Study On The Fatigue Behavior And Fatigue Life Improvement Mechanism Of Welded Joints Under Water Mist And H₂S Environment

The welded joints of steel bridges and submarine pipelines are not only subjected to the effect of alternating load,but also are affected by the corrosive environment.It is of great significance to investigate the fatigue behavior of welded joints and apply appropriate fatigue life improvement technology to ensure the safe service and improve the fatigue performance of welded structures in corrosive environments.In this paper,the fatigue behavior and life extension mechanism of welded joints in corrosive environment are investigated.Meanwhile,the fracture mechanism of welded joints subjected to the coupling effect of fatigue damage and hydrogen is explored.The corresponding fatigue curve of welded joints in corrosive environment has been analyzed to provide a certain amount of fatigue data for the correction of the standard design curve.The main conclusions are as follows:For dry air and water-spray environment,the fatigue performance of welded joints shows for medium-scale welded joints with high welding residual stress,the design curves recommended by IIW and BS7608 are conservative in dry air,while the standard design curve are dangerous in water-spray environment.For welded joints with low welding residual stress,the standard design curve is safe.For medium-scale welded joints,the post-weld heat treatment（PWHT）,high-frequency mechanical impact（HFMI）and PWHT+HFMI increase the fatigue strength of welded joints by more than60%,90%and 160%,respectively.For small-scale welded joints,PWHT and HFMI treatment can improve the fatigue strength of welded joints in water-spray environment by about 10%and more than 50%,respectively.The fatigue performance showed that the fatigue life of welded joints in H₂S environment is much lower than that in air,and the difference between them is more than one order of magnitude.With the decrease of stress range,the difference of fatigue life of welded joints in H₂S and air environment increases gradually.The S-N curve in H₂S environment is far lower than the standard design curve.PWHT can increase the fatigue life of joints by 40%in H₂S solution.HFMI treatment can improve the fatigue life of the joint by more than 4 times,while the PWHT+HFMI treatment can improve the fatigue life of the joint by more than 2 times.The improvement effect of HFMI is better than that of the PWHT+HFMI treatment.This is because HFMI introduces higher compressive residual stress and inhibits the initiation and propagation of cracks.The fracture mechanism of welded joints in corrosive environment is analyzed,which revealed that the coupling effect of fatigue damage and hydrogen from corrosive medium is an important factor in the fatigue life reduction of joints in corrosive environment.Meanwhile,fatigue damage would further enhance the hydrogen concentration in welded joint,and then promotes the hydrogen embrittlement sensitivity of welded joints.Moreover,the evolution of dislocation and grain boundary caused by microstructure heterogeneity and fatigue damage would affect the hydrogen distribution and hydrogen embrittlement sensitivity of the joint.The result proves that hydrogen-enhanced decohesion and hydrogen-enhanced localised plasticity are the main mechanisms affecting the hydrogen embrittlement of welded joints.The result of hydrogen penetration of welded joints shows that after fatigue damage,the hydrogen diffusion coefficient decreased and the hydrogen concentration captured by hydrogen traps increased.The PWHT treatment increases the hydrogen diffusion coefficient of welded joint,and reduces the hydrogen concentration and hydrogen trap density.The critical factors affecting the hydrogen distribution in welded joints are analyzed by numerical analysis.The results show that the interaction of hydrogen and stress would give rise to the non-uniformity of hydrogen distribution in welded joints.The results show that the hydrogen concentration at the weld toe increased with the rise of external load.Moreover,the presence of tensile welding residual stress further increases the hydrogen concentration at the surface area of weld toe and base metal.However,HFMI-treatment can effectively reduce the hydrogen concentration near the surface of the weld toe.Although the compressive residual stress of welded joints introduced by PWHT+HFMI treatment is lower than that of HFMI treatment,it can still inhibit the accumulation and diffusion of hydrogen at the weld toe.Even though the effect of external load would cause the release of an amount of compressive residual stress of welded joints treated with HFMI,resulting in the change of hydrogen distribution of welded joint,the hydrogen concentration at the weld toe is still very low,which demonstrates the effectiveness of HFMI-treatment in reducing the hydrogen concentration at the weld toe.