Research On Process And Mechanism Of 304L Stainless Steel Underwater Wet Welding Process And Mechanism

The tainless steel own good mechanical properties and chemical stability,and its construction can meet the requirements of many working environments.With the development of marine resources,stainless steel materials are gradually being applied to underwater oil pipelines and underwater platforms.However,underwater welding has problems such as the loss of alloying elements,too excessive cooling rate,spattering,high arc breaking frequency,poor arc stability,etc.Which results in the decline of welding quality.The application of longitudinal magnetic field can affect the arc movement in the welding process,change the shape of the arc and perform periodic rotational motion,and alter the droplet transfer mode.In this way,the liquid metal in the molten pool is stirred,which could break the small dendrites,spread the weld seam,refine the grain,improve the welding forming and the quality of weld joint.We choose309 L stainless steel self-shielded flux-cored welding wire as the filler,and applied the longitudinal magnetic field-assisted on the weld progress.The magnetic field strength and magnetic field distribution under different parameters was measured,which was used on the analysis of magnetic control device.We explored the effects of the longitudinal magnetic field on the arc and molten pool during welding process,and the underwater welding bubble was observed.The research shows that by the effect of electromagnetic force,the fluidity of the molten pool is greatly improved,the welding seam spread,the penetration depth decreased and the melting width increased.Mechanical properties such as tensile and impact toughness tests were performed.As the frequency of the magnetic field increases,the dendrites near the weld fusion line are broken up and transformed into smaller size equiaxed crystals.By the analysis of tensile fractures,the weld fracture transformed from brittle fracture to ductile fracture.We recorded the data of electrical signals and arc bubble patterns during magnetic underwater welding process.The result shows that the effect of the magnetic field is different if the welding voltages.When the welding voltage is low,the short-circuit transition is the main droplet transition mode.The low-frequency magnetic field can break the droplets to reduce the heat accumulation and reduce thearc breaking ratio.For the middle welding voltage,the droplet transfer is mainly consisted by mixed transition,the use of high-frequency magnetic field would increase the arc stability.When the voltage is high,the droplet transfer mode is a free transition.The magnetic field might cause the short circuit transition and arc off,which increase the coefficient of current and voltage variation,and reduce the stability of the arc.The behavior of arc and bubble with the application of the magnetic field was investigated.The welding arc rotate in a certain degree under the longitudinal magnetic field,which expanded the effective area of the arc in the welding process.As the magnetic field strength increases,the arc rotation period decreased and the radius increased.The molten pool also swirls around the wire with the arc swirls.Finally,the mechanism of the magnetic field on the bubble,molten pool and arc was analyzed.