Investigation On The Novel Flux-cored Wires For Underwater Submerged Are Welding At Water Depth Of 80m

Manufacturing and maintainance of marine engineering is an important part of the development and utilization of marine resources.Underwater welding is a high quality joining method,which plays an important role in the manufacturing and maintainance of marine engineering.However,a number of negative effects may inevitably be produced in underwater welding due to the nature of surrounding water.Especially,underwater welding can be adversely affected by both water and pressure in deep water environment.In order to develop high-quality and low-cost underwater welding technology,new welding methods and materials for underwater welding for deep water environment are investigated.This study would be of great theoretical significance and application value.Compared with the traditional wet welding,underwater submerged arc welding(USAW)proposed in recent years has notable strengths.In order to apply the method to extreme environments at 80 m deep water,this paper focuses on the key physical processes and the changing rules of the joint microstructure and properties for USAW at deep water.A variety of new are develops.The effects of welding parameters and flux-cored wire composition on the joint microstructure and properties are revealed.In this paper,the effect of welding parameters on weld appearance and porosity at water depth of 80 m is investigated,the reasons for the high porosity of the joint are analyzed.USAW experiments using the same welding parameters are conducted,bead-on-plate weld appearance at the water depth of 80 m is worse than 0.3 m and porosity is higher.It is found that slowing down the welding speed have the greatest effect on reducing the porosity.However,little pores still exist at the top of the bead-on-plate weld using optimal welding parameters,and the porosity is 0.71%.At the same time,the micro-pores inside the butt joint result in poor overall mechanical properties.It is found that the volume fraction of H2 for the gas produced during the welding process is the largest,77.3%.That indicates that the pores in the weld are mainly hydrogen pores.To avoid the hydrogen pores,the effect of CaF2 contents in the core of flux-cored wire on USAW at deep water is investigated,a novel flux-cored wire with CaF2 for 80 m deep water was developed.It is found that with the increasing CaF2 content in the core of flux-cored wire,the volume fraction of H2 produced during the welding process shows a trend of first decreasing and then increasing.The minimum volume fraction of H2 is 27.1%when 55 wt%CaF2 is added to the core of flux-cored wire.The arc stability first enhances and then weakens,and the gas production rate during the welding process first decreases and then increases.When 45 wt%CaF2 is added to the core of flux-cored wire,the arc stability is the best,the gas production rate is the slowest,the weld appearance is the smoothest,the average tensile strength of the welded joints is 587.10 MPa,the average elongation is 16.31%,the face and root bending reached 180°.However,the grains in the weld zone of the butt joint are coarse and there is a large amount of pearlit,so the impact toughness is not ideal,the Charpy impact toughness at 20℃ is 36.04 J/mm2.Exploratory experiments for improving the impact toughness of butt joints are conducted,the effect of nanoparticles contents in the core of flux-cored wire on the microstructure and mechanical properties is investigated.It is found that the grain size of bead-on-plate welds and butt joints are reduced when nanoparticles are added in the wire.Particularly,the grain size of the fine grain area of butt joint is reduced from 7.02 μm to 4.11 μm.The best overall mechanical properties and impact toughness at 20℃ increase by 12.8%when 5 wt%nanoparticles are added in the wire.