Hydrogen Embrittlement Susceptibility And Critical Embrittlement Parameter Of Multi-layer And Multi-pass Welded Joint In High-pressure Hydrogen-containing Gas Transmission Pipeline

High-pressure hydrogen-containing pipes are assembled by high strength steels using multi-pass and multi-layer welding technique.However,the complexion of microstructure of the heat affected zone(HAZ)can be severe due to multiple thermal cycle process.Based on what has been studied,the mechanism and influence of hydrogen embrittlement(HE)on secondary HAZ of X80 pipeline steel are still unknown.Thus,the protection of the weakest part can be hardly developed.In this thesis,thermal simulation technique was used to reproduce large volume of sub-regions of HAZ.Slow Strain Rate testing was applied to investigate the HE susceptibility of each sub-region and identify the most susceptible region.Optical Microscope,Electron Back-Scattered Diffraction,Transmission Electron Microscope and X ray Diffraction were applied to differentiate the microstructure features and their synergistic effect on the HE.Finally,SSRT under different hydrogen embrittlement was investigated to test the critical pressure value of the highest HE index zone,providing engineering guidance and reference for pipeline service under hydrogen environment.Microstructure morphology and the HE susceptibility of sub-regions of reheated HAZ were studied first through optical microscope observation and SSRT respectively.The relationship between microstructure and the HE susceptibility was analyzed.The results show that all the sub-regions exhibited plasticity loss under hydrogen environment to different degree caused by different phase composition,distribution as well as grain size.In comparison,reheated HAZs have relatively higher HE index among all the sub-regions.The sub-regions of reheated coarse grain HAZ were then chosen to compare HE index difference.Typical microstructure features and their synergistic effect were discussedwhether they can be influential factors for the HE.The most influential factor was confirmed.Lath bainite microstructure is more susceptive to HE relatively,and other factors such as grain boundary area,grain orientation,high angle grain boundaries and MA constituents also affect the HE behavior of microstructure.It turns out that the content and distribution of MA constituents play an important role on the HE.The MA constituents which are massive in size and segregate along the grain boundaries can deteriorate the HE resistance of ICCGHAZ and become the area with high possibility of crack initiation and propagation.According to the cask theory,the tolerance limit of hydrogen pressure of the whole weld joint can be obtained by determining the critical value of the weakest part.Therefore,ICCGHAZ was selected to test the critical pressure value through SSRT under different hydrogen pressure.The results exhibit that the plasticity loss becomes worse with the hydrogen pressure increase,indicating the rise of the HE susceptibility.By comparing the HE index,increment as well as the proportion of brittle zone and its increment trend,the critical value was finally confirmed to be 3.5 MPa.