| Welded joints are the weak link of pipeline systems and the location of oil and gas pipeline failure accidents.The China-Russia world-class natural gas pipeline project,which is under construction in China,will have a total mileage of 3,371 km after full completion and commissioning.The number of pipeline welds is nearly 300,000,which is the key to the intrinsic safety of the entire pipeline system.Laser shock peening has the advantages of reducing residual stress,improving surface quality and increasing fatigue life.It has a broad application prospect as a post-welding treatment technology.Biomimetic Laser Shock Peening(BLSP)technology has the potential to further improve the efficiency and overall performance of material processing.At present,the practical application of BLSP treatment for high-grade steel pipeline welds is in a predicament due to the lack of process standard guidance.The BLSP strengthening mechanism for weld fatigue failure is still not clear,and the lack of process application specification basis has become the main bottleneck restricting its application in weld treatment process.Driven by national demand,this study focuses on investigating the fatigue crack propagation(FCP)mechanism of BLSP-treated welded joints in highgrade steel pipelines.A fatigue life extension controlling method based on BLSP is then proposed.Taking the pipeline steel X80 of China-Russia East Route Project as the object,with the help of numerical simulation technology,connecting the theoretical analysis and experimental research method,it is investigated the residual stress field distribution and microstructure evolution rules of the weld strengthened by BLSP.On this basis,the influence mechanism of BLSP on the fatigue characteristics and fatigue life of structural components is investigated.The main work and conclusions are as follows:(1)Optimization of the laser welding process and the evolution of the residual stress field in X80 steel.The laser welding orthogonal experiments with laser power,welding speed and defocusing amount as the main factors were carried out,and the laser welding process parameters were optimized in connection with the range analysis.Comparative analysis of mechanical properties and microstructure differences between laser and metal inert-gas(MIG)welding of X80 steel.The results show that the laser welding performance is better than MIG welding.The average hardness of the laser weld region is about 293.1 HV,which is 35.1% and 11.8% higher than that of the base metal and MIG welding,respectively.The tensile strength of the laser welded joint is 659.3 MPa,and toughness fracture has occurred at the base material.The tensile strength of the MIG welded joint is 578.4 MPa,and brittle fracture has occurred at the weld.A three-dimensional laser welding numerical model is established based on the optimal laser welding process,revealing the temperature field and stress field distribution characteristics of X80 steel during the laser welding process.(2)Mechanisms of residual stress field and microstructure evolution in Laser Shock Peening(LSP)X80 steel and welds.A finite element model of LSP was established,with a focus on investigating the influence rules of nine-point laser shock on the residual stress field distribution of X80 steel material by changing the laser process parameters.A tensile specimen model of LSP weld zone was established.Through the stress mapping method,the stresses in the weld region were successfully transferred to the tensile specimen model.The mechanism of LSP on the weld stress field and the microstructural characteristics such as strength,dislocation density and configuration,and grain size distribution in the weld region of X80 steel were investigated.The results show that by adjusting different LSP parameters to increase the peak laser pressure,the overlap rates and impact times,as well as selecting a smaller diameter spot,a more uniform residual stress is obtained and the maximum residual stress on the material surface and the depth of the impact layer are increased.After LSP treatment,the tensile stress in the weld zone is gradually converted to compressive stress,which is significant in promoting grain refinement on the material surface.(3)Parametric analysis of biomimetic characteristics and fatigue life prediction of BLSP X80 steel.The effect of residual stresses induced by ordinary BLSP and combined coupled BLSP on the stress intensity factor(SIF)and effective stress ratio in FCP was quantified by extended finite element method(XFEM).The dynamic FCP behavior of single/double-sided LSP was analyzed and fatigue life was predicted by using the XFEM and the SIGINI subroutine.The results show that the residual compressive stress induced by BLSP reduces part of the tensile stress of the fatigue load when the crack length is propagated from 7 mm to 27 mm.It is beneficial to reduce the crack driving force and inhibit the FCP.The average effective stress ratios under different biomimetic intervals are about 104%～177%.The combined coupled biomimetic specimen has a faster contact with the impact region,which further improves the BLSP effect.Compared to the untreated specimens,the fatigue life under single/double-sided LSP residual stresses is improved by about 35.7%and 41%,respectively.(4)Research on the fatigue properties of BLSP X80 steel welded joints.The FCP behaviour of different BLSP welded joints was analyzed based on the XFEM.The fracture morphology and surface dislocation distribution characteristics of different biomimetic fatigue specimens were analyzed experimentally,revealing the anti-fatigue life extension mechanism of BLSP.The results show that the fatigue source is transferred from the surface to the subsurface by BLSP,which inhibits crack initiation compared with the untreated specimens.The fatigue life of the BLSP(3#),the combined coupled BLSP and the large-area LSP specimens are improved by 42.46%,56.40% and 55.51%,respectively.The fatigue striations are reduced by 33.67%,43.88% and 46.94 %.The transient fracture zone has a larger size and depth of the dimples,which is indicated that the FCP rate after LSP is lower.The performance of combined coupled BLSP specimen is similar to that of large-area LSP.It is indicated that similar fatigue properties are obtained by impacting a specific area on the specimen surface,while improving the machining efficiency. |
