Research On The Environment Assisted Fatigue Failure Of X80 Pipeline Steel Welded Joint

With the rapid development of national economy and increasing demands for energy supply by citizens,the transportation of oil and natural gas has become a vital national security issue.X80 pipeline steel,as a series of excellent pipeline steels with both low economic costs and desired service performances,has been widely used in the national oil and natural gas transportation pipelines.Therefore,the security and integrity of the X80 pipeline in service are extremely important.During their service periods,the pipelines may suffer degradation resulted from the environment and fatigue damage caused by stress fluctuations.Therefore,it is of great significance to elucidate the fatigue performance of X80 pipeline steel,especially the weakest welded joint,under environments at different level of degraded susceptibilities(such as the near-neutral pH environment and hydrogen enriched environment).In this article,the fatigue behavior of the CO₂ Arc Welding(a kind of gas metal arc welding)welded joint of X80 pipeline steel under water environment and electrochemical hydrogen charging condition was studied.Through comprehensive testing methods such as metallographic observation(OM),scanning electron microscope(SEM)analysis,energy dispersive spectrum(EDS)analysis and electron backscatter diffraction(EBSD)analysis,the fatigue S-N curves and fatigue crack growth rates of X80 pipeline steel welded joints both in the air environment and water environment were studied.Furthermore,the mechanisms in the crack initiation and propagation stages of the water induced fatigue were discussed.The mathematical models between the diffusion hydrogen concentration and the hydrogen charging time as well as the hydrogen charging current density of the X80 pipeline steel welded joints were established,respectively,and so does that of the fatigue life.In addition,the mechanisms of hydrogen-induced cracking as well as hydrogen-induced fatigue cracking were investigated after electrochemical hydrogen charging treatment.The main conclusions are as follows:Different from fatigue in the air,the existence of the water environment causes the pipeline steel to transform from a single cyclic stress behavior to a synergistic effect between the environmental medium and the cyclic load.The water environment accelerates the initiation process of fatigue cracks by promoting the formation of oxides.Meanwhile,water accelerates the fatigue crack growth rate through the coalescence of water and the stress intensity factor,which specifically changed the orientations of the grains near the crack tip to the crack preferred<001>//ND,increased the relative proportions of low-?CSL boundaries and triggered the secondary slip system of bcc crystalline structure in X80 pipeline steel welded joint.After electrochemical hydrogen charging,the fatigue life of X80 pipeline steel welded joint decreased with the increase of charging time and charging current density.In addition,the fatigue fracture mode changed from transgranular to intergranular with the increase of diffusion hydrogen content in the joint.Brittle and hard inclusions such as MnO produced by welding process are blamed for the fatigue cracks'turning around.The fusion line of the welded joint becomes the preferential propagation path of hydrogen-induced fatigue cracks and hydrogen-induced cracks.The decrease of the small-angle grain boundary at the crack tip,the increase of the grain orientation gradient and the difference in the capability of grain deformation between the grains from the opposite side of the crack are the main factors for the degradation of fatigue performances in the welded joint with the increase of hydrogen concentration.