Study on root strain concentration of girth weld joint with variable wall thickness for X80 pipe

In this study, considering the particularity of variable wall thickness joints, a strain concentration characterization method for variable wall thickness joint is proposed. The finite element simulation method is used to establish a finite element model consistent with the physical joint, and calculate the root strain concentration under different misalignment, different weld strength matching, different pipes strength collocation and different heat affected zone (HAZ) softening conditions. Based on the aforementioned analysis, it is found that the misalignment, the weld strength matching, the pipes strength configuration and HAZ softening all affect the strain concentration at the root of the joint. In particular, the misalignment is easy to cause geometric structure mutation of root, which is the main factor leading to strain concentration for root of weld. The larger misalignment, the lower strength matching of the weld, and the higher pipes strength can increase the strain concentration at the root. However, a large root strain concentration can induce strain aging embrittlement or coupling with welding defects to cause material damage, providing conditions for root crack initiation and propagation. Considering the special strain distribution of the variable wall thickness joint, a strain concentration characterization method for the variable wall thickness joint is proposed, and on this basis, the influence of misalignment, weld strength matching, pipe strength matching and heat affected zone softening on the strain concentration at the root of the variable wall thickness joint is explored, and effective measures are also proposed to alleviate the strain concentration.