Study On The Cyclic Plastic Deformation Behavior And Hydrogen Embrittlement Mechanism Of Deep-sea Pipeline Welded Joint

As a new type of submarine pipeline laying technology,reel-lay method has obvious advantages such as economy,safety and high efficiency.However,due to its technical characteristics,this method will cause the pipeline steel and its girth-welded joints to undergo repeated plastic deformation during the installation process,which maybe damage the structural integrity of the entire pipeline system and threaten its service safety in the hydrogen-rich environment.Therefore,carrying out the research on the key technology of reel-lay method can provide theoretical support for the practical application of this new method in China.This is of great significance to break the monopoly of foreign technology and improve the level of my country’s marine engineering equipment.In this study,in order to understand the cyclic plastic deformation（CPD）behavior of X60 pipeline steel and its welded joints during reel-lay installation,several repeated tensile-compression experiments were used to simulate the reel-lay process of the actual pipeline,and the microstructures at different deformation stages were characterized.The results showed that the compressive yield strength during the first cycle of plastic deformation was about 110 MPa lower than the tensile yield strength.As the number of cycles increased,the proportion of small-angle grain boundaries decreases,the dislocation configuration gradually evolves into dislocation walls/cells.The tensile and compressive yield strengths tended to be stable,which is due to the competition between the Bauschinger effect and the dislocation strengthening effect.CPD did not affect the tensile strength of the material,but reduced the yield-to-strength ratio.Under the action of welding thermal cycle,there was softening phenomenon in the strength overmatched welded joint of X60 pipeline,and the softened area was located in the intercritical heat-affected zone and part of the fine-grained heat-affected zone.The strain contours obtained by digital image correlation technique showed that the softened zone was the region with the most serious strain concentration in the welded joint,and the local strain reached 3.9%,far exceeding the nominal strain of3%.As the nominal strain of the welded joint increased,the strain gradient and geometrically necessary dislocation density in the softened zone increased correspondingly.The tensile fracture position of welded joint was the base metal adjacent to the softened zone,which was the result of the combined effect of the higher work hardening rate in the softened zone,the V-shaped welding groove and the restraint effect created by the“hard-soft-hard”sandwich structure.The Mises stress and strain distribution of welded joints with different mismatched strength during CPD process was preliminarily analyzed by using software of Abaqus.The result was that the strain of the strength evenmatched welded joint of X65 pipeline was lower during CPD,and the residual strain distribution in the sub-zones was also lower,which indicating that the strength evenmatched welded joint was more suitable for Reel-lay method.Furthermore,in order to understand the hydrogen embrittlement mechanism of strength evenmatched welded joints of X65 pipelines after CPD when they served in a hydrogen-rich environment,the electrochemical hydrogen charging and hydrogen permeation experiments were used to investigate the hydrogen damage in surface of welded joints and the hydrogen diffusion in sub-zones of welded joints comprehensively,systematically and deeply.In addition to capturing diffusible hydrogen,the dislocation walls/cells formed by CPD with 3%nominal strain could evenly distribute it to avoid excessive local enrichment,thus reducing the hydrogen embrittlement susceptibility of welded joints.The diffusion coefficient of the welded joint decreased from 2.63×10^-6cm²/s before CPD to 1.36×10^-6cm²/s after CPD,which was due to the increased density of dislocations acting as hydrogen traps,limiting the diffusion of hydrogen.The hydrogen embrittlement susceptibility of different sub-zones of welded joints in order from high to low was heat-affected zone,base metal and weld metal.The tensile experimental results of hydrogen embrittlement susceptibility of welded joints based on pre-charged hydrogen showed that the CPD enhanced the hydrogen capture capability of reversible traps,so that the degree of hydrogen-induced plastic damage of welded joints was lower than that of original joints.The binding energy of dislocation walls/cells to hydrogen was calculated to be31.22 k J/mol from thermal analysis spectra（TDS）,so they belonged to the strong traps among the types of reversible hydrogen traps.The greater the degree of CPD,the more serious the hydrogen-induced ductility degradation of the welded joint.Welded joints of pipeline steel will face aging problems when they are used for a long time in the deep sea.Strain aging resulted in a reduction in the number of vacancies and dislocations that acted as reversible hydrogen traps,with a corresponding reduction in hydrogen concentration from 1.57 wppm before aging to1.12 wppm after aging.Although the hydrogen concentration was reduced,the hydrogen-induced plastic damage of the welded joints was more severe.The aging caused the precipitation of C atoms from the matrix and segregation near the dislocations,thus forming the Cottrell gas clusters.Meanwhile the entangled dislocations were relaxed,and destroyed the original uniform distribution of dislocation walls/cells.That was just the essential reason of deterioration of the hydrogen embrittlement resistance of the material by strain aging.