OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 2 2022 Introduction An important goal of modern materials science is to increase the strength and wear resistance of tools and various machine parts by means of diffusion saturation of the metals and alloys with various chemical elements. It is hard to achieve the specifi ed mechanical and operational properties using common heat treatment (quenching and tempering). Abetter alternative is thermochemical treatment (TCT), when various chemical elements diffuse into the surface of metals and alloys. Metal working parts, subjected to TCT, can replace products made of expensive special steels and alloys [1-2]. Currently, there are several methods of TCT, depending on the saturating medium: gas, liquid and solidphase (in powders and pastes) [3-4]. It is known that borited layers have high hardness, corrosion and wear resistance. Saturating mixtures based on boron carbide are widely used for boriding in powder mixtures and pastes [5]. Aluminizing is the process of surface saturating with aluminum to improve oxidation resistance at high temperatures and atmospheric corrosion resistance. Various mixtures based on the powders of aluminum or ferroaluminum, aluminum oxide, etc. are used for aluminizing [6, 7]. It should be noted that the solid-phase TCT methods require long-term exposure at elevated temperatures, which adversely affects the structure and properties of the base metal. There are other methods for improving the surface properties of machine parts that do not require longterm exposure at elevated temperatures, such as concentrated energy streams (CES). Laser and electron beam treatment (EBT) are capable to heat a material’s surface rapidly and avoid the mentioned drawback [8-10]. There are also methods of the combined treatment, where common TCT is followed by subsequent laser treatments and EBT [11-13]. The latter method allows modifying the previously obtained diffusion layer and eliminating its defects (layering and phase inhomogeneity along the layer depth, brittleness, high surface roughness). It should be noted that the CES methods require costly equipment. Its use is justifi ed when the necessary properties cannot be attained through common surface techniques. Thus, it is reasonable to carry out a combined treatment with the TCT as a fi rst stage of the treatment to obtain a continuous protective layer over the entire surface area. Next, the most critical areas are additionally subjected to EBT to modify the obtained diffusion layers. Another opportunity is to carry out electron beam alloying (EBA) as a second treatment. For example, fi rst, powder aluminizing with furnace heating is carried out, followed by the EBA with boron carbide, or in reverse mode, i.e., common powder pack boriding followed by the EBA with aluminum. It is known, that the combined process of saturation with boron and aluminum (boroaluminizing) makes it possible to synthesize multifunctional layers [14, 15]. This paper contains the materials on the fi rst stage of treatment as independent processes that improve the set of physical and mechanical properties of steels over the entire surface area of a product. The purpose of this work is to determine the impact of boriding and aluminizing on the structure and properties of the diffusion layer on the surface of low-carbon steels. The paper presents the test results of the low-temperature modes of TCT. A comparative analysis of the structure and properties was carried out via examples of two steel grades. Methodology The following powders were used for saturating: boron carbide B4C of F-220 grade, aluminum powder PA-4 grade (6058-73 State Standard), aluminum oxide Al2O3 analytical pure class (8136-85 State Standard) and sodium fl uoride NaF analytical pure class (4463-76 State Standard). Amixture of 96% B4C + 4% NaF was used for the boriding process. The aluminizing mixture consisted of 48% Al + 48% Al2O3 + 4% NaF. The TCT in powders were carried out in the PM-16P-TD laboratory furnace at a temperature of 900 °C. The samples of St3 steel and 3Cr2W8V die steel with the size of 20×20×10 mm were subjected to TCT. The duration of the treatment process was 2 hours. As it is known, St3 steel is used for the bearing elements
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