Effects Of Welding Procedure On Residual Stress And Hydrogen Enrichment In Girth Joints Of Hydrogen-containing Gas Transmission X80 Steel Pipeline

Under the influence of internal hydrogen pressure,microstructure inhomogeneity and residual stress on welded joint of the high-pressure hydrogen-containing pipes,hydrogen atoms penetrate into the material easily and bring about hydrogen enrichment,causing a certain of hydrogen-induced failure risk.At present,multi-layer and multi-pass welding procedures has been adopted in assembly of long distance pipelines,different microstructures and residual stresses exist in welded joints under different welding processes.Therefore,it is necessary to study the hydrogen concentration distribution in service of welded joints under different welding processes,and the influence of different welding technologies on local hydrogen enrichment,providing engineering guidance for welding process design of hydrogen-containing pipeline.In this paper,the research is carried out by combining experiment with numerical simulation.Firstly,welding experiments were carried out with STT backing weld and semi-automatic filling capping weld(6 layers and 6 passes,6 layers and 9 passes),STT backing weld and automatic filling capping weld(6 layers and 7 passes),manual backing weld and semi-automatic filling capping weld(6 layers and 8 passes),establishing 3D model baesd on the obtained weld cross-section in welding experiments.Then the welding temperature field and welding residual stress field were simulated in turn,and the temperature field was verified by the shape and size of weld cross-section and the welding thermal cycle curve,the residual stress field was checked by the hole-drilling technique,it turns out that the results of simulation are reliable.In comparison,The maximum welding residual stress is obtained by STT backing weld and automatic filling capping weld(6 layers and 7 passes),and the welding residual stress value obtained by STT backing weld and semi-automatic filling capping weld(6 layers and 6 passes)is smallest.Hydrogen diffusion models under welding residual stress were established.Based on the parameter of each subregion on welded joint obtained from high pressure hydrogen permeation test,the stable hydrogen concentration in each zone of welded joints was calculated.The results show that the concentration of hydrogen enrichment increases significantly under the influence of different microstructures and welding residual stress at welded joints,and the former is more important than the latter,hydrogen distribution is consistent with hydrostatic stress distribution.While coupling with internal working pressure,the distribution of hydrostatic stress is more uniform and the peak stress is lower than that of considering welding residual stress only,the biggest decline is manual backing weld and semi-automatic filling capping weld(6 layers and 8 passes),reaching 11.5%,and the reduction of hydrogen concentration at its corresponding position is 39.1%.Therefore,under the premise of safe operation,increasing the internal working pressure of the pipeline can reduce the hydrogen enrichment at the joint.Finally,the hydrogen concentration values of the same region in four welded joints are compared.The results exhibit that the hydrogen concentration of each subregion in welded joint of the STT backing weld and automatic filling capping weld(6 layers and 7 passes)process is at the lowest value,and there is little area of hydrogen enrichment in the whole joint.Therefore,this weld technology is recommended,and two row welding should be used in capping weld in order to reduce hydrogen enrichment.