Underwater Wet Flux-cored Arc Welding Research Articles

A novel approach of ultra-low diffusible hydrogen welding wire for underwater wet flux-cored arc welding: a spontaneous barrier of molten slag

In underwater wet welding, a hydrogen-rich arc atmosphere generated by the thermal decomposition of water leads to extremely high diffusible hydrogen content and defects such as pores and cracks in the weld. In the present study, a neutral slag system was designed and verified for its protection in underwater wet flux-cored arc welding. This neutral slag system forms a stable protective layer of molten salt on the surface of the molten pool, mechanically isolating the molten metal from the water environment and effectively delaying heat loss. More crucially, it also retained the chemical metallurgical dehydrogenation component. When pure Ni belt was used as the outer metal, weld joints free of porosity were obtained, and the diffusible hydrogen content of the deposited metal was 3.61 mL/100 g, close to the level of onshore welding. This neutral slag system was further extended to the use of a pure Fe belt, where the diffused hydrogen content of the molten metal was 10.40 mL/100 g.

The Efficiency of Thermite-Assisted Underwater Wet Flux-Cored Arc Welding Process: Electrical Dependence, Microstructural Changes, and Mechanical Properties

Alumino-thermic additions to the flux-cored wire can be utilized to enhance welding heat input, improve welding productivity, and minimize the dependency on power sources in the air environment. In this article, the influence of Al/Fe2O3 thermite on electrical dependency, microstructural characteristics, and mechanical performance during the underwater wet welding of Q235 steel was investigated. The results revealed that adding exothermic flux enhanced the underwater wet welding appearance. The basicity of the slag steadily decreased from 1.77 to 0.73 because of the formation of Al2O3. Thermite increased the quantity and diameter of inclusions in the underwater wet weld metal. The excessive addition of Al/Fe2O3 thermite resulted in agglomerated inclusions in the weld metal. Significant microstructural changes from grain boundary ferrite to acicular ferrite were mainly caused by the de-oxidation products of the thermite process. When thermite was added to the flux-cored wire, the ultimate tensile strength first increased and then decreased, while the microhardness of the underwater wet welds showed an obviously increasing trend. The addition of exothermic flux additions to the flux-cored wire can help generate chemical heat and increase the energy required to melt the wire. This study established a link between thermite addition and joint performance, paving the way for the development and application of thermite-assisted self-shielded flux-cored wire.

A novel strategy to prevent hydrogen charging via spontaneously molten-slag-covering droplet transfer mode in underwater wet FCAW

Underwater wet welding is widely used in offshore platform maintenance, oil pipeline repair and wreck salvage operations. A high diffusible hydrogen content is recognized as one of the critical reasons behind the reduction in the reliability of underwater wet welded joints. In this study, the entry route of hydrogen during underwater wet flux-cored arc welding of 304 stainless steel at a depth of 0.5 m was first confirmed experimentally using an in-situ X-ray imaging system. The hydrogen charging was deemed to dominantly occur when pendant droplets were formed due to the extremely high temperature of molten metal and the absence of slag coverage. This resulted in 13.51 mL/100 g of diffusible hydrogen remaining in the droplet and 9.42 mL/100 g in the deposited metal. To prevent hydrogen charging, an unprecedented molten-slag-covering droplet transfer mode was developed by reducing the CaF2 content and maintaining the CaF2/TiO2 to an appropriate ratio in the flux, through which the molten slag spontaneously covered the droplet, electric arc and molten pool completely during the entire welding process and the entry of hydrogen was successfully blocked and the diffusible hydrogen content was reduced to the same level as that achieved via onshore welding (3.26 mL/100 g).

