OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 25 No. 1 2023 The elemental composition of the coated steels has a significant effect on the coating formation by DSLMMS. The formation of coatings is influenced by the content of such elements in the steel being coated, as carbon and chromium. When processing steels 20-Cr13, 12-Cr18-Ni10-Ti steels, containing chromium in a concentration of more than 10%, the formation of rather thin coatings is observed. This is due to the peculiarities of the interaction of diffusing chromium and the components of steels that already contain chromium. Thus, the formation of a coating requires the formation of chemical compounds, or solid solutions. In the presence of carbon bound into carbides as well as the presence of solid solutions with the participation of chromium, the chromium concentration gradient at the boundary material being coated/ diffusive element decreased, and as a result, the chromium diffusion rate decreases. In steels, containing chromium in small amounts, there is a different pattern of coatings formation. Carbon contained in steel in the form of a solid solution in iron, or in the cementite, actively diffuses to chromium due to the fact that chromium is a strong carbide-forming element, compared with iron, tending to form chromium carbides. The probable diffusion mechanism is grain-boundary diffusion. The results of X-ray microanalysis of diffusion-alloyed chromium CDA coatings are shown in figure 3. As studies have shown, after CDA, under the same modes, the elemental composition and elements distribution in the coatings being formed are determined both by the elemental composition of the alloyed steel and by a significant increase of the carbon concentration in the surface layers of the coated steels from 2.4 % carbon obtained on steel 40-Cr13 to 0.7 % carbon on austenitic steel 12-Cr18-Ni10-Ti (figure 3). The resulting coatings consist of several layers. Figure 4 shows a multilayer EDS image after the 40-Cr steel, CDA. It can be concluded that the coatings consist of several layers. The surface layer is characterized by high chromium content and is formed due to the diffusion of chromium into the surface layers of the material being coated. A transition zone formed between the coating and the base material is characterized by a decrease in the concentration of chromium, an increase in the concentration of iron and the presence of a sublayer with an increased concentration of nickel. The formation of a zone with high nickel content can be explained by the fact that nickel, having a low mutual solubility with chromium carbides formed during CDA, is pushed into the transition zone. As follows from the X-ray microanalysis of the distribution of chromium in the coating, despite the fact that all the steels under study were carburized under the same conditions, the distribution of chromium in the coating is determined by the amount of carbon in the surface layers of the steel being coated. Thus, comparing the percentage of chromium in the coatings obtained after the CDA and in the coatings obtained after the DSLMMS, it can be seen that the concentration of chromium is significantly reduced: for St3 – from 96.9 to 66.8 %; for 40-Cr – from 91.1 to 63.18 %; for 20-Cr13 – from 93.18 to 62.54 %; for 12-Cr18Ni10-Ti – from 92.92 to 64.77 %. Such decrease in the concentration of chromium in the surface layers can be explained by an increased content of carbon in the coatings obtained as a result of carburizing, which binds chromium into carbide compounds. The influence of the composition of steels being coated on the coating formation process is manifested due to the influence of its composition both on the carburizing process and on the DSLMMS. Carburization influence on the composition of the coated steels occurred according to the already sufficiently studied mechanism. But the effect of cementation to the DSLMMS technology is still practically unstudied. The carbon content after DSLMMS, despite the presence of high-temperature soaking, remains high in the range from 1.2% C to 0.8% C, throughout the entire thickness of the main coating layer. After that, the carbon concentration in the transition layer gradually decreased to the carbon concentration of the steel being coated. Such stability of the carbon concentration in the main layer is explained by the chromium carbides formation in this layer. In addition to carbon, the diffusion of chromium into the depth of the coating is also influenced by alloying elements. In the case of steel 20-Cr13 surface CDA with chromium, the chromium concentration remains almost constant (more than 60 %) at a depth of 15 µm. With CDA, the chromium concentration of more than 60 % in steel St3 does not exceed a depth of 8 µm, in steel 40-Cr – 7.5 µm, in steel 40-Cr13 – 7 µm, in steel 12-Cr18-Ni10-Ti – 6 µm. These phenomena occurring during the formation of diffusion-alloyed chromium coatings on steels containing carbide-forming elements, such as chromium, are explained
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