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Under Water Welding
Under Water Welding By- Guide-Amandeep Saini Prof. Dr. R. B. PatilT.E. MechT120220805
IntroductionThe fact that electric arc could operate under water was known for over a 100 years. The first ever underwater welding was carried out by British Admiralty Dockyard for sealing leaking ship rivets below the water line.
Underwater wet welding technique was misunderstood for a long time, and it was a synonym for low quality weld full of porosity and cracks with poor mechanical properties like low ductility and due to micro structural issues prone to cracking.
Through time, that status has been changed, and today underwater welding projects, both dry and wet, are used in most complex and difficult objects with a high level of quality assurance.
Classification of Under Water WeldingUnderwater welding can be divided in two main types with sub classification as follows:
Wet underwater welding is considered as welding at the ambient pressure where there is no mechanical barrier between welder-diver and surrounding water.
Dry underwater welding is considered as welding in dry ambient atmosphere, under atmospheric or hyperbaric pressure where welder-diver is divided from surrounding water by means of mechanical barriers.
Under Water Wet WeldingWet Welding indicates that welding is performed underwater, directly exposed to the wet environment.
A special electrode is used and welding is carried out manually just as one does in open air welding.
Welding power supply is located on the surface with connection to the diver/welder via cables and hoses.
In wet welding MMAW (manual metal arc welding), with a DC power supply is used. The electrode is kept in negative polarity.
Equipment NeededDiving equipment
Welding power sources
Welding cables and holders
Mechanical tools system
Advantages of Wet Under Water WeldingThe versatility and low cost of wet welding makes this method highly desirable. The increased freedom of movement makes wet welding the most effective, efficient and economical method.
The speed of the operation is extremely high.
As the equipments are very minimum, the welding can be performed in a shorter time with minimal planning.
Disadvantages of Wet Under Water WeldingElectric arc instability.
The rapid cooling leads to great hardness in the heat-affected zone, low toughness in the welded joint and the appearance of porosity due to the capture of gas bubbles.
This process is susceptible to Hydrogen Embrittlement.
This increases the susceptibility to the appearance of cold cracks, brings about porosity and degrades the mechanical properties of the joint.
The high oxygen content in the electric arc column, molten metal in the transfer and weld pool, which leads to oxidation, reduction of the proportion of alloy elements and the degradation of mechanical properties.
Under Water Dry WeldingDepending on water depth, shape and type of underwater structure as well as ease of arranging other necessary facilities underwater dry welding could be performed in several ways:
Dry welding at one atmosphere.
Dry welding in a habitat.
Dry chamber welding.
Dry Welding at One AtmosphereDry underwater welding in Cofferdam is characterized by the fact that the welder and the welding site are in dry atmosphere, without the presence of water. All the activities are performed at the atmospheric pressure.
Special problem in working in such a narrow confined space is the occurrence of vapors and gases which may have an explosive character.
The main advantage of such welding is better quality of weld related to wet underwater welding, flexibility in the selection of the procedure and the welding technology and higher working safety.
Dry Welding in a HabitatWelding at ambient pressure in a large chamber from which water was displaced and where such atmosphere is achieved that welder has no need to use diving equipment.
Weld properties are equivalent to one welded in normal conditions.
However, much more fit-up time is necessary to fix the habitat and prepare it for welding.
Dry Chamber WeldingWelding at ambient pressure in a simple open bottomed dry chamber, that at least accommodates the head and shoulders of a diver-welder in full diving equipment.
Welder-diver is partly immersed in water but welding is performed in a dry atmosphere.
Habitat is smaller and less complex than in case of dry habitat welding.
Due to smaller size of habitat other operating facilities are also less expensive.
Hyperbaric WeldingHyperbaric welding is carried out in chamber sealed around the structure to be welded.
The chamber is filled with a gas (commonly helium containing 0.5 bar of oxygen) at the prevailing pressure.
The habitat is sealed onto the pipeline and filled with a breathable mixture of helium and oxygen, at or slightly above the ambient pressure at which the welding is to take place.
This method produces high-quality weld joints that meet X-ray and code requirements.
The area under the floor of the Habitat is open to water. Thus, the welding is done in the dry but at the hydrostatic pressure of the sea water surrounding the Habitat.
Advantages of Dry WeldingWelder/diver safety
Good quality welds
Non-destructive testing (NDT)
Disadvantages of Dry Under Water WeldingThe habitat welding requires large quantities of complex equipment and much support equipment on the surface.
The chamber is extremely complex.
Cost of habitat welding is extremely high and increases with depth.
Work depth has an effect on habitat welding. At greater depths, the arc constricts and corresponding higher voltages are required. One cannot use the same chamber for another job, if it is a different one.
AdvancementsFriction Welding:Friction welding is a solid state welding process which produces coalescence of materials by the heat obtained from mechanically-induced sliding motion between rubbing surfaces.
When a suitable high temperature has been reached, rotational motion ceases and additional pressure is applied and coalescence occurs.
It has the ability to produce high quality welds in a short cycle time. No filler metal is required and flux is not used.
The process is capable of welding most of the similar or dissimilar metals. It also produces a fine-grained forged weld without any weld dilution, or weld inclusions.
Laser WeldingBecause of its coherent nature, Laser has found its application here too.
The focused laser beam is made toirradiate the work piece or joint at the given level and speed.
A shroud gas protects the weld pool from undue oxidation and provides with the required oxygen flow.
Laser heating fuses the work piece or plate edges and joins once the beam is withdrawn.
ConclusionAll the possible ways to weld under water were studied and it was concluded that if the points like weld quality, safety and the provision for further heat treatments are considered. Then, Dry Welding is the best way to weld. There is a lot of scope in the latest technologies like THOR-I(TIG Hyperbaric Orbital Rotator), Friction Welding & Laser Welding.
ReferencesVerma, Ketan and Garg, Harish K., Underwater Welding-Recent Trends and Future Scope, International Journal on Emerging Technologies, Chandigarh Group of Colleges (Gharuan Campus) Mohali,(PB)Doc.dr.sc. Ivica Garai, Prof.dr.sc. Zoran Kouh, Underwater Welding and Cutting, Advanced production technologiesDavies, A.C., Welding, 10th Edition, Cambridge University Press, 1996, pp 343-482Colligan, K., Material Flow Behavior during Friction Welding of Aluminum, Supplement To The Welding Journal, July 1999, Sponsored by the American Welding Society and the Welding Research Council, Welding Research