OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 The data in Table 2 shows that there is a non-monotonic change in the hardness value. The increase in hardness is observed at the medium and low tempering. It is associated with a decrease in the number of grains and an increase in its average size [16, 17], causing changes in internal stresses. Then phase transitions occur, leading to the new phases’ grains appearance due to the decomposition process of the martensite structure into ferrite and perlite. The number of grains increases but average size decreases. As a result of the polished section microanalysis, it is found that after quenching on the studied samples made of steel 09Mn2Si, a martensitic structure is observed with a slight presence of the ferrite and pearlite phases. In the case of quenching, the main initial structure observed in microphotographs is martensite. It occurs as a result of the steel heating to the intercritical interval. The martensite nucleus formation occurs when the alloy is cooled from the austenitic state and nucleuses are located at the interphase boundaries of the initial ferrite-cementite phase and at the boundaries of ferrite grains [30]. While heating the unstable martensite, obtained as a result of quenching, it decomposes into a mixture of ferrite and cementite. At the same time, Mn is concentrated mainly in the carbide phase [29] which is cementite in the structure under consideration. The martensite formed during quenching has a lath or packet (dislocational) structure. Crystals of such martensite are thin laths 0.2-2 µm thick, elongated in one direction. A set of elongated martensite crystallites parallel to each other forms packets. Martensite is separated by thin layers of residual austenite with a thickness of 10-20 nm. Both phases have a high density of defects in the crystal lattice structure [25, 27, 31–32]. The defects in the form of non-metallic inclusions of manganese sulfide [14] in most cases have a spherical shape (Figure 2) in such structure. The MnS compound formation occurs in the presence of manganese and sulfur in the steel composition. This process occurs due to the fact that sulfur, participating in the chemical process, forms a FeS compound with iron at a melting temperature of 988 ºС. [18, 19]. The manganese presented in the steel (09Mn2Si) is slightly soluble in iron alloys and replaces it in the compound, forming manganese sulfide. The cavities filled with manganese sulfide are formed in the metal due to diffusion processes and the dissolution of large inclusions during the smelting and manufacture of rolled products. The study [25] indicates that with an increase in the manganese content in a solid solution, the solubility of sulfur decreases due to the chemical reaction between sulfur and manganese. The sulfide is formed consequently. The inclusions size and number of manganese sulfide increases [26] with a sulfur content of about 0.023 %. Such inclusions are corrosive areas that contribute to an increase in the rate of metal corrosion in the local area. The connection between such inclusions and the metal matrix of the material is weak. It leads to the removal of this compound and the cavity formation on the surface under external influence. The aggressive effect of the corrosive medium in this area increases [20] due to the weak diffusion backoff. Figure 3 shows a pipe fragment made of 09Mn2Si steel with observed corrosion damage, which has a characteristic pitting shape. The process of martensite decomposition occurs during tempering. It leads to the formation of a ferritecarbide mixture with a granular carbide morphology [20]. At the same time, the ongoing processes lead to a change in the shape of inclusions from rounded to lamellar. The approach of the structure to the equilibrium state is accompanied by the elements’ redistribution. It occurs as a result of diffuse processes when the initial quenched structure is heated i.e. under conditions of high density of interfacial boundaries and small diffusion paths through an acicular mixture of phases [30]. The martensite grain-size number increases from 2 to 5 during low tempering (200 °C). The areas with the ferrite and perlite phases practically do not change. The carbon atoms in tempering and other impurities presented in the steel diffuse from the supersaturated solid solution of martensite into structural imperfections of the crystal structure (dislocations and intergranular boundaries). The formation of carbide phase components occurs by the interaction of carbon and the boundary layer, which is a depleted martensite or ferritic phase. The occurrence of regions with a reduced carbon content leads to a decrease in the overall hardness of the steel. Due to the high density of defects in the crystalline structure of the primary phase (martensite), the resulting pearlite-ferrite structure will also has a high density of defects and is highly distorted. The shape of manganese sulfide inclusions is distorted in such structure. It has an elliptical shape (Figure 4). The carbon begins to diffuse into the area where the sulfide is located and forms clouds around it.
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