| With the increasing global energy consumption,countries around the world are gradually making the development and utilization of the ocean an important strategy for their economic development.The extraction and transportation of marine resources have significant strategic significance for national energy security.As a permanent connection method,underwater welding has many advantages such as wide applicability to water depths and high connection quality,representing an important trend and main development direction in the field of marine engineering.It can be divided into three categories:wet,dry,and local dry welding.Among them,wet welding has gradually become a mainstream method with the advantages of economy and easy operation.However,due to the presence of water and bubbles,the stability of welding process and the mechanical properties of joint are greatly affected,ultimately leading to low joining quality.Although numerous researchers have conducted extensive research,the improvement in welding quality is limited.How to further improve the process stability and joint quality of underwater wet welding(UWW)is an urgent problem that needs to be solved.For this purpose,underwater submerged arc welding(USAW)was employed to separate the welding area from the surrounding water and optimize the combustion environment of the arc.The physical process of US AW,such as the droplet transfer and cavity evolution were analyzed with X-ray imaging method.The microstructure and mechanical properties of CCS-EH40 welds and gas composition under different water depth conditions were studied,and the mechanism of water depth on the special metallurgical process of USAW was clarified.Then,the microstructure of weld was optimized by adjusting the wire flux core,and joint with excellent comprehensive mechanical properties of the deep water USAW was obtained,which was of great significance for the engineering application of this method.It was found that the water-soluble nature of water glass made it unable to form effective protection around the welding area.However,epoxy resin,due to its high viscosity and water insoluble properties,could form a close bond with the base material,squeezing the surrounding water out of the welding area,providing long-lasting and effective protection to it.Therefore,the mixture of epoxy resin and flux could be used in the latter USAW process.Results of further research showed that the thickness of mixture has a significant impact on the geometric size and microstructure of USAW joints.As it increased,the fusion width and heat affected zone(HAZ)width of the joint increased,while the fusion depth showed an overall downward trend.The bulk ferrite in the weld gradually decreased and finally disappeared with the increase of mixture thickness,while the number of acicular ferrites gradually increased.The employment of the mixture of epoxy resin and flux during US AW could significantly improve the stability of welding process and weld formation,reduce the cooling rate,and promote the formation of acicular ferrite in the weld.Compared with wet welding,the tensile strength and elongation of the weld was increased by 22%and 6.97%,respectively,reaching to 472.93 MPa and 10.52%.Meanwhile,the bending angle was increased significantly to 180°,and the low temperature impact toughness was also greatly improved.In shallow water conditions,the gases released from the four kinds of underwater welding processes were mainly H2.Besides the compositions like those in the welding processes without epoxy resin,there were some hydrocarbons only detected in the gas released from US AW with epoxy resin,indicating the decomposition reaction of epoxy resin occurred in this process.Based on X-ray imaging system and synchronous electrical signals,the mass transfer dynamics for USAW was studied.It was found that the droplet transfer mode of USAW in shallow water was mainly repelled transfer with a small amount of surface tension transfer,while the repelled transfer could be divided into indirect repelled transfer and direct repelled transfer.Under these three droplet transfer modes,the welding process was stable without obvious arc break and short circuit.As for the gas cavity in USAW,it could be divided into two types:vertical cavity and inclined cavity.And the vertical cavity could gradually change into the inclined cavity due to the pressure of the mixed flux at the back of the welding torch.Droplets could easily fly out of the vertical cavity to become escaped spatters,while only a small number of them in the inclined cavity would fall into the unmelted flux and stay there as flux-trapped spatters.The proportion of indirect repelled transfer reduced with the increase of wire feed speed,while that of direct repelled transfer and surface tension transfer increased with it.Obviously,the effect of wire feed speed on the repelled transfer was stronger than its effect on the surface tension transfer.In addition,increasing the arc voltage could increase the proportion of repelled transfer and decrease that of surface tension transfer.Then,the influence of environment on droplet transfer in different welding processes was investigated.It was found that the droplet transfer of both UWW and land welding were dominated by repelled transfer.Due to the periodically bubble rising and necking,the UWW process was unstable and the arc broke currently,which did not occur in land welding.The droplet transfer of land submerged arc welding(SAW)was mainly flux wall guided transfer with a small amount of repelled transfer.According to the analysis of droplet force state,it was confirmed that the strong gas flow dragging force promoted the repelled transfer in US AW.Water depth has significant influence on the process stability,weld formation,microstructure,and comprehensive mechanical properties of joint during underwater welding process.Owing to the protection provided by the mixture,the arc voltage fluctuation in USAW was small,and the welding process stability did not change much with the increase of water depth,while the stability of UWW process decreased significantly as the water depth increased.Meanwhile,many pores appeared randomly in the middle and upper part of the USAW weld.For underwater wet welding,the weld formation gradually deteriorated with the increase of water depth.Under deep water conditions,there were more massive spatter-like metal distribution on both sides of the weld,and the inside of which was full of pores and non-fusion defect.The microstructure of USAW weld changed significantly with the increase of water depth.While the weld in shallow water was mainly composed of acicular ferrite,proeutectoid ferrite and side plate ferrite,the content of acicular ferrite decreased greatly in the weld of deep water,and a large amount of pearlite and massive ferrite appeared there.In addition,most mechanical properties of the joint deteriorated significantly as water depth increased,except for the tensile strength.Increasing the water depth could promote the decomposition of epoxy resin in the mixture and the volume fraction of carbon monoxide and methane except hydrogen in the main gas products of USAW.The decomposition rate constant of methane increased with the increase of water depth,which further increased the content of C in the decomposition products.Finally,the content of C in weld metal increased,and the micro structure changed significantly.By adding Fe2O3 and KMnO4 to the core flux of wire,the content of C in weld metal could be reduced to a certain level,and the formation of brittle and hard structures such as martensite could be also decreased.However,the process performance was poor,which was not conducive to the butt-welding experiment.Then,adding a certain amount of water glass to the mixture could inhibit the decomposition of epoxy resin,which effectively reduced the content of C in the weld metal,generating acicular ferrite and other beneficial structures,and appropriately improved the comprehensive mechanical properties of the joint.Under the water depth of 80 m,the stability of welding process did not change with the increase of Ni content,except for a certain proportion of short circuit.When the content of Ni in the weld was less than 2.32%,increasing it could promote the formation of acicular ferrite.While the content of Ni exceeded this level,cracks and brittle structures such as martensite would appear in the weld with the increase of Ni content.The results of EBSD analysis showed that although the grain orientation of weld metal was randomly distributed and did not change with the increase of Ni content,the increase of it could refine the grains and increase the proportion of large angle grain boundaries and the lattice distortion degree of weld matrix at the same time,resulting high density dislocation and residual stress,which increased the cracking possibility of weld metal.However,the microstructure of HAZ in USAW did not change as the Ni content varied.Ni content has significant influence on mechanical properties of deep water USAW joint.With the increase of Ni content in weld metal,the tensile strength and impact toughness of weld firstly increased and then decreased.When it was less than 2.88%,the bending angle of the joint could reach the maximum value of 180°,but it would decrease sharply as the content of Ni continiuosly increased.And the microhardness of weld metal increased with the increase of Ni content all the time.Based on the research results under the water depth of 80 m,then USAW process under the depth of 120 m was carried out,and joint with uniform shape,reliable fusion and excellent comprehensive mechanical properties was obtained.Specifically,the tensile strength and low temperature(-40℃)of weld reached to 901.6 MPa and 36.4 J/cm2,respectively,and the bending angle of joint increased to 180° without cracking. |
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