PSI - Issue 72

Halyna Krechkovska et al. / Procedia Structural Integrity 72 (2025) 149–156

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remained quite high (19.2% and 20%). Indeed, although hydrogen absorbed by the restored steel reduced its plastic deformation capacity (Fig. 2b compared to Fig. 1), the positive effect of its restoration on both elongation and RA values was still preserved. Thus, using the proposed RHT mode makes it possible to stabilize the structural and mechanical state of steel at an acceptable level even after a high degree of damage during its long-term operation. Thus, the analysis of the mechanical properties of heat-resistant steel under tension both after its restoration and after long-term operation showed a significant positive effect of restoration on all analyzed indicators of strength and ductility, as well as the preservation of the positive effect of the proposed RHT mode on all analyzed strength and plasticity indicators, as well as the preservation of this positive effect with additional hydrogenation. On the contrary, all mechanical properties of long-term operated steel deteriorated after its additional hydrogenation. This indicates that ignoring the negative effects of hydrogen absorption on the performance characteristics of steel, operated for a long time in steam pipeline components increases the risk of unpredictable failure. However, the use of the proposed RHT mode offers promising prospects for extending the service life of these elements. Fractographic signs of the influence of hydrogenation on the destruction of operated and restored 12Kh1MF steel from stretched bend zones of a steam pipeline. A characteristic feature of the specimen of the operational steel tested by tension in the air after preliminary electrolytic hydrogenation was traces of intense cracking on the lateral surface (Fig. 3a). After RHT, only traces of intense plastic deformation were observed in the vicinity of its fracture (Fig. 3b). A comparison of the results of this observation demonstrates the preservation of the positive effects of steel restoration, even with the additional negative influence of hydrogen. Fig.3. Features of the relief on the side surface of the specimen made of 12Kh1MF steel, operated for 2.86∙10 5 hours in the stretched bend zone of the main steam pipeline (a) and after applying RHT to it (b), revealed near their fractures. Both samples were tested for tension in air after preliminary electrolytic hydrogenation. It was noted that the negative signs of hydrogenation in the form of surface cracking were more pronounced on specimens cut near the outer surface of the pipe than near its inner surface, which confirmed the more intense degradation of steel. The described feature is consistent with the previously identified effect of the number of unit shutdowns on the intensity of degradation of the outer surface of steam pipe steels (Nykyforchyn (2010). To study the features of the fracture micromechanism of both variants of 12Kh1MF steel (operated and restored), fractures of samples cut near the outer surface of the pipe and hydrogenated before tensile testing in the air were analyzed. Fractographic analysis showed that in both cases the fractures were dominated by a ductile dimple relief. The presence of nanosized particles at the bottom of the dimples was an exclusive feature of the operated steel (Fig. 4a, yellow arrows). It was assumed that these particles were fragments of large carbides that retained (at least partially) cohesion with the matrix even after long-term operation of the steel. However, the connection between them was finally lost during the tensile testing of the sample. Compared to the relief of the operated steel, the dimples on the fracture of the restored steel were flatter and small carbides were more often visualized at their bottoms, which indicates the preservation of their cohesion with the matrix (Fig. 4b). A special feature of the fracture of the operated steel was also the areas of transgranular cleavage, which stood out against the background of the predominantly dimple relief. Their occurrence was due to the large number of operational voids, which were energetically favorable traps for the accumulation of hydrogen (Fig. 4a, red arrows). In the exploited steel, such pores were located mainly along the grain boundaries, whereas in the restored steel, they were more often found inside the grains in the form of deep pits at the bottom of the dimples. These deep pits were considered not healed defects during the RHT since the inclusions that initiated them had completely lost their connection with the matrix. Thus, possible paths for diffusion redistribution of elements between them were lost. As a result, they remained in the structure of the restored steel in the same form as they were in the operated steel. Larger