Creep relaxation to relieve residual stress in girth-butt welded X80 pipelines: Simulation and experiment

The residual stress in girth-butt weld presents safety risks for large-diameter oil-gas pipelines, necessitating an in-depth investigation into the welding residual stress and the development of effective methods to mitigate these stresses, thereby enhancing structural integrity. In this work, a finite element girth-butt welding model was developed to predict the residual stress of X80 pipelines. The residual stress relief resulting from local post-weld heat treatment (PWHT) was simulated based on the Norton creep model applicable to X80 steel. The simulation results, encompassing the residual stress both before and after PWHT, were validated through blind-hole drilling measurements. The results demonstrate that the welding residual stresses across all orientations were significantly reduced following PWHT, with a maximum stress reduction of approximately 360 MPa. The primary mechanical mechanism for residual stress relief was identified as high-temperature creep, and it was concluded that the PWHT alleviated welding residual stress effectively when the heating temperature exceeded the creep activation temperature. The consistency between the finite element analysis results and the experimentally measured residual stresses affirms the validity and feasibility of the finite-element-based approach for predicting welding residual stresses.