °C for 2 hours. Equipment for plasma-jet hard-facing is schematically shown in figure 1. In all processing modes, voltage (30 V), transporter table movement speed (4 mm/s), distance between plate surface and electrode (3 mm), nozzle diameter (5 mm) and shielding gas flow rate (18 l/min) are permanent. The current is used as a variable parameter (Table). The microstructure of the hard-faced layers was studied using an optical microscope MET-2 and a twobeam scanning microscope (multibeam system) LV-4500. To determine the boron content in the hard-faced layer, the electron probe micro-analyzer method was used. The electron probe microanalyzer method is as follows: a beam of highly accelerated electrons is incident on a small surface (∼1 μm2) of the sample, then the outgoing X-rays are selected based on its wavelength using the received crystal diffraction condition, and then the element concentration is quantified by
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