Porosity @ Welded Joint (Source: http://www.thefabricator.com/article/arcwelding/22-possible-causes-of-weld-metal-porosity)
Meticulously Slow Developments
Underwater welding has attracted many researchers over the past fifty years. The process is however loaded with numerous inherent limitations that slow down the pace of advances to a trot.
Above water or below it, technicians prefer welding over other joining methods because welding offers better joint efficiency and mechanical properties with greater application impact. Today’s welders have to confront the twin challenges of providing higher quality welds while cutting costs.
Research focuses on improving the output of conventional underwater welding processes, checking the performance of new processes, minimizing the effect of wet environs on weld-joints, testing the behavior of fresh materials, and ushering in greater automation, mechanization, and improved inspection techniques.
Underwater Welding Developments: Processes, Materials, & Hazards
Welding underwater is tougher than in air because of higher pressures, higher cooling rates, and hydrogen content in weld-joints that destabilize the arc, introduce porosity in weld-joints, and lower the toughness of weld-joints.
Wet Welding uses direct current (DC) only. Alternating current (AC) can electrocute diver-welders. Plus, maintaining underwater AC arcs is tougher. DC power sources with 300-400 ampere rating are best for wet welding while motor-generator welding machines are most suitable for all underwater welding.
Microscopic View of Hydrogen Embrittlement
(Source: https://abduh137.wordpress.com/page/4/)
Advanced Underwater Welding Techniques:
Friction Resistance Welding (FRW)
Laser Welding
Conventional Underwater Welding Techniques:
Wet Welding: Shielded Metal Arc Welding (SMAW) / Stick Welding and Flux Cored Arc Welding (FCAW)
Dry Welding: Gas Tungsten Arc Welding (GTAW) or Tungsten Inert Gas (TIG) Welding
Local Cavity Welding: Gas Metal Arc Welding (GMAW)
Apart from providing a shield, flux-coated electrodes in SMAW generate bubbles that displace water from the weld-pool to provide better welds. Technicians use straight polarity DC for wet SMAW. Ferrite electrodes with iron-oxide-based coating resist hydrogen cracking.
FCAW is semi-automatic and used for high deposition rate welding. Burnback and Porosity are FCAW’s downside. Burnback is the fusion of the electrode wire with the current contact tube due to sudden lengthening of the arc.
Recent advancements have improved the wet weld-ability of FCAW and provided halogen-free flux formation for nickel-based flux-cored filler materials and stainless steel flux-cored wires.
Frictional Resistance Welding Schematic
(Source: http://www.researchtrend.net/pdf/17%20HARISH%20GARG.pdf)
GTAW or TIG Welding offer less porous welds and more stable arcs for underwater dry welding. TIG with AC is economical for welding sheets up to 5mm thick. Welding plates over 5mm thickness requires TIG with DC.
FRW joins parts using heat generated by friction resulting from the relative motion between these parts. Welders apply additional pressure after temperature rises substantially. FRW is now used to repair underwater cracks on pipelines and marine structures
Rotational time, pressure, and welding time are the three control parameters in FRW. Flashes occur on the outside perimeter of the weld. Flash characteristics indicate if the three control parameters are correctly applied.
FRW is expensive and requires complex equipment, but:
provides excellent welds in short cycle times without using flux, filler metal, or shielding gas
offers fine-grained forged welds without weld-inclusion or weld-dilution
are free of hydrogen embrittlement because there is no weld pool
can join dissimilar metals, even aluminum with ceramic
Laser Welding Schematic (Source: http://www.researchtrend.net/pdf/17%20HARISH%20GARG.pdf)
Laser Welding is the most important operation among laser joining processes because of recent developments and the immense volume of work involved. Lasers are extremely coherent energy sources and offer quick, high quality welds with minimum heat affected zone (HAZ).
Hazard Management:
Electric Shocks: minimized through proper insulation, limiting open circuit voltage of welding sets, shutting-off power supply immediately after extinguishing the arc, using double-insulated cables and single/double circuit breakers
Open circuit voltage is the voltage at the output terminals of the welding power source when welding is not being executed
Explosion: nullified by preventing the buildup of explosive hydrogen and oxygen gas pockets
Nitrogen Narcosis: the drowsiness due to inhaling air under pressure below 100feet depth when nitrogen enters the bloodstream. Decompression chambers, emergency air supply, and stand-by divers slash this hazard
Materials: Underwater welding normally joins low-alloy steels, carbon steels, and duplex and austenitic steels:
high-strength steels with over 0.4% carbon equivalents demonstrate worst weld-ability
cold or hydrogen cracking occurs when welding high-strength-low-alloy steels and dissimilar metals
fully austenitic stainless steels are hot-cracking-prone
Elimination of cracking requires minimizing:
Amount of Diffusible Hydrogen using consumables that generate less hydrogen and adjusting weld parameters to reduce hydrogen pickup by weld-pools
Hard Microstructures in Heat Affected Zones (HAZ) by controlling the zone’s cooling rate
Residual Stresses in Weld Joint using edge preparations that reduce weld deposit, small weld deposits, and choosing consumables whose thermal expansion coefficients are compatible with the base material
Finally
With the number of underwater welded structures rising by the day, the future importance of underwater welding cannot be overstated. Despite the advances over the past five decades, a lot still remains to be achieved.