PSI - Issue 68

Halyna Krechkovska et al. / Procedia Structural Integrity 68 (2025) 762–768 Halyna Krechkovska et al. / Structural Integrity Procedia 00 (2025) 000–000

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The effect of RHT on the brittle fracture resistance of the operated steel. The impact toughness tests of the operated steel revealed a clear decrease in the KCU index from the inner to the outer surface of the pipe (Fig. 3e, columns I). It was believed that the more than 5-fold decrease in the steel's brittle fracture resistance near the pipe's outer surface compared to the inner surface was due to more intensive degradation of its microstructure under the influence of high temperatures, stresses and hydrogenation. Their long-term impact contributed to changes in the structural-phase state and an increase in steel damage near the pipe's outer surface due to favourable conditions for creep here. The same tendency to decrease both the brittle fracture resistance and the strength and ductility characteristics of steel, operated for a long time near the outer surface of the pipe in the stretched bending zone, was considered a sign of its more intensive degradation. Typically, a decrease in the strength of steel under the influence of various factors (heat treatment, alloying, plastic deformation, etc.) is accompanied by an increase in its plasticity and resistance to brittle fracture. And vice versa, these indicators tend to decrease with the growth of steel strength. And only the decrease in steel strength due to its degradation is manifested by a simultaneous, symbatical decrease in all its properties. The tendency to decrease the impact toughness of the steel from the inner to the outer surface of the pipe also remained after applying the proposed RHT mode to the operated steel (Fig. 3e, columns II). Moreover, at all pipe wall thickness analysis levels, the KCU values of RHT-treated steel were almost twice as high as those of in-service steel. However, applying RHT to steel in service did not eliminate the gradient of brittle failure resistance across the pipe wall thickness. The KCU values of the restored steel decreased significantly (almost four times) from the inner to the outer surface of the pipe. It is important to note that, despite the lowest resistance to brittle fracture of the restored metal near the outer surface of the pipe, this value satisfied the requirements of the regulatory document. Thus, using the proposed RHT mode not only effectively restores the steel microstructure from the stretched bending zone but also significantly increases its hardness, strength and plasticity characteristics, as well as resistance to dynamic loads. All this creates optimistic prospects regarding the suitability of the proposed restoration method for extending the service life of TPP steam pipeline elements in cases where the metal's hardness on the pipe's outer surface becomes close to unacceptable values. 4. Conclusions The optimal mode of RHT for the heat-resistant steel 12Kh1MF after 2.86·10 5 h hours of operation in the stretched bending zone of the main steam pipeline of a thermal power plant has been certified. The distribution of grains of different sizes across the pipe wall thickness for steel after operation and subsequent RHT was analyzed. It was established that the proportion of large grains in the operated steel from the stretched bending zone was greatest near the outer surface of the pipe, which is due to more favourable conditions for creep here compared to other bending zones. The effectiveness of the influence of the optimal RHT mode (holding for 150 min at 1100 °C with air cooling and repeated normalization from 960 °C followed by tempering at 740 °C) of the operated steel on the distribution of grains with different sizes in the cross-section of the pipe was shown. It was shown that after such RHT, the proportion of large grains in the steel structure over the entire pipe wall thickness decreased to 10%, while the proportion of small grains increased to 55%. It was believed that these changes would have a positive effect on the creep resistance of steel. It was found that the hardness of the steel after RHT increased to 170 HB (compared to 120 HB for the operated steel). Applying the optimal RHT mode demonstrated its effectiveness in restoring both strength indicators of 12Kh1MF steel, with an even more pronounced positive effect on plasticity characteristics. The effect of restoring 12Kh1MF steel was maximally manifested near the outer surface of the pipe, where the increase in both plasticity indicators reached 30%. Additionally, using the RHT mode made it possible to almost double the impact toughness of the steel across the entire thickness of the pipe wall. The increase in hardness, strength, plasticity and resistance to brittle fracture of the restored steel relative to the corresponding characteristics of the exploited steel justifies the validity of using the proposed mode of restorative heat treatment to extend the service life of long-operated critical elements of TPP steam pipelines.