Underwater Wet welding

Welder-divers perform wet welding at ambient pressure in the water, with no barrier between water and weld. Gas and slag produced as the fluxcoated electrode is consumed shields the weld from the water. Basicwelding equipment, tools, and supplies for shielded-metal-arc wet welding underwater are essentially the same as those needed for welding above water, with the addition of dive gear with life support and communications, a welder's lens-holder assembly, and a waterproofed electrode holder designed for the wet-welding electrode. Above water, an operator called a tender communicates to the welder by radio, responding to the welder's requests for adjustments to welding amperage or for more electrodes. The tender monitors the welding power supply to verify voltage and amperage readings. He also has access to a 400-A single-pole knife switch to interrupt current to the electrode holder upon welder request.

Wet welding is used during installation and repair of offshore structures. Maintenance and repair applications include repair or replacement of structural members damaged by corrosion or fatigue; damaged pipeline sections and manifolds; sheet and H piling; and tubular braces and supports of docks and tanker-mooring dolphins. Welder-divers also wet-weld to make temporary and permanent repairs to holes in ship and barge hulls and to repair nuclear powerplant equipment.

Mechanical properties of wet welds compare favorably to those of dry welds, in spite of the rapid quench-cooling rate caused by the surrounding water. The following comparisons are based on AWS D3.6, Specification for Underwater Welding, for Class A (dry) welds vs. test results for wet welds.

Reduced-section and all-weldmetal tensile-test and fillet-weld shear-strength tests on wet welds meet AWS D3.6 Class A requirements. Charpy V-notch test results on both the weldmetal and heat-affected zone (HAZ) of wet welds significantly exceeded Class A requirements. Fatigue properties of the HAZ exceed requirements of the American Petroleum Institute Recommended Practice for Designing and Constructing Fixed Offshore Structures, API RP 2A). For wet weldmetal, fatigue properties are similar to those of dry welds using identical electrodes.

Guided bend tests and all-weldmetal elongation tests demonstrate that wet weldmetal lacks the ductility required by AWS D3.6 for Class A dry welds. Wet welds pass tests with a 3-1/3T bend radius; a 2T radius is specified for Class A welds. For elongation, D3.6 requires a minimum of 12, 14, or 19 percent depending on base-material yield strength. For a steel rated 50,000-PSIyield strength, a Class A weld must score a 19 percent elongation; wet welds on these materials exhibit elongation of 10 to 14 percent.

Regarding hardness, the HAZ of wet weldmetal, depending on the carbon equivalent (CE) of the base metal, is susceptible to hardness significantly greater than that specified by D3.6-HV 325.

Also, porosity in wet welds often exceeds the amount found in dry welds; the level of pososity in wet welds increases with depth, particularly when welding in water deeper than 100 feet. However, recent process improvements have resulted in porosity-free welds at depths from 3 to 33 feet. Other recent advancements, results of an ongoing Joint Industry Underwater Welding Development Program, include development of a wet-welding technique to prevent hydrogen-induced cracking, and excessive hardness, in the HAZ of highCE (greater than 0.40) base metal. Welds were made on A537 Class 1 str\eel plate with a CE of 0.462 weight percent, including 0.20 carbon.

Ongoing work includes reformulating welding electrodes to mitigate pressureinduced changes in composition and microstructure of wet welds, and the level of porosity in the welds.

Sumber : Grubbs, C E. "Underwater Wet welding". 29 Januari 2014. http://search.proquest.com/docview/213296971?accountid=31562