OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 4 2022 The martensite grain-size number increases to 7 with a further raise in the tempering temperature to 350 °C. At the same time, a certain martensite quantity begins to decompose into ferrite and perlite. There is a diffusive carbon outflow from the martensitic matrix [22, 25]. These processes occurring in the structure lead to softening, which is associated with a decrease in internal stresses and, as a result, a decrease in the defectiveness of the crystal lattice due to a decrease in the dislocation density and various structural defects, as well as a lower hardness of the resulting ferrite phase [27–30, 31–34]. This process clearly reflects the dependence of the value of internal stresses on the tempering temperature. These results are presented in [21] and are based on the analysis of X-ray diffraction patterns taken on a DRON-7 X-ray diffractometer [35]. The results show that the value of internal stresses decreases with an increase in the tempering temperature in this temperature range. The removal of distortions of the cementite crystal lattice, which is part of perlite, leads to its transition to an equilibrium state. The cementite becomes “highly coercivity material” overall. However, a decrease in the amount of martensite and an increase in depleted phases (both martensite and ferrite) leads to a decrease in the overall level of both hardness and coercitive force. In accordance with Kersten’s theory of “inclusions” it is associated with a small contribution to the total value. The manganese sulfide is expanded by internal forces in the direction of internal stresses into the form of elongated inclusions or chains [21] when the temperature rises. The coefficient of thermal expansion of manganese sulfide is higher than that of iron [22, 23]. Therefore, this compound experiences greater compression than the matrix [24] when the material is cooled. As a result, the appearance of elongated manganese sulfide particles is observed Fig. 4. Inclusions of manganese sulfide in a 09Mn2Si steel sample after low-temperature tempering: a – spherical inclusions in a micrograph obtained using a scanning electron microscope; b – the distribution of manganese in the micrograph shown in a; c – the distribution of carbon in the micrograph shown in a; d – the distribution of sulfur in the micrograph shown in a а b c d
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