Study On The Microstructure And Properties Of Welded Joints Of Steel For High-Wire-Energy Welding

In recent years,China’s shipbuilding industry has developed rapidly,and a relatively complete steel system for ship and offshore engineering has been initially established.EH420 is widely used in marine engineering due to its high strength,strong toughness,and other advantages.In the construction of large ships,welding processes directly affect shipbuilding efficiency,with welding hours accounting for about 30%-40%of the total shipbuilding hours.Therefore,while ensuring the performance of welded steel,improving welding line energy and reducing welding passes have become the focus of research.The increase in linear energy affects the microstructure,mechanical properties,and corrosion resistance of welded joints,thereby affecting the safety performance of ships and offshore platforms.This study investigates the influence of welding processes on the microstructure and properties of welded joints,as well as the corrosion behavior of high energy welded joints in 3.5%NaCl solution.The main research content and results of this paper are as follows:1)Two different high energy welding methods,170 k J/cm and 250 k J/cm,were used to weld 70 mm thick EH420 marine steel.The 170 k J/cm line energy was used for two pass welding,and the 250 k J/cm line energy was used for single pass welding.The effects of welding pass and line energy on the microstructure and properties of the welded joint were studied.The results show that at 170 k J/cm T/4,due to the influence of both welding thermal cycles on the microstructure transformation here,the microstructure has"heritability",and the size of M-A component increases,reducing the impact energy,which is about 49 J lower than the upper surface of 170 k J/cm.With the increase of line energy,the size of M-A components on the surface of 250 k J/cm is larger than that on the surface of 170 k J/cm,and the impact energy on the surface of 250 k J/cm is reduced by about 18 J compared to that on the surface of 170 k J/cm.After welding,the inclusion of Ti N wrapped Mg O still exists in the Heat-affected zone.Mg O has high temperature stability,and the mismatch between Mg O and Ti N is 0.34%,which can better adhere together to improve the stability of pinned particles Ti N;the mismatch degree of between Ti N andα-Fe is 4.47%,which belongs to effective heterogeneous nucleation and can induce the formation of needle like ferrite within the crystal.2)The welded joint under 250 k J/cm line energy was taken as the research object to carry out corrosion experiments,and the weld,Heat-affected zone and base metal were studied by immersion experiments and electrochemical experiments.The results show that the corrosion rates of weld,Heat-affected zone and base metal are 3.7812,4.6406 and5.0458 g/（m²·d）respectively after 20 days of immersion,and the corrosion rates gradually increase.The corrosion products are composed of Fe OOH,Fe₃O₄,and Fe₂O₃.The open circuit potential difference between the weld seam and the base metal is 54.43 m V,and galvanic corrosion can occur inside the sample;Self corrosion current density of weld,Heat-affected zone and base metal is 4.7688×10^-6、8.8854×10^-6 and 1.0056×10^-5 A/cm²,and the charge transfer resistance is 356.2,229.6,208.1Ω/cm² respectively.The fitting data shows that the corrosion resistance of the weld is the strongest,followed by the Heat-affected zone,and the corrosion resistance of the base metal is the worst.The initial stage of corrosion is mainly pitting corrosion,which starts around inclusions.The inclusions in the weld are mainly high Al₂O₃,with high electrochemical stability;The inclusions in the Heat-affected zone and the base metal are mainly high Mg O,and the electrochemical stability is poor,so the pitting corrosion behavior in the weld is light,and the pitting corrosion behavior in the Heat-affected zone and the base metal is poor.