Effect of heat input on microstructure and mechanical properties in underwater wet flux-cored arc welding of structural steels

Effect of heat input on microstructure and mechanical properties in underwater wet flux-cored arc welding of structural steels

Improvement of microstructure and mechanical properties for underwater wet 16Mn/304L dissimilar steel welded joints assisted by presetting butter layer

Underwater wet flux-cored arc welding (FCAW) is a simple and economic method to join dissimilar steel structures widely used in offshore applications. The available research on underwater wet welding of dissimilar steel joints is inadequate. In this paper, a butter layer was applied to eliminate the formation of hardened martensite at heat affected zone of 16Mn side and to reduce the stress concentration at the fusion boundary. The influence of ERNiCrMo-3 butter layer on the microstructure, elemental distribution, and mechanical performance of underwater wet 16Mn/304 L dissimilar steel welded joints was investigated. The grain boundary type and grain growth orientation across the fusion boundary were particularly investigated. The use of the ERNiCrMo-3 butter layer helped to eliminate the formation of hardened martensite resulting from the rapid cooling rate. Type I boundary and type II grain boundary were high angle grain boundaries, which was found at the Ni-based weld metal/ferritic steel (butter layer/ferritic steel) interfaces. The impact toughness of HAZ at 16Mn side was improved by the use of butter layer because of the absence of martensite. The butter layer was accompanied by an increase in ultimate tensile strength in the orders of ~80 MPa. The expected research results can enrich the underwater welding/repair technology of dissimilar steel structures and guide the future design of dissimilar steel welded joints.

The mechanisms of underwater wet flux-cored arc welding assisted by ultrasonic frequency pulse high-current

An ultrasonic frequency pulse high-current (UFPHC) was utilized to improve the stability of underwater wet flux-cored arc welding on Q235 steel. Based on the studies of the bubble evolution, arc behavior, droplet geometry, weld formation, and mechanical properties of the joint, the arc extinguishing phenomenon was suppressed effectively by the additional UFPHC, which decreased the amount of spatter on the workpiece and improved the appearance of the weld. The additional UFPHC reduced the average droplet radius by 40% and increased the average transition frequency by 99%, compared with the same value of additional welding current. The UFPHC enabled to decrease the hardness of the heat-affected zone by 15% due to the production of less martensite compared to results obtained by simply increasing the welding current. The vibration effect of the UFPHC in the welding pool was indirectly confirmed by the significantly decreased porosities in the final joint. The morphology of the impact fracture of different welding cases showed that the UFPHC was helpful to decrease the brittle fracture area in the weld joint and increase the value of the impact toughness for the weld joint.

Investigation of arc stability, microstructure evolution and corrosion resistance in underwater wet FCAW of duplex stainless steel

Underwater wet flux-cored arc welding (FCAW) process was used to join 2205 duplex stainless steel using Ni-based filler under three different heat inputs of 16.9 kJ/cm, 23.5 kJ/cm and 32 kJ/cm. The arc stability, microstructure and corrosion resistance with different heat inputs were studied. The results showed that the welding process at the heat input of 23.5 kJ/cm was stable with less spatter and good weld appearance. The weld width, weld penetration and dilution ratio increased with increasing heat input. A single-phase austenitic microstructure was obtained for all welds with varied parameters. The heat-affected zone consisted of coarse ferrite and austenite in grain boundary. The grain size of weld zone and heat-affected zone coarsened as the heat input rose. Compared with the base 2205 base metal, the hardness of HAZ increased about 15% while the weld metal changed little. Electrochemical test in 3.5% NaCl solution indicated that the corrosion resistance of the weld metal under high heat input was better, but its corrosion resistance was inferior to that of base materials. Weld metal obtained at 23.5 kJ/cm possessed comprehensive properties in terms of arc stability and corrosion resistance. These results are helpful to promote the application of duplex stainless steel in the fields of nuclear power fields.

Investigation of Process Stability and Weld Quality of Underwater Wet Flux-Cored Arc Welding of Low-Alloy High-Strength Steel with Oxy-Rutile Wire

Abstract The paper described the experimental findings of underwater wet welding of E40 steel using self-shielded flux-cored wire with a TiO2-FeO-MnO slag system. The arc stability, weld quality and corrosion resistance with different heat inputs were studied. The results showed that the wet welding process of the designed wire displayed good operability in the range of investigated parameters. The microstructure and mechanical properties of the weld metal depended on the heat input. Due to the high fraction of acicular ferrite in the weld metal, the mechanical properties of the weld metal under low heat input had better tensile strength and impact toughness. Fracture morphologies at low heat input had uniform and small dimples, which exhibited a ductile characteristic. The diffusible hydrogen content in the deposited metal obtained at a heat input of 26 kJ/cm significantly reduced to 14.6 ml/100g due to the combined effects of Fe2O3 addition and the slow solidification rate of molten metal. The microstructure also had a significant effect on the corrosion resistance of the weld metal. The weld metal with high proportions of acicular ferrite at low heat input exhibited the lowest corrosion rate, while the base metal possessed a reduced corrosion resistance. These results were helpful to promote the application of low alloy high strength steel in the marine fields.

