PSI - Issue 59

V. Makarenko et al. / Procedia Structural Integrity 59 (2024) 385–390 V. Makarenko et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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The data in Table 1 shows that during the long-term operation of underground sewage facilities, intense metal hydrogen saturation occurs, directly leading to its embrittlement (increase in hardness), which is confirmed by the results of the now conducted studies. In particular, Table 1 shows that pipes with a long service life (more than ten years) are particularly exposed to hydrogen saturation of surface layers (up to ≈ 1 mm). Thus, the steel 09G2S with a service life of 10 years in the middle of the wall has a hydrogen content of about 5.5 ppm, and after 40 years of operation, it is 16.7 ppm. At the same time, 17G1S grade steel with a service life of, for example, 20 and 30 years has a hydrogen content of 9.5 ppm and 11.7 ppm, respectively. The same trend persists for low-carbon steel grade 20, that is, depending on the service life, along with hydrogen saturation, simultaneous embrittlement of the structure occurs – microhardness increases significantly (by 2.3...2.7 times), which reduces the resistance of pipe metal to cracking in underground pipe structures. X-ray spectral analysis showed the presence of sulfur (up to 2.95%), oxygen up to 22%, as well as aluminium (up to 0.70%), silicon (up to 1.75%) and manganese (up to 2.21%) found in the outer layers of corrosion products of pipe structures with a service life of 10 years (steel 17G1S and 09G2S). Sulfur and manganese were found in flake like particles of brownish colour under the outer layer (3.23% and 4.08% by weight, respectively). A similar pattern is observed for pipe steel grade 20: the sulfur content in corrosion products reaches almost 2.8%, and oxygen up to 19.2% by weight. In general, in our opinion, the mechanism of hydrogen destruction in the presence of sulfur and oxygen can be explained as follows. Atomic hydrogen can penetrate into metal before molecules are formed. The condition for this, in addition to the small radius of hydrogen, maybe the presence of non-metallic inclusions (sulfides and oxides), which turn out to be collectors of hydrogen and places of its intensive penetration into the metal. These processes are also influenced by the sulfur content, which slows down the recombination of hydrogen atoms, increases the effective concentration of hydrogen absorbed by the metal and reduces crack resistance in the zone of maximum biaxial stresses. The main role of inclusions such as SiO 2 , Al 2 O 3 and MnS is reduction of the formation of cavities at the interface “matrix - inclusion”, which turn out to be collectors of molecular hydrogen. At the same time, the nature of the inclusion is essential – brittle silicates and alumina are destroyed during thermomechanical processing in the manufacture of profile rolled products, and the number of traps increases. In contrast, lamellar sulfides are able to deform without disturbing the interface. The presence of ductile, well-deformed, non-metallic inclusi ons in steel (primarily FeS•MnS and MnS) causes the production of a stitch structure characterized by a pronounced anisotropy of corrosion-mechanical properties. Moreover, the shape of non-metallic inclusions is almost impossible to change by subsequent heat treatment. Makarenko et al. (2023) found that some sulfide inclusions in carbon and low-alloy steels act as initiators of the formation of corrosion cracks, while others do not affect this process. The occurrence of cracks is mainly due to the placement of specific non-metallic inclusions; as they grow, the cracks become intergranular. In our research, the metallographic data obtained were used as parameters to contrast the initial state of steels (not in use) and pipe structures of sewer structures with their operational state, as well as to contrast the structural features of the destruction of samples, the nature of the propagation of fatigue cracks in steels in the initial (not in use) and use. Hydraulic steel's corrosion and mechanical characteristics depend on the composition, shape, size, and number of carbide phases. Using X- ray diffraction analysis methods, the parameters of the crystal lattice of the α -matrix were measured, and the level of its elastic curvatures (microstresses of curvatures), as well as the distribution of carbon in ferrite and steel, were estimated (Table 2). As can be seen from the data obtained, an increase in the service life of steel sewer pipe structures leads to an increase in the values of the volume-centred cubic cryst al lattice of the α -solid solution and the growth of microstresses in the structure. At the same time, part of the carbon from the decomposed cementite passes to the boundary of the α -matrix. The second part, apparently, remains on dislocations, passing into microcracks, and also goes to the formation of new finely dispersed carbide particles; relatively large carbide particles are formed at the grain boundaries between perlite and ferrite. Special experiments have established that with an increase in the service life of sewer structures, the mass fraction of cementite (Fe 3 C) in the metal significantly decreases (Fig. 2). As a rule, the above processes lead to local embrittlement of the metal of sewer metal structures, and under favourable conditions (with alternating cyclic loads), micropores form near these particles, coagulating and forming cracks.