PSI - Issue 72

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

155

dimples formed around these deep pits. It was assumed that they were formed under the influence of hydrogen absorbed by the steel during the electrolytic hydrogenation of the sample and accumulated in operational defects that were not eliminated during the RHT. Another positive aspect was noted, namely, operational defects initiated by inclusions that had completely separated from the matrix during operation moved from the boundaries into the body of the grains due to the complete recrystallization of the steel during the RHT. Thus, in the centre of large, but at the same time flat dimples on the fractures of the restored steel samples tested by tension after hydrogenation, not eliminated during the RTH operational defects were visualised (Fig. 4b, red arrows).

Fig. 4. Microfractographic features of fractures of 12Kh1MF steel specimen, long-term operated in the bend of the main steam pipeline (a), and after restoration (b), tested for tension in the air after preliminary electrolytic hydrogenation. Summarizing the obtained results, it should be said that improving the properties of 12Kh1MF steel and eliminating the negative impact of hydrogenation after its restorative heat treatment increases the prospects for using the proposed RHT mode to extend the service life of TPP steam pipeline elements. 4. Conclusions An analysis of the mechanical properties of heat-resistant steel operated in the stretched bend zone of the main steam pipeline of the TPP and after restorative RHT was carried out. A positive effect of RHT on both the strength and plasticity characteristics of the operated steel was established, confirming the effectiveness of using the optimal RHT mode. In particular, the σUTS value after RHT increased by 18.5%, and σYS by 11% for steel near the outer surface of the pipe, where the conditions for creep are most favorable and, consequently, signs of steel degradation are most clearly manifested. In terms of plasticity characteristics, an elongation of the restored steel increased by 50%, and RA by 60% compared to the corresponding values for operated steel. It is shown that the positive effect of RHT on the mechanical characteristics of steel (relative to the operated steel) under the influence of hydrogenation is somewhat reduced, but in general, it remains positive. The deterioration of the properties of the operated steel under the influence of hydrogenation was fractographically manifested in the presence of fragments of transcrystalline cleavage on the fracture, clearly visible against the background of the ductile dimple relief of the fracture. No such traces of hydrogen embrittlement were found on the fracture of the steel after RHT. This indicates that ignoring the negative impact of hydrogenation on steel's operability in long-term exploited steam pipeline elements can lead to their unpredictable failures. At the same time, the use of RHT offers promising prospects for extending the service life of such components. References

Babii, L.O., Student, O.Z., Zagórski, A., Markov, A.D., 2007. Creep of degraded 2.25 Cr-Mo steel in hydrogen. Materials Science 43(5), 701 – 707