No abstract

Pipeline transmission is an essential means of oil and gas transportation, but its safety is threatened by fatigue fracture that generally occurs in the heat-affected zone (HAZ) adjacent to welded joints of the pipeline. Therefore, it is important to determine the fatigue properties of HAZ. In this work, the microstructure and mechanical properties in the HAZ of a X80 pipe were accurately simulated by thermal simulation. The fatigue life and crack growth rate of typical sub-HAZ were tested. The results showed that fine grain HAZ exhibited the lowest strength and fatigue life. In contrast, coarse grain HAZ had the highest strength, but its fatigue life was lower than that of the intercritical HAZ. The microstructure of each sub-HAZ and the effect of this structure on fatigue properties were analysed and discussed in detail. The results provide insight into the effects of microstructure and mechanical properties on the service safety of pipeline transportation.

The weld joint is the weakest zone of a longitudinal-seam submerged arc welded (LSAW) pipe, which has great effects on its in-service properties and safety. The weld geometry and shape of the weld joint are important factors that affect the mechanical properties of the pipe. In this work, two-dimensional (2D) and three-dimensional (3D) finite element models (FEMs) of a large-diameter, heavy-wall API X80 LSAW pipe were established, and the stress distribution of the pipe was calculated to simulate the in-service gas transmission pipeline. Results showed that the stress distribution calculated by both 2D and 3D models are similar. Consequently, the effects of weld geometry, softening of heat affected zone (HAZ), and strength matching of the weld joint on bearing pressure capability of the pipe were analyzed, and the results showed that the bearing pressure capability of the pipe can be improved by controlling these parameters. These results are beneficial in providing references for welding parameter design and improved properties of the X80 LSAW pipe.

To obtain high strength and excellent deformability for ferrite/bainite dual-phase (F/B DP) pipeline steel for gas pipelines based on strain-based design, the volume fractions of ferrite and bainite should be considered first. In this work, abstract representative volume elements (RVE) of finite element models (FEMs) of mesostructure for F/B DP pipeline steel with volume fractions of bainite between 30% and 58% were established, and the effects of volume fraction of bainite on the tensile properties and deformation compatibility were studied. Results show that the stress and strain in the mesostructure were primarily distributed in the bainite and ferrite, respectively, and strain concentration occurs at the ferrite/bainite interface. With increasing volume fractions of bainite, the strain localization factor (SLF) and strain ratio of ferrite with bainite ( ε F / ε B ) decrease, which can improve the deformation compatibility of the F/B DP pipeline steel. However, the stress ratio of bainite with ferrite ( σ B / σ F ) and the contributions of bainite to stress and strain sequentially increase, and, as a result, the strength increases and the ductility decreases. Therefore, a balance of strength and deformability can be obtained when the optimal volume fraction of bainite is in the range of 40% to 48%.