Significance of heat-affected zone softening on the structural integrity of undermatched X70 pipeline girth welds

Weld strength undermatching and heat-affected zone (HAZ) softening are critical factors contributing to girth weld failure, both of which significantly compromise structural integrity. This study evaluates the strain evolution and structural integrity of girth welds in thin-walled X70 gas transmission pipelines in accordance with current Australian and international standards. The sample investigated was a girth weld produced using a qualified welding procedure for a real pipeline, representing thousands of kilometers of X70 pipelines currently in service. The coated X70 pipe used in this work presents much higher strength than the minimum required 485 MPa for X70 grade steel, making it unlikely to achieve matching weld strength using cellulosic electrodes. This study conducted cross-weld tensile tests with and without weld reinforcement using digital image correlation (DIC) with particular attention to HAZ softening and strain evolution in the HAZ subregions. The results show that fine-grain heat-affected zone (FGHAZ) has the most significant softening effect on the HAZ, followed by inter-critical heat-affected zone (ICHAZ), coarse-grain heat-affected zone (CGHAZ) and sub-critical heat-affected zone (SCHAZ). Both weld strength undermatching and HAZ softening significantly reduce the tensile strain capacity of girth welds. The softened HAZ diminishes the strengthening effect of reinforcement, increasing the risk of fracture within the HAZ. This finding is important given that welding standards do not specify minimum levels of reinforcement, and the presence of undercut or other workmanship imperfections would further reduce the observed low tensile strain capacity. • Heat-affected zone (HAZ) subregion samples were physically simulated. • Combined effects of strength undermatching and HAZ softening on X70 thin pipe integrity. • Critical insights into inhomogeneous strain behavior of pipeline girth welds. • Recommendations provided for existing pipeline welding procedure qualification.