Assessment of the effect of the steels structure dispersion on its magnetic and mechanical properties

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 4 2021 Fig. 15. Test statistics obtained from the analysis of the considered samples obtained for H c The microstructure research of the samples under study and the information analysis about the values of internal stresses allow to explain the observed phenomena. The dropout of a hardened sample made of 15KhSND steel is apparently associated with the formation of such a state of the structure in which the violation of the coherence of the lattices of martensite and cementite is not observed [23, 38-41]. Being in this state, both phases have a low density of defects in the structure of the crystal lattice [39, 42–45], which affects the value of internal stresses, making it suf fi ciently low compared to the internal stresses observed during a similar heat treatment for steels 09G2S and St3. The dropout in the values of the analyzed parameters for a sample made of 09G2S steel tempered at a temperature of 350 o C can be explained by the processes of steel softening arising from a decrease in the density of dislocations and various structural defects that accumulate on carbide inclusions, which are the compound of manganese with carbon [23, 43–47], and leading to a decrease in internal stresses. In addition, it reduces the magnitude of internal stresses and the process of decomposition of martensite into ferrite and cementite, which occurs during medium tempering, and diffusion of carbon from carbon-enriched martensite regions [42, 45]. The phases of ferrite and depleted martensite formed during such a process have a lower hardness compared to the initial phase of martensite, which causes a decrease in the magnitude of internal stresses and, as a consequence, softening [38, 47]. The deviation from the found dependencies may be due to the process of the cementite particles coagu- lation and an increase in the average grain size for a sample made of St3 steel tempered at 650 o C. These processes lead to the equilibrium state of the structure [23, 38–42]. An increase in the average grain size and a decrease in the number of grains observed on a microsection leads to an increase in the length of high-angle boundaries, which leads to a decrease in the magnitude of internal stresses, and therefore, the distortions of the crystal lattice that they cause. The process of grain enlargement stops when the “critical size” is reached. The steel softening and the softer phases formation signi fi cantly affect the values of the coercive force and the ultimate strength. The processes occurring during this heat treatment also lead to a decrease in the value of the grain size factor. Conclusions 1. The general satisfactory correlations are observed for the ultimate strength, internal stresses, coer- cive force and the grain size factor for steels 09G2S, 15KhSND and St3. The mathematical analysis of the obtained dependencies is carried out. Its results indicate the relationship between the analysed parameters.