OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 25 No. 4 2023 for 480 minutes in the melt at a temperature of 1,050 °C in an isothermal mode, while argon was above the surface of the bath. After DALMMS, the surface of the specimens was cleaned of melt residues by etching in a mixture of acetic acid and hydrogen peroxide. The chemical composition of the diffusion coatings was analyzed using X-ray microanalysis. An INCA x-sight X-ray spectrometer by Oxford Instruments Analytical was installed on a JEOL JSM-7500F scanning electron microscope for this purpose. To identify the structure of specimens made of steel St3, 40Cr, etching was carried out in a 4 % solution of nitric acid in ethyl alcohol. Grechko reagent was used for specimens made of 40Cr13, 30CrMnSiNi2 steels. Microhardness was measured with a Dura Scan Falcon 500 electronic hardness tester. The microhardness of the coatings, as well as the transition layers and the base material, was measured at a load of 10 grams (GOST 9450). Results and discussion It was revealed that DALMMS with nickel and chromium results in the formation of diffusion coatings on the surface of all the materials under study. Fig. 1 shows micrographs of the specimens. It was revealed that diffusion coatings were formed on the surface of all the specimens under study during DALMMS. The coatings consist of a surface layer and a transition layer. The transition zone is characterized by different elemental composition, structure and microhardness from both the coating and the base material (figs. 1–3). However, the structure and elemental composition of these coatings were different and were determined by the elemental composition of the coated steel. Thus, a carbide layer is formed on the surface of 40Cr and 30CrMnSiNi2 steels, which is confirmed by the results of microhardness measurement and the results of X-ray microanalysis (figs. 2, 3). In this case, the carbide grains are oriented perpendicular to the surface of the specimen (fig.1 a, d). The formation of a carbide layer did not occur on St3 and 40Cr13 steels. The formation of the carbide layer is due to the fact that the carbon contained in the steels diffuses to chromium, which is a strong carbide-forming element. At the same time, the carbon contained in the steel St3 was not enough to form a carbide layer. In the case of 40Cr13 steel, the absence of a carbide layer is explained by the fact that in this steel carbon is bound into chromium carbides, which does not allow it to actively diffuse to the surface of the specimen, as in the case of forming a coating on 40Cr steel. The results of the microhardness measurement are shown in fig. 2. The specimens were subjected to X-ray microanalysis to identify the features of the formation and structure of coatings. The results are shown in fig. 3. From the X-ray microanalysis results presented above, it was revealed that the elemental composition of the coated materials had a significant impact on the concentration distribution of elements in the surface layers of products subjected to DLLHR. From the point of view of elemental composition, coatings on steels 40Cr and 30CrMnSiNi2 can be considered as consisting of two layers: a surface layer enriched with chromium and an intermediate layer enriched with nickel. Thus, the chromium content in the surface layer of the coating on steel 40Cr was 80 %, on steel 30CrMnSiNi2 the chromium content was 78 %. It is also worth noting the nature of the distribution of chromium over the coating. On steels 40Cr and 30CrMnSiNi2, Elemental composition of specimens Steel grade С Si Mn Ni S P Cr Cu St3 0.14–0.22 0.15–0.3 0.4–0.65 ≤ 0.3 ≤ 0.05 ≤ 0.04 ≤ 0.3 ≤ 0.3 40Cr 0.36–0.44 0.17–0.37 0.5–0.8 ≤ 0.3 ≤ 0.035 ≤ 0.035 0.8–1.1 ≤ 0.3 40Cr13 0.35–0.44 ≤ 0.6 ≤ 0.6 ≤ 0.6 ≤ 0.025 ≤ 0.03 12–14 – 30CrMnSiNi2 0.27–0.34 0.9–1.2 1–1.3 1.4–1.8 ≤ 0.025 ≤ 0.025 0.9–1.2 ≤ 0.3
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