How does underwater welding work?
Underwater welding works just like a tropical island which is have 2 seasons,wet and dry, and all welder divers are required to learn both techniques in their training.As technology progresses,wet and dry welding undergo major changes in research and application.Most people picture a diver floating in clear water with a gentle glow at the end of his or her electrode. In reality, very little underwater welding takes place directly exposed to water (wet welding),most is done in habitats (dry welding).
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The diver(welder) has a tank of nitrogen or argon and blows a big bubble of gas around the spot that is welded.Inside the bubble,welding is performed in the usual fashion.Some welding techniques do not require a gas bubble,because the burning electrode produces enough gas.Waterproof electrodes are required however.Ambient pressure greatly affects welding conditions though,so welding at great depths requires a welder with special knowledge.
If the bubble collapses, the arc is extinguished,so the art is in keeping the bubble stable,in addition to the movements a diver does to keep his hand stable on the task and to the movements a welder does to ensure a good weld (maintaining a small and constant arc length, weaving motions,keeping the electrode at an angle,preserving constant speed). Also the welder has to make a durable weld despite being unable to properly prepare the seam (to clean the oxides or prepare an elaborate cut) or use preheating and with increased chances of defects such as hot cracking, stress-corrosion cracking and, most importantly, hydrogen embrittlement.
Underwater welding process in a nutshell
There are three main types of underwater welding used in the industry for many years. They are the wet underwater welding, coffer dam welding and hyperbaric welding.
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In wet underwater welding,the Metal Arc Welding (MAW) is the most common process.For deep water repair, the friction welding is of great use.This is because it has the advantage of being relatively insensitive to depth and lends itself to robotic operation. In the former Soviet Union,the flux core arc welder (FCAW) had been widely used.The coffer dam welding is carried out in the dry in air.A rigid steel structure to house the welders would be sealed against the side of the structure to be welded.This would be open to the atmosphere.In hyperbaric welding a chamber would be sealed around the structure to be welded and is filled with a gas at the prevailing pressure.Helium containing 0.5 bar of oxygen is usually used as the gas.
The welder or diver would be usually works from outside the enclosure.He welds with each of several electrodes in turn,each electrode having been positioned in advance through a flexible port.The advanced welding technology involves the fabrication of a transparent enclosure around the intended weld.An appropriate gas is introduced to exclude water. Underwater welding offers a great deal of use for assembly or repair underwater.It has found to be more effective compared to the alternatives like clamped and grouted repairs. Wet underwater welding has been widely used in the repair of offshore platforms.
Underwater welder benefits
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Underwater welding refers to a number of distinct welding processes that are performed underwater.The two main categories of underwater welding techniques are wet underwater welding and dry underwater welding,both are classified as hyperbaric welding.
In wet underwater welding,a variation of shielded metal arc welding is commonly used, employing a waterproof electrode.Other processes that are used include flux-cored arc welding and friction welding.In each of these cases,the welding power supply is connected to the welding equipment through cables and hoses.The process is generally limited to low carbon equivalent steels,especially at greater depths,because of hydrogen-caused cracking.
In dry underwater welding the weld is performed at the prevailing pressure in a chamber filled with a gas mixture sealed around the structure being welded.For this process, gas tungsten arc welding is often used, and the resulting welds are of high integrity.
The applications of underwater welding are diverse it is often used to repair ships,offshore oil platforms,and pipelines. Steel is the most common material welded.For deepwater welds and other applications where high strength is necessary, dry underwater welding is most commonly used.Research into using dry underwater welding at depths of up to 1000 m are ongoing.In general, assuring the integrity of underwater welds can be difficult (but is possible using various non-destructive testing applications),especially for wet underwater welds,because defects are difficult to detect if the defects are beneath the surface of the weld.
The underwater welding technology
There are many ways to use water to cut,braze,weld etc.Multiplaz 3500 is an interesting device that needing only a 20 amp 220V plug.The unit produces a plasma (8,000C) using only water to cut or mixture of water-IPA to solder.However we are talking about under water,so this is another story.
Arc welding is achieved by arranging an electrical circuit in which current passes from the welding power source through the welding electrode,across an arc to the workpiece,and through a return lead to the power source.For the safety of the welder/diver during wet underwater welding, it is vital that he or she is completely insulated from the welding circuit.
An insulated electrode holder or welding gun is used,and the welder/diver wears rubber gloves. For safety, on the verbal directions of the welder/diver, the power supply (which is controlled from the surface, via a double-pole ‘knife’ switch) is switched off immediately welding stops. For a discourse on underwater electrical safety, see Ref.1.
Once the welding circuit is completed by striking an arc, the heat of the arc is sufficient to vaporise the surrounding water. Hence, the arc is surrounded by a vapour shield, similar to the gas shield which surrounds an arc during welding in air. However, reaction with the molten metal influences the composition of this vapour, and it comprises typically ~70% hydrogen, ~25% carbon dioxide and ~5% carbon monoxide.
The resulting weld metal diffusible hydrogen level can be extremely high (up to 100 ml/100g of deposited metal for rutile electrodes), but is much lower for oxidising electrodes (~25 ml/100g of deposited metal).These levels increase as the welding depth increases, thereby increasing the risk of hydrogen-assisted cracking. See Refs [2,3] for information on the metallurgy of wet underwater welding.