| With the continuous consumption of land resources,human dependence on marine resources continues to increase.Therefore,marine engineering in the 21st century will form a new huge industry.The construction,post-maintenance and repair of many offshore engineering structures such as oil drilling platforms,gas and oil pipelines,sea-crossing bridges,and marine factories and cities are inseparable from the application of underwater welding technology.However,the current development of underwater welding technology was slow and the main problem of the welding are high implementation difficulty and the poor quality.The causes of these problems are inseparably related to the water environment in which underwater welding is located.Generally speaking,the presence of the water environment can easily lead to poor stability of the welding process,and greatly increase the cooling rate after welding,which seriously weaken the quality of underwater welding.Therefore,in order to eliminate the interference of the water environment to the underwater welding process,a mixture composed of epoxy resin and submerged arc flux was added to the underwater wet welding area in this study,and the research of underwater submerged arc welding was carried out.In terms of exploring the process and welds performance of underwater submerged arc welding,this study compared the welds morphology,welding electrical parameters,welding thermal cycle process and finally determine the best solvent to flux ratio of 4:6.When epoxy resin was used as the solvent,its weld appearance,microstructure and welding process stability are better than WG-USAW,which proves that epoxy resin is more suitable for underwater Submerged arc welding solvent composition.In the research of welding mechanism,the process of underwater submerged arc welding was successfully photographed by designing a suitable visual acquisition scheme.It was found that the reason why the underwater submerged arc welding process is relatively stable is that the large amount of gas generated during the welding process supported a stable arc cavity structure in the welding area,allowing the arc to burn stably in the arc cavity,and the excess gas was discharged in the air flow channel,which further maintains the stable existence of the arc cavity structure.In addition,the droplet size of underwater submerged arc welding was much smaller than the droplet size of underwater wet welding.Due to the drag force of the air flow,the droplets transition direction of underwater submerged arc welding are between the upper left and lower left of the welding area..In the research of butt weld performance,it was found that no matter whether Q235,Q345B(the new national standard is Q355)or CCS-EH40 steel was used as the base metal,the underwater submerged arc welding performance of PPS-AN2 flux cored wire is much better than that in underwater wet welding.The coarse grain zone of the weld was mostly composed of tight acicular ferrite,while the grain size of the fine grain zone was significantly refined compared with the underwater wet welded joints.The indicators of tensile properties,bending properties and impact toughness have been greatly improved.In the development of flux-cored welding wire,the tensile strength of the newly designed seven flux-cored welding wires in underwater submerged arc welding far exceeds the related performance of the original PPS-AN2 welding wire.Part of the welding wire can reach a maximum of 572MPa,even exceed the tensile strength of the base metal,which provides the necessary foundation for achieving equal strength matching or high-assembly welding.In addition,when 5%Al2O3 nanoparticles were added to the No.0 welding wire,the results obtained show that the welding wire has high strength and excellent plastic properties,which is of great significance for further improving the quality of underwater submerged arc welding. |
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