Residual stress assessment of multiple welding repairs of girth welds in pipeline used in oil industry

摘要

This work presents the residual stress assessment of multiple welding repairs in the same area in seamless API X52 low carbon pipeline. Four conditions of shielded metal arc welding (SMAW) repairs and one as welded specimen of the girth weld were evaluated to determine changes in the microstructure (metal base, weld bead and heat affected zone) to evaluate their effect on the residuals stresses and mechanical properties of the welded joints. One of the mainly adverse effect of residuals stresses are in the susceptibility of stress corrosion cracking (SCC) of buried pipelines. The residual stresses were measured through X-ray diffraction (XRD). Samples were evaluated on the internal side of the pipe in longitudinal and circumferential direction. Circumferential residual stresses are greater than longitudinal stresses. Microstructural characterization of the welding joints through scanning electron microscopy (SEM) was performed. Relation between microstructure, mechanical properties and residual stresses was carried out. In general, the grain size increases with the number of repairs, and consequently there is an increase in residual stresses. Significant reduction in Charpy-V impact resistance with the number of weld repairs was observed overall in the weld fusion line. The hardness and strength increase in the first repair and in subsequent repairs decrease. As increasing the average grain size, the hardness and the absorbed energy decreases. Generally, the residual stresses showed a tendency to decrease in the first repair and after showed an increase with the number of repairs. It is clear that residual stresses depend more than the position of measurement than the welding repair number, which is directly relate with the microstructure and phases presented.