Numerical simulation of bubble and arc dynamics during underwater wet flux-cored arc welding

The dynamic behaviors of underwater bubbles and arcs are critical to welding processes and weld quality in underwater wet flux-cored arc welding. In this study, bubble evolution promoted by an arc plasma and flux decomposition was simulated using hydrodynamic equations. The influence of water on bubble and arc behavior was considered with regard to ionization of hydrogen in the arc, gas-liquid interfacial surface tension, and the absolute pressure. The interface between the water and bubble was determined via the volume of fluid and level-set methods. Physical parameters of mixed gas or plasma in the bubble were calculated by mass weighted average method based on the values of pure gas or plasma. The arc and bubble temperature distributions, fluid flows, and internal pressures were analyzed to reveal heat and mass transfer in the arc. The cycle of arc bubble evolution was about 0.056−0.063 s in calculation. The velocity of arc plasma could reach up to 500 m/s, which was one of the major factors to influence the arc bubble behaviors. Experiments were performed and the results showed that the simulated bubble shape, growth cycle, and evolution process agree well with the experimental data.

Influence of welding speed on weld pool dynamics and welding quality in underwater wet FCAW

The effects of welding speed on the molten pool dynamics, the weld forming and the diffusible hydrogen content in deposited metal during underwater wet flux cored arc welding (FCAW) were studied. The molten pool was elongated with a higher solidification speed, as the welding speed was increased. The faster solidification improved the stability of the molten pool by reducing the bubbles and gas disturbance during FCAW, whereas it caused more hydrogen remaining in the deposited metal and increased the diffusible hydrogen content of the weld. According to the variation coefficient of weld reinforcement, the weld formation was improved at first and then deteriorated with the increasing welding speed. More bulges appeared in the weld at a low welding speed and pits were formed more easily with a high welding speed. The occurrence of the defects was closely related to the molten pool behaviors, which were determined by the pool angles during FACW.

Investigating the Advantages of Ultrasonic-assisted Welding Technique Applied in Underwater Wet Welding by in-situ X-ray Imaging Method.

In this study, the effects of ultrasonic on melt pool dynamic, microstructure, and properties of underwater wet flux-cored arc welding (FCAW) joints were investigated. Ultrasonic vibration enhanced melt flow and weld pool oscillation. Grain fragmentation caused by cavitation changed microstructure morphology and decreased microstructure size. The proportion of polygonal ferrite (PF) reduced or even disappeared. The width of grain boundary ferrite (GBF) decreased from 34 to 10 μm, and the hardness increased from 204 to 276 HV. The tensile strength of the joint increased from 545 to 610 MPa, and the impact toughness increased from 65 to 71 J/mm2 due to the microstructure refinement at the optimum ultrasonic power.

Numerical Analysis of the Metal Transfer and Welding Arc Behaviors in Underwater Flux-cored Arc Welding

Based on the VOF method, a three-dimensional numerical analysis model coupling both droplets and arc behaviors is developed to simulate the metal transfer and arc plasma flow processes in underwater wet flux-cored arc welding. The relationships between several physical quantities such as droplet shape and temperature, electromagnetic force and current density of droplets and arc at different periods are analyzed. As it turns out, the special arc plasma component is the main reason why the arc concentrates on the bottom of the droplet, rather than being wrapped. As the volume of the droplet increases, the arc temperature decreases. The constriction of arc shape leads to the result that the welding current flow goes through from the droplet bottom–a small area to subjacent arc. Therefore, at the anode spot an intense electromagnetic spot force is formed. The droplets are also affected by the gas flow resistance inside the bubble. Under the combinative actions of the spot force and the gas flow resistance, the main mode of droplet transfer is large droplet repelled transfer. For verifying the reliability of the model, the real-time metal transfer process is captured by a high-speed camera, the experimental values and the simulated values are found to be in good agreement.

