Research on Thermite Assisted Underwater Wet Welding Metallurgy and Arc Stability of Low-alloy Steel

Author:Li Hong Liang

Supervisor:feng ji cai liu duo


Degree Year:2018





Underwater wet welding,an important technology in ocean engineering,is widely used in the fields such as wreck salvage,marine rescue,submarine oil pipeline installation and so on.To improve the applicability of available underwater wet welding consumables in complicated ocean environment,reduce electric energy consumption and improve welding efficiency,self-shielded flux-cored wires with exothermic additions were developed based on the advantage of self-propagating reaction in heat generation.The characteristics of heat transfer process in underwater wet welding with exothermically self-shielded flux-cored wire were particularly analyzed,and influence mechanism of thermite types and contents on arc stability,molten droplet transfer process and metallurgical behavior were systematically studied in this paper.Underwater wet welding of E40 steel and Q460steel could be achieved under low heat input,resepectively,and wet welded joints with good quality were obtained.The basic formulation of underwater wet self-shielding flux-cored wire was determined by theoretical analysis and experiment verification,and the principle of selecting thermite was clarified.For low-alloy steel E40 with a carbon equivalent0.39wt.%,a low carbon steel flux-cored wire with TiO2 as the main component of slag system was selected.On this basis,a low carbon steel flux-cored wire with the addition of Al/CuO was designed.For low-alloy steel Q460 with a carbon equivalent0.46wt.%,a nickel-based self-shielded flux-cored wire with CaF2 as the main component of slag system was selected,and Ni-based flux-cored wire with Al/NiO addition was also designed.The influence mechanism of thermite on the arc stability and droplet transfer behavior of underwater wet welding was investigated using X-ray high-speed camera system and real-time welding electrical signal acquisition system.The relationship between the type and content of thermite and the arc stability of underwater wet welding was clarified.The force behavior model of liquid droplets was established based on the static equilibrium theory.The influence mechanism of Al/CuO system on droplets transfer behavior of underwater wet welding was emphatically analyzed.It was found that the arc stability could be improved in a certain range of heat flux content.When the content exceeded 30wt.%,the amount of spatter increased and the arc stability became worse.The Al/CuO system could reduce the repelled transfer ratio,refine the droplet and improve welding appearances.Effects of heat input on arc stability and microstructure and properties of E40steel underwater wet welding was studied by using low carbon steel self-shielded flux-cored wire.On this basis,effects of Al/CuO on solidification and crystallization behavior of weld metal was analyzed,and the relationship between tensile strength,impact toughness of weld metal and thermite content was established.It is found that the reaction product Cu was the main factor to change the microstructure of Al/CuO system weld metal.Copper significantly improved the undercooled austenite stability,made matching of strength and toughness of weld metal poor,and increased the hot cracking tendency.The distribution of copper in weld metal was further analyzed.Copper-rich phases with continuous distribution along grain boundaries were susceptible to form at weld end and hot cracking occured under residual tensile stress.In order to realize the underwater wet welding process of Q460 steel with carbon equivalent greater than 0.45wt.%,the effect of welding parameters on arc stability and welding quality was systematically analyzed using developed nickel-based self-shielded flux-cored wire.The optimum heat input range of nickel-based self-shielded flux-cored wire was determined,and Q460 steel wet welded joints with good welding quality was obtained.The results revealed the superiority of Ni-based filler metals in welding steel with Ceqq larger than 0.45wt.%.However,the range of acceptable welding parameters for Ni-based self-shielded flux-cored wire was narrow,and was restricted by heat input.Therefore,the effect of Al/NiO on underwater wet welding process properties was further analyzed in this paper,and the influence of Al/NiO on the metallurgical behavior of underwater wet welds was clarified.The effect of thermite on heat transfer process of underwater wet welding was studied,and the relationship between cooling rate of welded joint,welding arc thermal efficiency and thermite was revealed.Combining with the experimental measurement and theoretical calculation,effects of the thermite on the chemical heat by thermite reaction,arc heat and wire metling coefficient were studied.It was found that the addition of heat flux changed the welding thermal process of flux-cored wire,Al/CuO and Al/NiO series flux-cored wire increased the melting coefficient of flux-cored wire,increased the proportion of chemical heat,decreased electric energy consumption,and increased the flux-cored wire melting coefficient.Effects of heat flux content on the cooling rate of welded joints were analyzed by measuring the thermal cycle curve of heat affected zone,and the corresponding relationship between cooling rate and heat input of welding process was established.Based on the influence mechanism of thermite on underwater wet welding process,the formulation of exothermicaly self-shielded flux-cored wire was optimized.The thermite content in flux core of exothermic low carbon steel wire was 20wt.%.The weld metal maintained a good impact toughness of 35J/cm2 while gaining 672MPa strength.The heat flux content in flux core of exothermic Ni-based wire was 30wt.%.The welding arc stability was improved remarkably.The tensile strength of weld metal was 522MPa and the impact toughness value was up to 115J/cm2.In order to further study the heat transfer process of exothermically self-shielded flux cored wire and classify the role of chemical heat produced by thermite reaction in welding wire melting,the balance between heat production and heat dissipation of wire extensition in underwater wet welding were quantitatively analyzed.Arc heat,chemical heat and resistance heat were the main energy sources for welding wire melting.The convective heat transfer between liquid droplets and arc bubbles was analyzed.On this basis,the effect of thermite on welding arc efficiency was analyzed by comparing welding process in the air with the same welding parameters underwater.Finally,the cooling rate of UWW joint with exothermically self-shielded flux-cored wire was predicted by finite element numerical simulation.The reliability of the model was verified by comparing the simulation results with the measured values.