OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 25 No. 1 2023 Minor changes in the structure of the diffusion layer after 5-hour borocoppering undergo specimens of 0.5C-Cr-Ni-Mn steel (Fig. 4, c). The thickness of the layer was increased by 10 µm (Fig. 5). The needle-like structure of the layer remains unchanged, but the needles’ growth is observed. It is also worth noting that directly adjacent to the boride needles are some secretions, presumably of a carboboride structure, which have a direction at some angle relative to the needles themselves. The microhardness and the nature of its distribution remain unchanged (Fig. 6, c). An increase in the carbon content in Steel 45 (0.45%C) and Steel U10 (1.0%C) reduces the average layer thickness at both soaking modes. The thickness of the layer is greatest on 0.5C-Cr-Ni-Mn steel specimens, despite the intermediate carbon content (Fig. 4). It is likely that alloying elements in steel take part in the intensification of diffusion during borocoppering. As can be seen in Fig. 6, the distribution of microhardness after borocoppering for 3, 4 and 5 hours on all steels is similar and is characterized by a gradual decrease in values from the surface to the base metal. It should be noted that the microhardness of all specimens over the entire thickness of the layer after 5-hour borocoppering is higher by 100–150 HV, compared with the microhardness of specimens after borocoppering for 3 and 4 hours. Presumably, this is due to an increase in the content of the harder phase of FeB after a 5-hour borocoppering. The data given in Table 2 (Fig. 7, a) confirm the presence of boron and copper in the diffusion layer on the test specimen made of Steel 45 (0.45% C). There is a decrease in the concentration of boron and copper in the direction from the surface to the interface with the base metal. Carbon is pushed into the transition zone, where its concentration is maximum – 0.56%. Nickel and manganese are almost evenly distributed over the entire thickness of the diffusion layer. The presence of chromium was detected in the transition zone. Consequently, the elemental analysis shows the nature of the distribution of alloying elements corresponding to the chemical composition of Steel 45 (0.45% C). The results presented in Table 3 (Fig. 7, b) for Steel U10 (1.0% C) indicate the presence of boron on the surface in the amount of 16.81 % and a gradual decrease in its concentration to 0.68 %. The maximum amount of copper is observed on the surface of the diffusion layer and directly under the boride needles. Carbon is pushed under the boride layer, where its content reaches 1.69 %. Chromium and manganese are evenly distributed over the entire thickness of the diffusion layer. Table 4 shows the elemental composition of 0.5C-Cr-Ni-Mn steel after borocoppering for 4 hours (Fig. 7, c). As in the previous specimens, the maximum concentration of boron is observed on the surface, followed by its decrease towards the boundary with the base. The maximum carbon concentration is visible on the surface and in the transition zone. Aluminum, chromium, nickel, molybdenum and copper are concentrated in the same zones as carbon. X-ray phase analysis performed on the surface of Steel 45 (0.45% C) (see Fig. 8) after borocoppering demonstrates the presence of phases FeB, Fe2B. The inability to determine copper is most likely due to its small amount. Ta b l e 2 The elemental composition of the diffusion layer on Steel 45 (0.45% C) after 4 hours of borocoppering (Fig. 7, a) Points of the spectrum Chemical elements, mass % B C Mn Ni Cr Cu Fe 1 16.73 0.2 0.29 0.41 – 2.39 79.98 2 11.37 0.06 0.38 0.44 – – 87.75 3 6.2 0.32 0.22 0.51 – – 92.75 4 – 0.56 0.24 0.31 – 0.36 98.53 5 – 0.47 0.35 0.51 0.12 0.17 98.38
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