OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 25 No. 4 2023 nickel content on the surface was 40 %. The depth of diffusion of nickel and chromium on steel St3 was 14 µm, and on steel 40Cr13 it was 9 µm. At the same time, on steel 40Cr13 the chromium content in the coating was 9 %. The chromium concentration corresponding to the concentration of the uncoated material was detected at a depth of 7 µm. On St3 steel, the maximum chromium concentration was 14.5 %. The chromium concentration corresponding to the concentration of the uncoated material was detected at a depth of 15 µm. It can be concluded that one of the main factors affecting the concentration distribution of elements in the coating, its structure and microhardness is the elemental composition of the coated material, especially the carbon content. Steels 40Cr and 30CrMnSiNi2 contain a fairly large amount of carbon, while the chromium content is about 1 %. Thus, most of the carbon in these steels is in the form of cementite. Chromium diffusing during the DALMMS process forms its own carbides on the surface of the coated material due to its greater affinity for carbon than iron. This factor causes more intense diffusion of chromium into materials, elemental-phase composition of which allows the formation of chromium carbides. In this case, a layer with an increased concentration of nickel is formed under the carbide layer. The formation of this layer is due to the low mutual solubility of nickel and carbides. Also, it is worth noting that on steels St3 and 40Cr13 the formation of a carbide layer did not occur. On St3 steel, the absence of a carbide layer is explained by the insufficient carbon content for its formation. On steel 40Cr13, due to the high chromium content, the absence of a carbide layer is explained by the fact that this steel contains carbide (Cr, Fe)23C6, which does not allow carbon to actively diffuse to the chromium obtained during DALMMS, and thereby form a carbide layer. Thus, comparing the chromium content in coatings on steel 40Cr and 40Cr13, a difference in concentration values was revealed by 8.8 times, which indicates a significant influence of the percentage of carbon and chromium in steel. In the case when carbon is bound into carbides of elements that have a lower affinity for carbon than chromium, diffusion of carbon occurs to the chromium obtained during DALMMS and the formation of chromium-based carbides. The distribution of nickel in the coatings also had its own characteristics. On specimens with a carbide layer on the surface, nickel was pushed into the zone under this layer. Thus, on a specimen made of steel 30CrMnSiNi2, the maximum concentration of nickel was observed at a depth of 5 μm and amounted to 21 %, while the nickel content in the carbide layer was 1.5 % (fig. 3). The low nickel content in the carbide layer is explained by the low mutual solubility of nickel and chromium carbides. A similar layer enriched with nickel was observed on 40Cr steel. The maximum nickel concentration was 13 % at a depth of 4.5 µm. Further, the nickel concentration gradually decreased to a concentration characteristic of the uncoated material. On steels St3 and 40Cr13, the nickel concentration was significantly higher and amounted to 40 % on the surface of the specimen. Then the concentration gradually decreased to concentration values characteristic of the uncoated material. The obtained data on the structure of the coatings are in good agreement with the already known results of diffusion saturation with nickel and chromium using the DALMMS technology for materials such as Armco, Steel 10, Cr6WV [21]. Thus, when forming coatings on steels 40Cr, 30CrMnSiNi2, in the context of elemental composition and structure, the coatings consist of several functional layers: surface carbide layer and transition one. At the same time, the layers have a clear interface. For steels 40Cr13 and St3, a single-layer coating is formed. Fig. 4 shows EDS images of steels 40Cr and St3, characterizing the elemental composition of diffusion coatings. The presented images show the distribution of elements in the coating and between the diffusion layers. Thus, for a specimen made of 40Cr steel with a Ni-Cr coating, the surface layer consists of carbide grains elongated in the direction of diffusion; the transition layer is a solid solution of Fe(Ni,Cr) (fig. 4 a). The coating on a specimen made of St3 steel is formed on the basis of solid solutions of the Fe(Ni,Cr) system (fig. 4 b). Thus, it was revealed that the formation of Ni-Cr coatings obtained using the DALMMS technology, provided that the technological conditions are constant, largely depends not only on the elemental composition of the coated materials, but also on its phase composition.
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