In the period of the last world war, the need for immediate welding was felt to save ships sinking at sea and this requirement had raised the status of underwater welding almost from a dream to a practical process.
In addition to developing and utilizing 70% of the earth for international interest, such as the development of offshore gas and oil fields, fisheries larges offshore construction and mining of mineral resources in the seabed, thereby underwater welding has developed. Underwater welding has been used primarily for temporary repair work due to shipwrecks and unexpected accidents or corrosion and other maintenance operations.
Underwater welding process inside and the problems involved in it can be understood with the help of the following facts:
- Underwater welding can be done on the surface in the same way that it is in place because the joint is difficult to prep joints as they are outside.
- Chilling action of water on weld metal and surrounding plate.
- A high pressure due to the water head under which welding occurs.
- Gaseous covering surrounding the arc area.
- Gas bubbles are formed surrounding the arc due to the burning of flux at the electrode and the segregation of water.
- As welding continues these bubbles travel one by one to the surface of the water, making the position around the arc unstable.
In addition, gas conduction arcs within the envelope generate dynamic pressure gradients around the column.
The size of the gas envelope increases with the salinity and temperature of seawater but decreases with depth. The effect of metal structures and therefore on the mechanical properties of a welded joint.
Welding of alloy steels due to the chilling effect of water on the weld metal can easily emit heat affected area. Therefore underwater welding is usually limited to mild steel.
Another problem with underwater welding is that deep water work is never easy because diving operations are dependent on the tide and weather, and difficulties arise from different positions in witch welding has to be done.The insulation of welding circuits is alsoan essential thing. Even the welding electrode coating is protected by a layer of wax varnish or cellulose.
Types of underwater welding
- Underwater welding can be classified as follows:
- Under Water Welding
- A.Wet Welding
- B. Dry Welding
- Dry welding can be classified as :
- a.Hyperbaric welding
- b.Cavity welding
- Wet welding is done directly in water using waterproof stick electrodes. In this method, welding arc is made underwater without any special precautions because the hardness and brittleness cause the effect of water to quench due to which the water in the arc separates and the arc develops further.
- The dry welding process requires a controlled atmosphere and pressurized enclosure.In this process, the weld metal is not allowed to come in direct contact with water. Good quality welding can be achieved through this welding.
Although this process is a bit expensive it has many benefits: like
1 Environmental Low Hydrogen Problem.
2 improvement of the stability in the welding operation
3 Low weld metal and base metal quenching rates.
4 Strength and flexibility comparable to weld welds.
Welding is done in a drought compartment, constructed around the joint to be welded. The water is expelled by providing a gaseous atmosphere in the chamber at a pressure equal to water pressure and so keeping the water out. This process is termed as housing-related welding when used to connect to a large pipeline, where the chamber is large, filled with a breathable environment and equipped with a life support system for the operator.
The hyperbaric welding process has some limitations like:
A practical difficulty is a need for a seal between the compartment and the structure to be welded.
As the pressure increases the depth increases introduces problems both for the welding process and for the divers. Weld chemistry, metallurgy and arc physics are all affected. The air soon becomes objectionable as the habitat atmosphere increases the pressure as it presents a potential fire hazard on ambient chairs exceeding 3 atmospheres. In addition, oxygen becomes an alcoholic air because the atmosphere is not recommended for depths greater than 15 m.
(Helium + oxygen) and argon (argon + oxygen test)
This is also known as a welding process wherein welding operation is done in a dried atmosphere.
the traditional order, for feeding filler metal and shielding gas, is occupied by means for introducing a cavity gas and the whole is surrounded by a trumpet-shaped nozzle through which a high-velocity water jet passes. The cavity welding process avoids the need for a dwelling compartment and is automated and can also be operated with remote control. Through this process, flat and overhead welding can be done easily for which a backup strip is used.
Cavity welding is not suitable for butt joints without pipeline nor for lap joints, accounting for the majority for underwater welding work..
A good alternative to a good working procedure:
A good underwater welding process should be:
a-Affordable welding equipment and low welding cost.
b-Reduction in electrical hazards
C- At least 20 cm/min welding speed.
d – allow good visibility
E-be easy to operate
f – Produce good quality and reliable welds.
G- Welding allowed at all positions.
h-such that the operator can support himself with one hand and weld with the other hand due to lack of gravity in the water.
Underwater Welding Fusion Process
Currently, the fusion welding process of greatest practical importance in underwater welding are:
SMAW welding which is used extensively as a wetting technique, but is also suitable for habitat welding.
TIG and MIG welding processes have also been used to a more limited extent in wet welding as well as in local enclosed gas shrouds.
SMAW: Shielded metal arc welding a suitable process for underwater welding, SMAW process is economically chippers also as compare to other processes.
GTAW: The GTAW process can be performed in all situations with the same success as welding outside of water.
The power source should be the direct current power source but it should have the capacity to provide each welding operators a minimum of 300 Amps for better performance.
All electrical leads must have lighting gear, electrode holders, gloves, etc., fully insulated and in good condition.
Ferritic electrodes with a coating based on iron oxide should be used as they resist hydrogen cracking.
The positive polarity of the work is preferred. This means that 65–75% of the heat is being welded to the metal. The weld pool is easy to handle and has sufficient fluidity to fill the undercut substantially
Electrodes with positive polarity can be used for overhead butt welding or welding cracks in vertical and overhead positions.
Applications of underwater welding
Salvaging vessels sunk in the sea.
Maintenance of ships.