Comprehensive Study On The Fatigue Properties Of Welded Joints Of X80 Welded Pipes

In order to realize the large transmission volume(30 billion m~3/n)and long-distance transmission,the large-diameter,thick-walled,high-strength X80 pipeline has become the preferred material for the construction of long-distance natural gas pipelines.With the continuous improvement of the pipeline steel grade and transportation pressure,high requirements are put forward for the pipeline transportation safety.At present,welded pipes are used in the construction of large-diameter,thick-walled,and high-strength X80 pipelines.A large number of studies have shown that long-term service pipeline failures mainly occur at welded joints and are related to fatigue and corrosion.Therefore,the research on the fatigue properties of the welded joints of pipeline is of great significance to the safety design of pipeline transportation.An X80 welded pipe used in the Third West-East Gas Pipeline Project is taken as the research object in this work.In view of the impact of welding,weld grinding at pipe end,diameter expansion and internal and external coating anti-corrosion processes during the manufacturing process on the structure and performance of the X80 welded pipe,thermal simulation,metallography,TEM,SEM,EBSD,X-ray stress tester and other methods are used to study the microstructure of the HAZ and fatigue property of welded joint.The influences of the microstructure and properties of the HAZ,weld reinforcement,plastic deformation and residual stress on the initiation and propagation of fatigue cracks at welded joints are comprehensively analyzed.The results of welding thermal simulation show that the fine-grained HAZ is softened by the welding thermal cycle,and its strength is the lowest,resulting in the lowest fatigue life;while the coarse-grained HAZ has high strength,and its fatigue life is higher than that of the fine-grained HAZ,but due to the coarsening of the microstructure and a large number of low-angle boundaries,the fatigue crack growth rate increases faster,thus reducing the fatigue life of the coarse-grained HAZ;the strength of the intercritical HAZ is higher than that of the fine-grained HAZ.The internal microstructure is small and the high-angle boundaries account for a higher proportion,therefore,its fatigue life is the highest.In order to study the changes in properties of the X80 welded pipe before and after the weld grinding,the finite element analysis model of the original welded joint of the X80welded pipe and the welded joint of the X80 welded pipe after removing the weld reinforcement is established.The effect of weld reinforcement on the fatigue properties of welded joints is studied.The results show that the fatigue crack initiate at the weld toe of the inner weld and growth along the coarse-grained HAZ,due to the stress concentration caused by the weld reinforcement;after the weld reinforcement is removed,fatigue crack initiate in the coarse-grained HAZ next to the fine-grained HAZ and growth along the fine-grained HAZ,due to the weakest strength in the fine-grained HAZ and the hardness gradient between the coarse-grained HAZ and the fine-grained HAZ.Therefore,the fatigue strength of the welded joint of the X80 welded pipe after removing the weld reinforcement is 1.31times that of the original X80 welded pipe.The fatigue test results of the actual welded joint of the X80 welded pipe and the welded joint of the X80 welded pipe after removing the reinforcement confirmed that the fatigue crack initiation position and fatigue life were consistent with the finite element analysis.Pre-strain method was used to study the effect of the plastic deformation at the weld joint caused by the diameter expansion on the property of the welded joint of the X80welded pipe.The results show that the strength and total fatigue life of welded joints increase with the increase of plastic deformation in the pre-strain range of 1%to 3%.The reason for the increase in the fatigue life is the increase in the fatigue crack initiation period caused by the increase in strength,but the fatigue crack growth period is shortened.For each1%increase in the pre-deformation at the welded joint,the fatigue crack growth period is decreased by about 22%,which seriously reduces the safety of pipeline transportation.The mechanism of fatigue crack growth acceleration at the welded joint after plastic deformation is analyzed and the results show that the cleavage fracture,occurring during the fatigue crack growth,accelerates the fatigue fracture.The residual stress distribution and fatigue life at the welded joints of X80 welded pipes after different pretreatments are measured to evaluate the impact of the coating process on the properties of the welded joints.Stress-relief annealing can significantly reduce the residual stress of the welded joint,improve the residual stress distribution at the welded joint,and increase the fatigue life of the welded joint.The X80 welded pipe after stress-relief annealing after plastic deformation had the highest fatigue life due to the increase of strength and the release of tensile residual stress.The results show that the fatigue life reaches 10~7 cycles when the maximum alternating stress is 550 MPa.