Effect of Filling Rate on Underwater Wet Welding Process and Weld Appearance.

Real-time electric signal, matter transfer mode and welding pool behavior were obtained to investigate the effect of wires’ filling rate on arc stability and joints’ appearance during underwater wet flux-cored arc welding (FCAW). The electric signal results showed that arc stability first decreased and then increased rapidly because the raise of filling rate affected the number of charged particles and the electrical conductivity of welding arc atmosphere. Two typical transfer modes, globular repelled transfer mode and surface tension transfer mode, were observed in this study. The ratio of surface tension transition could be increased by adding wires’ filling rate. Meanwhile, the geometry of molten pool was changed and the distance between droplets to welding pool reduced as the filling rate increased. The fusion line became more regular and the radius of curvature increased under the effect of bubbles in the molten pool. As the filling rate improving, more slags on the welds surface were acquired and the welds were much flatter and smoother.

Visual Investigation on the Arc Burning Behaviors and Features in Underwater Wet FCAW

AbstractThe dynamic arc behaviors have a significant effect on the process stability and welded joint quality in underwater wet flux-cored arc welding (FCAW). In this study, clear underwater arc images are obtained by using a high-speed camera, which further reveal the complex physical process of underwater wet welding. The mechanism of underwater contact arc ignition is described in detail. Gas ionization and electron emission processes are carried out in bubbles generated by resistance heat. The dynamic arc behaviors are obviously different between bubble growth and separation stages. The high-speed gas flow in the bubble separation stage has a great disturbance to the arc. The arc shapes under water and in air under the same parameters are compared, and the mechanism of arc column shrinkage and arc length shortening is revealed from the aspects of water environment cooling, gas composition in bubbles, disturbance of gas flow, water pressure, and electric field strength.

Effect of water flow on the microstructure, mechanical performance, and cracking susceptibility of underwater wet welded Q235 and E40 steel

Abstract Underwater wet flux-cored arc welding was investigated varying water flow velocities. Q235 and E40 steel, which have different carbon equivalent (CE) values, were used for these experiments. At the same flow conditions, the joint with E40 (a higher CE value steel) contained more side plate ferrite, acicular ferrite, and even martensite microstructures in the weld metal. The coarse grain heat-affected zone (CGHAZ) in the Q235 joints was dominated by Widmanstatten ferrite and some bainite, while lath martensite and acicular martensite were dominant in E40 joints. The side plate ferrite in the weld metal and acicular martensite in the CGHAZ were the most probable locations for the formation of cold cracks owing to their brittle characteristics. In the E40 weld joints, the mechanical properties showed a slight increase initially and then decreased dramatically with increasing flow velocity. In this study, the joints with higher CE values demonstrated a higher susceptibility for cold cracking, resulting in narrower process parameter ranges limited by cold cracking in the underwater environment.

Investigation on the mass transfer control, process stability and welding quality during underwater pulse current FCAW for Q235

Pulse current technology is employed to adjust the mass transfer and improve the welding quality, and effects of the pulse frequency (f), load duty cycle (D) and peak current (Ip) on the droplet were analyzed based on Taguchi method during underwater flux-cored arc welding (FCAW). In underwater wet pulse FCAW (FCAW-P), the mass transfer process is observed and the interaction mechanism of the pulse current on the droplet is investigated using in-situ X-ray imaging method. Within one pulse period, the decreasing repulsive force and the increasing gravitational assist the droplets detach from the wire or touch with the welding pool at the end of the peak pulse duration and reduce the droplet detachment angle, improving the mass transfer stability and thus increasing the arc stability and ameliorating the bead appearance. The stable mass transfer process and welding arc increase the effective heat input, and thus decreases the acicular ferrite and increases the pro-eutectoid ferrite in the bead. The hardness distribution and the tensile strength of the beads deposited in conventional and pulse current underwater wet FCAW are identical basically, demonstrating the application of pulse current technology does not deteriorate the mechanical properties.

