The Development And Alloying Additions Of High-strength Covered Electrode For Underwater Wet Welding

According to the characteristics and the problems of underwater wet welding，low-hydrogen rutile electrodes are developed and underwater wet welding tests arecarried out. With a neutral slag basicity of1.124, the diffusible hydrogen content ofthe joint is56ml/100g. Qualified seam formability, slag spreadability anddetachability are achieved. In an effort to improve weld properties, a test matrix of17coating formulations are prepared for the alloying addition of deposite metal.Meanwhile, the transition behaviors of alloy elements are studied. The microstructureand mechanical properties of the joints obtained by electrodes with different alloyingadditions are particularly analyzed.For the electrode coating with Mn-Si-Ni-Mo-Ti alloy system, the transferefficiency of Ni and Mo are up to90%, but because of the high partial pressure ofoxygen in UWW environment, Mn, Si and Ti are faced with a high oxidation loss.The transfer efficiency of Ti is only4.03%. When Ni content increases from0to3.16wt.%, the addition of Ni restrains the formation of coarse PF along the austenitegrain boundary.2.04~2.45wt.%of Ni content produces an as-deposited microstructureof more than30%AF. The tensile strength of all-weld metal increases with the theincrease of Ni, the highest tensile strength reaches640MPa. Impact toughness firstlyincreases and then decreases with Ni addition increases. The highest impact toughnessis archived when Ni content is in the range of2.2~2.5wt.%. When Ni content is2.45wt.%, the impact toughness at0℃reaches63.59J/cm2, elongation reaches14%,and the results represent good plasticity and toughness. Above-mentioned mechanicalperformance meets the requirement of Class A Welds Classification in the AWS D3.6Code.The addition of Mo further depresses the formation of coarse PF. But high levelof Mo content (0.609wt.%) result in the formation of lath-like bainite and brittlemartensite. AF nucleation increases with the combined addition of Ti and B. When Ticontent reaches0.016~0.018wt.%, microstructure of deposited metal is dominated byfine and homogeneous AF(80~95%.), which is expected microstructure for UWW.But when Ti and B contents continue to increase, there turns out to be large number ofbainite and M-A constituents in deposited metal, which exacerbate the mechanicalproperty of the weld.Micro-hardness and all-weld metal tensile strength increase with the addition ofNi, Mo and other alloying elements in deposited metal. On average, every0.3wt.%increase of Mo content leads to about60MPa increase of tensile strength, the highesttensile strength reaches682.5MPa. But high level of Mo content(0.609wt.%) has harmful effect on the plasticity and toughness of the weld. The deposited metal with0.652wt.%Mn,0.34wt.%Si,2.45wt.%Ni,0.357wt.%Mo and0.016wt.%Ti achievesgood plasticity and toughness while maintains its tensile strength592MPa, whoseimpact toughness at0℃and elongation reach53.97J/cm2and16.2%, respectively.