The Impact of Weld Repairs on the Microstructure and Mechanical Integrity of X80 Pipelines in Oil and Gas Transmission

The integrity of oil and gas pipelines is critical to energy transportation and has significant implications for national energy security. This study employs finite-element numerical simulation to investigate the impact of multiple repairs on the microstructure and mechanical integrity of X80 pipeline girth welds. The effects of varying repair iterations on the microstructure, toughness, and loading capacity of high-strength steel pipes were analyzed. The results revealed that the microstructure of the welded joint remained unchanged across different repair instances, but the toughness, optimized by welding heat input, diminished after three repairs due to grain growth from repeated thermal cycles. Specifically, the impact toughness of the welding line, fusion line, and adjacent areas decreased significantly after three repairs, with the toughness of the welding line dropping to 25 J and the fusion line dropping to 30 J. The hardness of the welded joint decreased with repairs, and the dispersion of hardness increased. The average hardness at the welding line decreased from 25 HV to 20 HV after three repairs. Residual stress in the repaired girth weld was highest in the filling welding layer, increasing with the number of repairs. The loading capacity of the girth weld significantly decreased after the first repair (by 12.1%) and continued to decrease with additional repairs (15.3% after the second repair and 16.7% after the third repair). It is concluded that X80 pipeline girth welds should be repaired no more than twice to maintain optimal structural integrity.