Optimization of Post-Weld Heat Treatment Process for Long-Term Used TP347 Pipe

Abstract During the repair of thick-walled TP347 pipes that have been in service for over a decade under high-temperature and high-pressure conditions, stabilization heat treatment is required following welding. Software simulation was performed to model and calculate the residual stress in the weld seam post-heat treatment. The simulation results demonstrated that stabilization heat treatment effectively reduces residual stress in the weld seam. Furthermore, as the heat treatment temperature increases, the residual stress values in the weld seam decrease, consistent with the theoretical understanding that stabilization heat treatment promotes stress relaxation and microstructural homogenization in the weld region. Samples from thick-walled TP347 pipes that had been in service for more than ten years were welded and subjected to stabilization heat treatment at varying temperatures (850°C, 900°C, and 950°C). These samples were then analyzed through microstructural observations, tensile testing, hardness testing, and corrosion testing. The results revealed that the hardness values of the samples exhibited minimal variation across different heat treatment temperatures. However, compared with the results given in references, the tensile strength values were lower than those of new pipes, with some values failing to meet standard requirements. Additionally, tensile strength decreased as the heat treatment temperature increased. Metallographic observations indicated an increase in precipitated phases within the material after heat treatment, with grain size becoming larger as the heat treatment temperature rose. Scanning electron microscopy (SEM) analysis confirmed the formation of NbC precipitates within the grains and along grain boundaries in the heat-treated samples. Corrosion testing, conducted in accordance with ASME G36-94 (2018), revealed that the crack propagation rate of the samples increased with higher heat treatment temperatures. These findings highlight the importance of strictly controlling the heat treatment temperature during the repair of austenitic stainless steel that has been exposed to prolonged high-temperature service. Excessive heat treatment temperatures can lead to material performance degradation, emphasizing the need for precise temperature management to maintain the integrity and performance of the repaired components.