Study On The Effect Mechanism Of Laser Heat Treatment On Fatigue And Stress Corrosion Of X80 Pipeline Steel Welded Joints

X80 pipeline steel,as a kind of pipeline steel,were mainly used in West-east gas transmitting project?.Pipeline steel,as a ferritic steel,has good welding performance.However,it still can form weld defects during welding,which make welded joint become the weak link in the whole pipeline system.At present,the advanced methods used to treat pipeline steel welding joint are ultrasonic peening and laser heat treatment.While the reliability and life of welded joints after laser treatment?LHT?are still lack deeply understanding and research.The aim of this paper is to research deeply on the corrosion properties and fatigue properties of LHT X80 pipeline steel welded joints,and systematically expound on the formation mechanism of the welded joints strengthening layer,corrosion resistance properties and fatigue properties,providing a safe and feasible test methods for the Post-Weld Heat Treatment,Main research results and originality are shown as follows:?1?Discussing the microstructure,porosity,hardness,residual stress and the residual austenite of joint surface before and after LHT,the results shows that:after LHT,the holes rate on the welding surface reduced,the hardness of welding surface overall increased and the weakening tendency of the heat affected zone reduced;the strengthening layer formed by LHT with grain refinement and it thickness is 0.5mm,besides,the strengthening layer formed compressive residual stress and with a more uniformly distributed residual austenite while it content did not changed.?2?On the basis of electrochemical etching method,for the X80pipeline steel weld joints,to study the effect of H₂S corrosion environment on the electrochemical corrosion properties in NACE solution?saturated H₂S?,and the mechanism of LHT on electrochemical corrosion was discussed.The results show that the self corrosion potential of X80 pipeline steel welded joint increased from-670 mv to-600 mv,the corrosion morphologies changes from total corrosion to local corrosion to improve corrosion resistance.The surface layer of grain refining and compressive residual stress by LHT offset hydrogen pressure at a certain extent to inhibit the generation of surface crack.?3?According to the welding joint of pipeline steel working in hydrogen sulfide environment,the effects of LHT on H₂S stress corrosion in NACE solution?saturated H₂S?were analyzed with SST?slow strain test?,and the mechanism of LHT on SCC?stress corrosion cracking?was discussed.The results show that the welded joints in primitive state is brittle fracture,while after LHT fracture transform from brittle to ductile.The tendencies of hydrogen induced corrosion are reduced,and stress corrosion sensitivity index decreased from 35.2%to 25.3%,by unsafe range drop to a safe range.?4?Aiming at the welding joint of X80 pipeline steel working in stable stage,the fatigue limits of X80 pipeline steel welded joints before and after LHT were obtained by tension fatigue experiments based on Locati method.The results show that a strengthened layer is introduced by LHT to increase fatigue strength of X80 pipeline steel welded joint by11.4%,from 542 MPa to 604 MPa.Fatigue crack of the as-received sample in primitive state is initiated on the surface,while fatigue cracks are initiated on sample surface after LHT.In summary,the article provide experimental basis for LHT to improve the fatigue properties and stress corrosion resistance of X80pipeline steel welded joint and technical support for practical application of laser heat treatment.