Research On Fatigue Properties Of Welded Joints Of X80 Longitudinal Submerged Arc Welded Pipe

Fatigue properties are important parameters for the safety design of pipelines.Owing to welded joint is the most vulnerable area of proreties in steel pipe,therefore,the study on fatigue behavior of welded joints is great significance to evaluate the security and service life of pipelines.In this thesis,the fatigue life of welded joints of X80 longitudinal submerged arc welded(SAWL)pipe with excellent deformability was meansured by using the tension fatigue test.The effects of weld reinforcement,residual stress of welded joints and pre-tension deformation on fatigue crack initiation location,propagation and the fatigue life of welded joints were explored.The results show that the fatigue crack initiation occurs at the inside weld toe,hence,the inside weld toe with welded joint specimens of pipeline steel pipe is the weakest area for fatigue.The fatigue life of welded joints is raised notably by removing welded reinforcement,meanwhile,the fatigue crack initiation changes and appears in the heat affected zone(HAZ).The severe local stress concentration resulting from the weld reinforcement is the main responsible for promoting fatigue crack initiation and reducing the fatigue life.The residual stress observably affects the fatigue life of welded joints.The fatigue life of welded joints arises after the stress relief annealing.However,the stress relief annealing has little effect on the position of fatigue fracture.Comparing of specimens with and without weld reinforcement,the different effects of the stress relief annealing on the fatigue life could be found,the rise of the fatigue life of welded joint specimens without weld reinforcement is more than that of welded joint specimens with weld reinforcement.Pre-deformation has also some effect on the fatigue properties of welded joints.In the range of 4% pre-deformation,the fatigue life of welded joints increases with the increase of deformation.In addition,the same law is observed for the test samples without weld reinforcement and after the stress relief annealing.