OBRABOTKAMETALLOV technology Vol. 25 No. 1 2023 It is established that chromium carbides Cr23C6 are responsible for intergranular type of corrosion. Formation of these carbides along the boundaries of austenitic grains is responsible for the reduction of chromium dissolved in the matrix to values (less than 13%), providing a local drop in corrosion resistance [12–14]. This process is intensified during long soaking at temperatures corresponding to the active formation of chromium carbides, while corrosion spreads into the depth of the grain. Some measures are implemented to reduce the tendency of 12-Cr18-Ni10-Ti steel to this type of corrosion like heat treatment of billets (quenching, annealing), as well as optimization of the chemical composition forming during smelting and ladle treatment of liquid steel [15–17]. The formation of a martensitic α-phase in the structure of stainless steel may occur during working at low temperatures or in the process of cold deformation, accompanied by an increase in the magnetic properties of the material. This transition is undesirable for austenitic steel, so the chemical composition and rolling parameters are optimized in order to prevent it [7, 18]. The formation of ferrite δ-phase in this type of steel starts at the beginning of the solidification of the melt, then with further cooling the δ-ferrite phase dissolves in austenite. Due to the significant cooling rates of ingots, this process is usually incomplete. Even after hot plastic deformation, there is a residual ferrite phase in the metal structure, which degrades the magnetic properties. Moreover, the ductility and crack resistance of steel are reduced [19–20]. Today the methods of thermodynamic simulation are widely used to assess the influence of the chemical composition of materials on the number and type of phase constituents. Such information allows clarifying recommendations and measures to improve the quality of metal products [21–23]. The purpose of work: – studying of the main defects typical for 12-Cr18-Ni10-Ti stainless steel; – performing thermodynamic simulation of the accompanying phase transformations; – making recommendations for improving the quality of the metal and reducing its defects based on the studies and calculations. The methods of investigation As part of this work, tests of 12-Cr18-Ni10-Ti steel for resistance to intergranular corrosion were carried out in accordance with GOST 6032-2017 “Corrosion-resistant steels and alloys”. Oxide scale was removed from the surface of the heat-treated specimens by chemical etching, and then specimens were kept in a boiling aqueous solution of copper sulfate and sulfuric acid in the presence of metallic copper. Depending on the method, the holding time was 24 hours or 8 hours. After the tests, bending by 90 ± 5° and examination for cracks were carried out. The presence of cracks on the specimens bent after the test and the absence of cracks on the control specimens bent in the same way indicated the tendency of the steel to intergranular corrosion. Preparation for metallographic analysis consisted of sequential grinding, polishing of stainless steel specimens and electrochemical etching in a 10% aqueous solution of oxalic acid. Buehler Ltd equipment was used for specimens preparation. An additional austenization operation of the specimens could be carried out if necessary, namely, a long soaking in the range of 1,000–1,200 °C to remove magnetization and dissolve the ferrite phase. Metallographic analysis was carried out directly using a microvizor Mkvizo-MET-221. Hardness measurements of the hard-worked metal specimens were carried out by a TB 5015-01 tester using Brinell scale. Thermodynamic simulation was performed using the Thermo-Calc software product. This program allows equilibrium calculations of multicomponent multiphase systems under different temperature conditions for various chemical compositions.
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