Wednesday, 15 August 2012

Underwater Welding Processes

The chosen welding processes of practical significance in underwater welding are:

  1. Manual shielded meal arc welding, that is extensively used as a wet technique but is also suitable for habitate welding.
  2. TIG welding.
  3. MIG welding.
TIG and MIG welding processed have also been used to a limited extend for wet welding as well as more commonly in local enclosed gas shrouds.

1. Shielded metal arc welding:

Manual shielded metal arc welding is an economical process for underwater welding. This process can be carried out in all positions with the same success as welding in air. The DC welding equipment used for underwater welding must have a capacity of at least 300 Amps for each welder. All electrical leads, lighting gear, electrode holder, gloves, etc must be fully insulated and in good condition. Ferritic electrodes with a coated based on iron oxide should be used as they resits cracking, Positive polarity of work is preferred. This means that 65% to 75% of the heat is in the metal being welded. The weld pool is easier to handle and has enough fluidity to fill in undercut to a large extend. Electrode having positive polarity may have to be used for overhead butt-welding or welding cracks in the vertical and overhead positions. Current setting for underwater welding are normally higher than for welding air because there is a loss of heat by conduction through water and the control of these settings must be accurate to ensure consistent work. 

2. TIG welding:

TIG welding has got the advantage that it gives a stable arc and less porous welds. TIG welding has been widely used, particularly for root runs. Although this process is relatively slow, it is very flexible and can accommodate variations in fit up and produce high quality penetration beads. TIG welding is preferred to MIG welding in dry welding as filler wire is manually controlled.

In TIG welding, as the depth (ie pressure) increases:
  1. The arc becomes constricted and the voltage increases for a given arc length.
  2. Tungsten tip starts getting eroded and this phenomenon influences weld bead width, and penetration. The erosion of the tip gives rise to arc instability when TIG welding is carried out at high pressure.
  3. Arc welding becomes more difficult. 
TIG welding becomes restricted as the operating depth is increased.

3. MIG welding:

Because of the high cost of diving operations, it is highly desirable to complete welds in the shortest possible time. This has directed attention to the use of semi-automatic processes using solid wires or flux code wires. Wires containing oxidising and reducing elements give good results even without shielding. MIG welding is faster and less expensive that TIG welding. Arc heat increases with the depth of water so filler metal melts fast CO2 or Argon is used as shielding gas. The shielding gas gets denser and may require flow rates upto 10 times the surface rates. MIG dry welding is preferred to MIG wet welding for better results.