Correlation between wire feed speed and external mechanical constraint for enhanced process stability in underwater wet flux-cored arc welding

It is a great challenge to improve the process stability in conventional underwater wet welding due to the formation of unstable bubble. In this study, mechanical constraint method was employed to interfere the bubble generated by underwater wet welding, and the new method was named as mechanical constraint assisted underwater wet welding. The aim of the study was to quantify the combined effect of wire feed speed and condition of mechanical constraint on the process stability in mechanical constraint assisted underwater wet welding. Experimental results demonstrated that the introduction of mechanical constraint not only suppressed the bubble without floating but also stabilized the arc burning process. The degree of influence of mechanical constraint, which changed with wire feed speed, played an important role during the mechanical constraint assisted underwater wet welding process. For all wire feed speeds, the fluctuations of welding electrical signal were decreased through introduction of mechanical constraint. The difference in the proportion of arc extinction process between underwater wet welding and mechanical constraint assisted underwater wet welding became less with increasing wire feed speed. At wire feed speed lower than 7.5 m/min, the improvement of process stability was very significant by mechanical constraint. However, the further improvement produced limited effect when the wire feed speed was greater than 7.5 m/min. The observation results showed that a better weld appearance was afforded at a large wire feed speed, corresponding to a lower variation coefficient.

Numerical analysis of molten pool behavior during underwater wet FCAW process

Underwater wet flux-cored arc welding is a candidate method for onsite fabrication and maintenance of underwater structures. For a systematic understanding of physical mechanisms of this welding process, transient three-dimensional CFD models encompassing the effects of slag and underwater environment on heat and mass transfer within the weld pool are developed. The roles of droplet impingement, gravity, electromagnetic force, surface tension and interface tension on weld pool evolution under wet environment are discussed and compared with those in conventional flux-cored arc welding. Results show that in the underwater process, there exists a considerable vortex flow on the longitudinal section of weld pool, which transfers a large amount of heat to the bottom of weld pool. The region of fluid flow on molten pool surface is more restrained in the underwater process because of the rapid water cooling effect as compared with the conventional process. Good agreement is found between the predicted and the experimental weld beads under different welding parameters. The presented models can be used for further study of the underwater process.

Effects of heat input on arc stability and weld quality in underwater wet flux-cored arc welding of E40 steel

Real-time electric signal data, microstructure characteristics and mechanical properties at six different heat inputs (16 kJ/cm ∼39 kJ/cm) in underwater wet flux-cored arc welding (FCAW) of E40 steel were investigated. The electric signal results showed that the arc stability would deteriorate at high heat input because of the formation of spatters. The heat input changed the chemical composition of weld metal. With increase in the heat input, the ultimate tensile strength and impact toughness of weld metal increased due to solid solution strengthening of alloying elements. Moderate heat input (22 kJ/cm and 25 kJ/cm) was optimum, good arc stability was obtained and mechanical properties was excellent. The heat input had limited effect on the hardness of CGHAZ although the cooling rate slowed down.

Effect of water flow on the arc stability and metal transfer in underwater flux-cored wet welding

The influences of flow rate and direction on metal transfer mode and molten pool shape in underwater wet flux-cored arc welding (FCAW) process were studied. Real-time electric signal data, metal transfer images and weld appearances at different flow rates and directions were obtained. The electric signal results showed that the increase of flow rate would deteriorate stability of the wet welding process. Two typical modes of metal transfer, globular repelled transfer mode and surface tension transfer mode were observed. Their proportion would change with increasing of flow rate. When flow rate increased from 0m/s to 2.5m/s, the proportion of globular repelled transfer mode increased from 71.5% to 100%. When welding direction was downstream, the proportion of spatter increased from 4.23% to 56% with the flow rate adding to 2.5m/s. The variation of flow direction would change position of molten pool, which result in the change of spatter lose rate. The depth-to-width radio of weld bead which was welded upstream increased from 0.13 to 0.8 with the increase of flow rate. As welding downstream, the depth-to-width radio didn’t change obviously. The method to overcome adverse effects of flow was discussed, and an acceptable appearance of welding seam was obtained.