Underwater welding, as the name implies, is “the welding produced inside water”. A decade back underwater welding was limited to the state of patching a hole in a sunken ship, just to get her afloat for major repairs to be carried out in dry docks.

One or two of the world’s great navies might have treasured secrets about sub-ocean welding but for most of us there was neither a need for welding structures under water nor was there a solution for it.

The recent intensification of efforts in the field of exploring the seas for the natural resources beneath its beds has aroused the interest of welding engineers to develop tools and techniques for obtaining reliable welds under water.

The present techniques for underwater welding are far from complete and have limited applications in salvaging operations. Because of the high cost of dry habitat welding the pri­mary thrust in research and development has been with open water (wet) welding.

Underwater welds suffer from defects like undercuts hard and brittle HAZ, microcracks due to hydrogen embrittlement, solidification cracking, stress corrosion cracking, etc.

15.1 COMPARISON OF UNDERWATER AND NORMAL AIR WELDING

Underwater arc welding differs from air welding in the following features:

1. Electrodes are painted for waterproofing.

2. Electrode core wire is usually the same as in air welding but in the case of the welding of high strength steels inside water using wet welding technique, a core wire of stainless steel or special steel is preferred.

3. The flux coating in common use is that of rutile type. Iron-oxide covering, which is not very common in air welding, has been found to be more advantageous (Khan, 1979).

4. In air welding a gap is maintained between the electrode and the parent plate. This gap cannot be maintained in water as soon as the electrode is lifted for maintaining a gap the arc extinguishes. For maintaining an arc in water, it is necessary to keep the electrode in contact with the plate. A slight pressure is also maintained. Cooling action of water on flux coating and waterproof paint results in the formation of a barrel at the end of the electrode. Arc burns inside this barrel space (see Fig. 15.1).

5. Underwater arc is surrounded by a bubble of steam and gases. The pressure on the arc equals the atmospheric pressure plus the pressure of the water column above the arc as shown in Fig. 15.2. The pressure around the arc, thus, increases with depth. This affects arc behaviour and equilibrium of chemical reactions which affects weld chemistry. Carbon, silicon and manganese content of the weld metal increases with depth with corresponding change in properties.

6. Cooling rates in air welding could be controlled by change in arc-energy input. There is far less scope for doing this as the voltage and current during underwater welding have a close range.

7. Hydrogen and oxygen levels are normal in air welding while weld-metal and heat affected zone hydrogen and oxygen levels are well in excess of those in air-welding. This is due to increased amounts of hydrogen and oxygen in arc bubble.

8. Electrode holder is insulated.

15.2 WELDING PROCEDURE

While welding in water the electrodes are first painted for water proofing, kept in waterproof containers and are taken to the place of welding in water by the diver-welder. During welding the electrode is held in a special (fully insulated) electrode holder. When the electrode is brought to the plate in the welding position, the welder gives an indication to the operator of the generator called “tender” to put the generator on Fig. 15.2. After weld bead is completed another signal is given to put the generator off. This precaution is taken for the safety of the welder.