Study On Corrosion Resistance Of EH40 Steel And Its Underwater Wet Welded Joints

With the rapid development of the shipbuilding industry,the material property requirements for offshore platforms are getting higher and higher.EH40 ship plate steel is a kind of micro-alloy high strength steel with fine grain,and the quality of its welded joints is an important factor to ensure the safety of welding engineering construction.The welded joints are greatly affected by working conditions in the marine environment,so higher technical requirements are put forward for the reliability of marine engineering.There are few studies on the corrosion behavior and failure mechanism of underwater wet welded joints,and the effect of the microstructure difference for the welded joint on the corrosion resistance is ignored.In view of the wide application prospects of underwater wet welding technology,this thesis takes the underwater wet welding joints obtained by self-developed stainless steel and carbon steel electrodes as examples to analyze the corrosion behaviors of base metal and the welded joints in the marine environment.Firstly,the microstructures of welded joints under the multilayer multi-pass welding process were observed: the base metal is mainly composed of bulk ferrite and pearlite;the shape of microstructure in the heat-affected zone is different and mainly includes mainly granular bainite and ferrite;the fusion zone in the underwater wet welding joint obtained by stainless steel electrode is obvious and exists many primary cracks and secondary cracks,and the weld zone is mainly austenite;no cracks exist in the fusion zone in the underwater wet welding joint obtained by carbon steel electrode,and the weld zone is mainly ferrite and pearlite microstructure.To study the effect of the microstructure on the electrochemical corrosion behavior of the underwater wet welded joints,the corrosion morphology observation and electrochemical test were carried out with the base metal immersed separately and the welded joint immersed as a whole.It can be seen from the corrosion morphology that the growth of corrosion holes gradually accelerated and then slowed down with the increase of immersion time.It can be seen from the electrochemical tests that the corrosion resistance of all samples is both: the corrosion resistance first weakens and then increases with the increase of immersion time,and the corrosion resistance is the worst at 360 h.The corrosion resistance of the base metal in the two welded joints immersed as a whole is better than that in the heat-affected zone,mainly because the high dislocation density in the heat-affected zone accelerates the electrochemical anodic dissolution rate.The corrosion resistance of the weld zone in the underwater wet welding joint obtained by the stainless steel electrode is the best,which can be attributed to the high content of Cr and Ni;the corrosion resistance of the weld zone in the underwater wet welding joint obtained by the carbon steel electrode is the worst,which can be attributed to the higher amount of cementite,increases the cathode area and accelerates the corrosion.Compared with the corrosion behavior of the base metal immersed separately,the overall immersed base metal in the underwater wet welding joint obtained by stainless steel electrode is used as an anode,which is greatly affected by the galvanic effect,resulting in poor corrosion resistance.However,the base metal immersed as a whole in the wet welded joint obtained by the carbon steel electrode is used as the cathode,and the corrosion is inhibited.To study the effect of microstructure on the stress corrosion behavior,the failure mechanism of stress corrosion cracking in the base metal and its underwater wet welded joints was analyzed by slow strain rate tensile test,and the mechanism of microstructure on stress corrosion cracking in various regions was discussed.The tensile test results showed that the stress corrosion resistance of the base metal in the seawater environment was better;the failure locations of the two welded joints in the seawater environment were the heat-affected zone,which was mainly because the heat-affected zone is subjected to rapid heating and cooling,resulting in coarse structure and residual stress;compared with the underwater wet welding joint obtained by the stainless steel electrode,the underwater wet welding joint obtained by the carbon steel electrode has a smaller loss of section shrinkage and a smaller loss of elongation,indicating the stress corrosion susceptibility is low.