Restoration of steel properties after long-term operation at bends of TPP steam pipelines by heat treatment

To improve the microstructure of long-term (2.86·10 5 h) operated heat-resistant steel 12Kh1MF in the tensile zone of the main steam pipeline of a thermal power plant, a mode of restorative heat treatment (RHT) is proposed by double normalization from 1100 and 960°C followed by tempering at 740°C. An analysis of grains of different size categories across the wall thickness of the pipe in the steel structure after its operation and after additional RHT is carried out. Based on the grain size distribution near the outer surface of the pipe after RHT, the optimal mode of restoring the steel structure is substantiated. It was shown that after RHT in the optimal mode, the proportion of small grains in the steel structure increased to 55% over the entire thickness of the pipe wall, and the proportion of large grains decreased to 10% relative to long-term operated steel. Certification of steel after RHT confirmed increased hardness and improved mechanical properties in tensile and impact tests. Thus, the RHT largely eliminated the negative consequences of 12Kh1MF steel degradation during long-term operation. On this basis, the proposed RHT regime was considered an effective way to restore steel properties during operation, necessary for extending the service life of elements of operated steam pipelines.