Effect Of Defects On Fatigue Life Of X80 Buried Gas Pipeline Steel Welded Joints

With the continuous growth of energy consumption demand in China,the number of natural gas transmission pipelines has been increasing year by year,and the service conditions of pipelines have become more complex.X80 pipeline steel with long distance transportation,large diameter,thick wall,and high strength has become the preferred choice for natural gas pipeline network construction.At present,natural gas pipelines are generally connected using welding methods.During the pipeline welding process,defects inevitably occur at the joints,making the welded joints the weakest part of the pipeline.The investigation results of pipeline failure show that the failure of in-service pipeline steel mainly occurs at the welded joints,with fatigue failure exceeding 30%.Therefore,it is of great significance for the safety design and maintenance of natural gas pipeline transportation to study the fatigue performance of pipeline steel welded joints,especially the effect of welding defect on the fatigue life of welded joints.In this paper,the effect of different welding defect on the fatigue life of X80 pipeline steel welded joints is studied by combining material test and finite element simulation.The work done and conclusions obtained in this paper are as follows:(1)Based on the Abaqus general finite element platform,the dflux heat source subroutine of multi pass welding was compiled using double ellipsoid heat source,and the temperature field changes during the welding process and the distribution of residual stress after welding were studied.At the beginning of welding,the heat input was small,and the weld and the base metal in contact with the weld could not be fully melted,which easily led to welding defect;When the welding heat source moves to one-third of the position,the temperature of the weld exceeds 2000℃,and the welding process enters a stable state,maintaining stable welding quality.The residual stress after welding is mainly concentrated in the weld and Heat-affected zone.The maximum residual stress after welding is 508MPa,which is less than the yield strength of the material;The analysis of residual stress on the path shows that both transverse and longitudinal residual stresses exhibit a symmetrical distribution;The accuracy of the simulation results was verified by monitoring welding temperature and measuring residual stress after welding.(2)The tensile and fatigue crack growth rate tests were carried out on X80 welded joints,and the statics parameters of welded joints and the material parameters related to fatigue crack growth were obtained:the yield strength of X80 welded joints was 540MPa,and the tensile strength was 630MPa;The material parameters obtained by fitting the Paris formula are C=3.57e-10,m=2.40.(3)Based on ASTM standards,a joint simulation was conducted using Abaqus-Franc3d-Matlab to study the effects of single cracks,double cracks at different welding root positions,and composite defects of cracks and pores on the fatigue performance of welded joints.The results showed that when there was no welding residual stress,the cracks basically propagated according to pure mode I cracks.When there was welding residual stress,K_Ⅱwould greatly increase,The crack propagation mode changed from pure mode I crack propagation to mixed mode I-II crack propagation mode,and the crack propagation path at the weld toe and Heat-affected zone was greatly deflected;Not every part of the welded joint will cause fatigue crack propagation,and the center of the weld joint is subjected to compressive stress,resulting in crack closure;The minimum fatigue life of the weld toe is 86003 cycles,making it the most dangerous part of the welded joint.(4)When there are collinear double cracks propagating in the joint,the side near the crack front will attract each other,leading to an increase in stress intensity factor.After crack fusion,the stress intensity factor of the new crack front will undergo a sudden change,and the crack propagation rate will gradually accelerate.As the crack continues to expand,the morphology of the crack front will gradually become smooth and smooth;As the initial spacing between collinear cracks increases,the crack propagation life gradually decreases.When the crack spacing is 5mm,the fatigue life decreases by 55.49%compared to a single crack.(5)When there are pores in the welded joint,they can cause stress concentration,but the degree of stress concentration caused by pores does not have a significant linear relationship with the diameter and volume ratio of pores.The size of stress concentration is related to the position and state of pores,specifically manifested as internal single pores<surface single pores<adhesive pores;As the pore diameter increases,the fatigue life continuously decreases.When the pore diameter increases from 0.6mm to 0.7mm,the life decreases by 24.6%.As the crack propagates,the stress concentration effect of the pore weakens;As the volume ratio of pores increases,except for surface pores,the fatigue life of joints shows a slow increasing trend,and pores can play a similar role as crack arrest holes.