Study On Gas Entry And Escape Behavior And Control Of Molten Pool In Underwater Wet Welding

In recent years,with the increase of exploration and development of marine resources,a large number of marine engineering equipment and platform equipment have been put into use.The particularity of the marine environment places higher demands on the strength(especially fatigue strength),toughness and corrosion resistance of marine engineering equipment.Underwater wet welding is widely used in the repair and maintenance of marine engineering equipment due to simple equipments,high efficiency and low cost.However,since the water environment is not isolated during wet welding,higher diffusion hydrogen content in the joint often leads to cold cracks in the weld,the toughness of the welded joint is seriously reduced,even the structure may suddenly fail,causing serious accidents.During the underwater wet welding process,bubbles are continuously formed and escape from the molten pool for a large amount of hydrogen.Therefore,in order to control the diffusion hydrogen content of the joint and improve the quality of the welded joint,the gas entry and escape behavior of the underwater wet welding pool are explored in this paper.By adjusting the wire formulation,the droplet transfer and the pool oscillation process are adjusted,thereby reducing the diffusion hydrogen content in the wet-welded joint.It provides reliable data for the research and material development of stainless steel underwater wet welding with flux cored wire.The main ways of diffusing hydrogen in stainless steel wet-welded joints were explored.The welding process in different hydrogen content environments such as air and CCl4 was taken by an X-ray high-speed camera system.Wet-welded arc balloon gas and droplet cavity gas were collected and analyzed for their composition.The entry path of hydrogen in wet welded joint is determined: the droplet absorbs the hydrogen element in the arc balloon during the formation of the wire end,and the two transition together into the molten pool,and after cooling,diffusion hydrogen is formed in the joint.Different droplet transfer forms have a significant effect on the diffusion hydrogen content of the welded joint.When the heat input is constant,the surface tension transition ratio is reduced by changing the welding current and the arc voltage to reduce the repulsive transition ratio,thereby effectively reducing the diffusion hydrogen content of the welded joint.The underwater wet welding pool and the dynamic behavior of the weld pool under different hydrogen contents were taken by X-ray high-speed camera system.It is determined that the main route for the escape of hydrogen in the wet-welded joint is the bubble rupture process in the liquid pool metal.Compared with onshore welding,the main reason for the strong vibration of the wet weld pool is that the dynamic behavior of the weld pool is affected by the droplet impact and the bubble motion of the weld pool.The dynamic behavior evaluation index of the molten pool was established.The standard deviation of the height change in the molten pool was taken as the parameter.It was found that with the increase of the welding speed,the standard deviation of the wet welding pool oscillation was reduced from 2.41 mm to 1.35 mm,in the welded joint.The diffusible hydrogen content increased from 1.85 ml/100 g to 8.18 ml/100 g.The reason is that as the welding speed increases,the amount of exhaust gas in the molten pool decreases,and the number and volume of bubbles in the molten pool decrease,and more gas remains inside the molten pool.Therefore,the diffusion hydrogen content of the welded joint increases and the pool oscillation amplitude decreases.In addition,the increase in the length of the molten pool also increases the surface stability of the molten pool.By changing the ratio of calcium fluoride to rutile in the welding wire,the alkalinity of the wire is changed,and the form of the droplet transfer and the oscillating form of the molten pool during the welding process are adjusted to obtain a low hydrogen content and weld formation of the welded joint.In the welding process of the welding wire with a basicity of 0.91,the liquid slag will completely cover the surface of the molten pool,and the protection effect on the molten pool is enhanced.The droplet transfer process is all carried out inside the slag,so that the contact of the droplets with the arc bulb during the formation process is greatly reduced,so the amount of hydrogen entering is greatly reduced,and the diffused hydrogen content of the welded joint is lowered to 3.06 ml/100 g.