PSI - Issue 20

Sleptsov O.I. et al. / Procedia Structural Integrity 20 (2019) 130–135 Sleptsov O.I. et al./ Structural Integrity Procedia 00 (2019) 000–000

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was established that the role of silicon, which also, like phosphorus, is prone to the formation of grain-boundary segregations (captures) is extremely unfavorable. Firstly, as the silicon concentration increases, the process of nucleation of intergranular corrosion defects within the grain boundaries accelerates, and the growth rate of corrosion damage increases by more than an order of magnitude when phosphorus concentration changes from 0.1 to 1.1%. The tendency of Fe-C-Cr-Ni alloys to IGC also increases, and τ min time decreases. Since neither X-ray structural analysis nor metallographic studies of the specimens revealed any silicon containing phases in their structure, marked by Ermakov B.S. and Solntsev Y.P. (2008), the increase in the rate of penetration of corrosion damage from the surface into the specimens should probably be attributed to the appearance of concentration fluctuations of silicon in the volume of austenitic grains and redistribution of its atoms from the body of the grain to its surface, as a result of which non-equilibrium grain-boundary silicon segregations occur within the boundaries of austenite grains. The assumption about the occurrence of segregations of silicon atoms within the grain boundaries and their role in the development of corrosion grain boundary defects was tested by two methods - the analysis of the microhardness of a solid solution along the austenitic grain and the emission spectral microanalysis method. The average grain size in the studied specimens was in the range of 60-80 microns. It has been established that in the volumes of grains more than 20 μ m distant from the boundary, the hardness is almost unchanged, and at distances of about 10-20 μ m it increases dramatically. The increase in microhardness correlates with the silicon content, the more silicon is contained in the steel, the more intensely it increases. The results of the studies of grain boundary surfaces of the destruction of specimens confirm that an increase in microhardness can be associated precisely with grain boundary silicon segregations. It follows from Table 3 that even taking into account the averaging of the data obtained by the method of emission spectroscopy, the concentration of silicon within the steel grain boundaries is sharply increased, and the size of the silicon segregation thickness is in the range of 10000-20000 Å. Table 3. The content of elements within the boundaries and volumes of grains of steel 12X18H12T with different silicon content Si content, % Element The average content of the element in wt. % in a layer with a thickness of 1000 Å at a distance from the grain boundary, Å

0-1000

1000-2000

10000-11000

20000-21000

Cr Ni C Si Cr Ni C Si

17,9 11,2 0,12 1,14 17,9 11,6 0,11 4,70

18,0 11,4 0,11 1,12 18,2 11,8 0,11 4,21

17,3 12,0 0,12 0,36 18,0 11,7 0,12 1,94

17,6 11,8 0,12 0,14 18,1 11,6 0,12 0,82

0,1

1,1

4. Conclusions

The mechanism for the formation of nonequilibrium silicon segregation is apparently based on the process of accumulation of impurities near the grain boundaries of the metal due to the drift towards the boundaries of the “vacancy-impurity atom” complexes and the fixation of this state during thermal processing, which is confirmed by the data in Table 3. In this case, the possibility of increase of defectiveness in the structure of the border areas is assumed due to an in-crease in the density of dislocations and the accumulation of vacancies in them. Indeed, it was shown in the study Lozovatskaya L.P. (1982) that in steel 03X18H11 with a low silicon content (less than 0.1%) with-in the boundaries and body of the grains, dislocations have a cellular appearance and uniform arrangement, and in high-silicon steels (silicon content 0.8%) the presence of coplaner clusters of dislocations is indicated within the grain boundaries, the number of which is more than twice the number of those in the grain body. The calculation of the activation energy of the diffusion process, made on the basis of emission spectral microanalysis data, showed that with an increase in the silicon content from 0.1 to 1.1%, the activation energy decreases from 129.3 to 108.9 kJ / mol, and the increase in its con-tent, within the grain boundaries, of austenitized