OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 (Figure 5). The processes caused by the containment of dislocations on impurity elements of the structure during its interaction through Cottrell atmospheres should be taking into account. Fig. 5. Inclusions of manganese sulfide in a 09Mn2Si steel sample after mediumtemperature tempering (350 °С) The mobility of crystal lattice’s defects in the form of dislocations is very important in steels with a low content of alloying elements. It strongly affects the hardness value, which decreases quite quickly with an increase in the tempering temperature. It is also worth mentioning that the presence of alloying elements, such as Mn, in the steel composition during the tempering process can alloy cementite [27]. The structure transforms into ferrite-pearlite with an insignificant percentage of the observed phase of residual martensite at a tempering temperature of 500 °С. This tempering reduces the density of dislocations and plane defects of the crystal structure. As a result, the distorted cementite passes into a more equilibrium state. The proportion of ferrite in this mixture is 60.9 % and the proportion of perlite is 39.1 %. The total grain-size number of the structure is 7. At this temperature, further stretching of manganese sulfides occurs along the boundaries of the grain structure, which is associated with an increase in the plasticity of this compound. There are atmospheres formed by diffusing carbon atoms, which form additional areas with high corrosion activity around the inclusions (Figure 6). The structure acquires an equilibrium state with the further temperature increasing to 650 °C. The appearance of granular pearlite is observed. The proportion of ferrite is 64.6 %, the proportion of perlite is 35.4 % and the structure grain-size number is 7. The perlite grain-size number increases due to the process of coagulation of the cementite particles that are part of the mechanical mixture. The structure approaches the equilibrium state [22–25]. It causes a decrease in the magnitude of internal stresses. The increase in the number of grains is due to the ferrite phase fragmentation. Although the pearlite grain size increases, there is a decrease in the average grain size observed on the microsection due to the appearance of smaller ferrite grains. An increase in the number of grains and the system dispersion leads to an increase in intergranular boundaries. Fragmentation continues until the grain reaches a “critical size”. The reduction of internal stresses in this situation is associated with a decrease in the crystal lattice distortion because of the increase in the length of boundaries between the grains. The hardness of the material is reduced due to these processes. Carbon diffused in the area of MnS inclusions is redistributed in the matrix between the formed phases (Figure 7). The corrosion rate of such material decreases (Figure 8) as a result of these processes. More details on the results of corrosion studies of the steel under consideration can be found in [16].
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