Welding-underwater,Problems, Progress and Automation.SOLUTIONS with Effective, Powerful Advice.
Welding-underwater A general overview of the main types of processes employed is presented in our other page that can be seen by clicking on Underwater Welding. In the present Page we are going to deal more deeply with some subjects of Welding Underwater not covered sufficiently in the previous page above. We think that anyone wishing to prepare for a Career in Welding Underwater should study very thoroughly theory and practice of this demanding job, by absorbing all available information as we are presenting in our pages.
An Important Book on Welding-underwater Please be advised that we were authorized by the Author, David J. Keats, of Speciality Welds Ltd., to publish in our Welding Advisers Website, As it is essential reading for anyone involved underwater and a perfect reference for all welders, we urge our interested readers to read the first excerpt from Chapter 1 by clicking on A Case for Wet Welding. You will find there the links to other two previews and to the book itself.
Specifications A well known Specification for Welding-underwater is ANSI/AWS D3.6. which provides essential information to all involved:
This Specification presents four different Types or Classes for Welding-underwater: Type A welds are characterized by requirements ensuring a weld, performed by Welding underwater, comparable to a surface weld. Usually dry (hyperbaric) Welding-underwater meets these requirements. Type B welds refer to wet Welding-underwater weldments. They are appropriate for less critical applications with reduced ductility and increased porosity. Type C welds have even less requirements than type B welds for applications without load-bearing functions. Type O welds present requirements of surface dry welds, as well as those of other Codes and Specifications. AWS D3.6M-99 Other Welding underwater documents have been developed by different agencies.
Economic Importance We may remind that the main drive to the development of Welding-underwater technology is economic, because in spite of the complexity, it permits to save much time and expense when compared with the need of pulling to a dry dock the structure to build or to repair. Offshore platforms and structures, used for the exploitation of submarine oil fields, are constructions whose continuous availability has important economic consequences. It is customary to provide a thorough inspection at least once every five years if unscheduled accidents do not require immediate intervention. Damages requiring repair may be due to corrosion or fatigue, or even to impact from massive moving objects. Physical Conditions influence Here we wish to provide some more insight into the influence of the physical conditions of Welding underwater onto the metallurgical microstructures obtained and on the soundness of the welded constructions. The main factors influencing Welding-underwater outcome are the following for wet Welding-underwater:
Electric Arc The process most commonly used for Welding-underwater is SMAW (Shielded Metal Arc Welding with covered Electrodes of special types) although FCAW (Flux Cored Arc Welding) has been also employed. Wet Welding-underwater has been carried out as a building or repair technique at depths of 100 m (330 ft) and down to 200 m (660 ft) under sea level and over. As already remarked in the previous page (see above), Welding-underwater by the Electric Arc is performed in either the wet environment or in a protective chamber to be constructed around the weld zone and evacuated of water by compressed helium gas, to create the dry conditions. The cost of hyperbaric Welding-underwater is much higher than that of wet welding. Assembling and dismantling the equipment can be very expensive in a deep-sea environment. Depth Influence In both cases pressure increases with depths: this fact brings about unexpected consequences like a decrease of Silicon and Manganese and an increase of Carbon in the weld pool, correlated to an increase of Oxygen. Also Hydrogen content may increase with depth, with increased danger of cracking. And the arc is less stable, with increasing depth because the high ionization potential of Hydrogen makes it more difficult to sustain the arc. Hydrogen Cracking Wet Welding-underwater of higher strength steels is very susceptible to Hydrogen cracking because Hydrogen is more available in presence of water. To limit the tendency to Hydrogen induced cracking certain provisions may be attempted:
Porosity Another considerable concern in wet Welding-underwater is weld metal pore formation which increases with pressure (depth). Weld metal porosity due to Hydrogen absorption, in the case of wet welding, can be minimized by altering welding parameters as follows:
Alternating current is not used in wet Welding-underwater.
CE and Quench Rate For wet repair welding, the Carbon Equivalent of the base metal (steel) should be limited to max. 0.40 %, when calculated using the following accepted formula: CE = %C + %Mn/6 + %(Cr+Mo+V)/5 + %(Ni+Cu)/15 But the rapid cooling rate provided by the surrounding water quench must be considered. Whereas in normal dry conditions the time for cooling from 800 to 500 oC is generally between 8 and 16 s, under water the same cooling interval is covered in 1 to 6 s, depending on thickness and heat input. Obviously this rapid quench rate in wet Welding-underwater, produces significant amounts of heat affected zone (HAZ) martensite in carbon steels and has measurable consequences on microstructure and properties of welds. Hardness of 400 Vickers measured under a load of 10 kg can be found in the weld when the Carbon Equivalent is about 0.40% The major changes in microstructure and in mechanical properties occur in the first 50 m (165 ft) of depth. At greater depths, the weld metal composition and microstructure remain fairly constant. If necessary, compositional modifications of consumables for wet Welding- underwater must be studied to provide for suitable properties. Friction Welding Developments The application of friction welding as a repair procedure to be conducted underwater was attempted in the last decades, especially in England by The Welding Institute, (www.twi.co.uk) with progressive involvement and success. One of the most important advantages of friction welding is the absence of a liquid phase with the complete elimination of the influence of gases (Hydrogen and Nitrogen) and the fact that the metal composition is not altered, with consequent uniformity of results independent of depth. Therefore a procedure demonstrated at a certain depth need not be requalified at another one, which is a requirement for all other Welding-underwater processes. A process was developed with the intent of providing a method of repair for linear cracks. A sort of stitch welding was employed by which subsequent friction welds of tapered studs (short conical rods) were performed in precise holes to bridge the gap between the two sides of a crack. The continuity was assured by a certain overlap of the rods along the crack. And the integrity was demonstrated by destructive bending tests. Future Automatic Equipment Special Welding equipment and procedures were developed to perform this kind of repairs. Most of the functions of the welding head were designed to be performed automatically. The trend of these developments is for increasing the independent autonomy of the systems in order to dispose of the requirement for manual operation of divers. The programmed developments will enable hopefully to perform pipe laying down and Welding-underwater even at extreme depths still not heard of. Additional information, including the Reference to an important Book on Welding-underwater, can be found in our other page available by clicking here. An Article on Developments in Underwater wet Welding was published In Section 11 in Issue 29 of Practical Welding Letter for January 2006. To read the article click on PWL#029. An Article on Filler Metals for Underwater Welding (2) was published in Issue 48 of Practical Welding Letter for August 2007. An Article on Electrodes for Underwater Welding was published (4) in Issue 61 of Practical Welding Letter for September 2008. In the same issue there is also a note (11) on Underwater Welders Pay.
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