Characteristics of Some Hyperbaric Welding Processes | Metallurgy

In this article we will discuss about the characteristics of some hyperbaric welding processes.

Characteristics of Shielded Metal Arc Welding (SMAW):

This is most widely used underwater welding process and can be used in both wet and dry environments. Wet welding is used for lightly stressed welds only. Hyperbaric SMAW process is most commonly used as it requires simple equipment and the deposition rate is high. The major factor influencing the use of SMAW is hydrogen. A combination of susceptible microstructure, applied stress and hydrogen can result in cracking.

It is important to understand the chemistry of hyperbaric SMAW and study the process behaviour with changes in pressure. For instance, the electrode burn off rate increases with increase in pressure (using positive polarity) but it can be controlled by using negative polarity.

The environmental pressure has a direct effect on the equilibrium point of the chemical reactions occurring in the arc of weld pool. Electrode coatings contain carbonates etc. which are broken down by the arc to form CO and CO₂ for shielding.

But at high pressures, less gaseous product is formed, increasing carbon levels and making oxygen available to combine with the available silicon and manganese, increasing slag and reducing the levels of these elements in the weld pool. For this reason, special low carbon electrodes are used.

Characteristics of Gas Tungsten Arc Welding (GTAW):

It finds considerable use in hyperbaric environments. The only significant difference between surface and hyperbaric GTAW is the technique to arc initiation. Under hyperbaric conditions, higher voltages are required to ionise a path between the tungsten electrode and the workpiece.

Since high voltage is hazardous to the welder in humid chamber, the programmed contact start technique is used. The tungsten electrode is lightly touched to the workpiece and a low current passed. As the torch is lifted away, the current is increased to the required welding level.

The capability of this technique to deposit metal is found to increase at depths beyond 150 metres. Since it is more controllable at depths beyond 250 m, it is attractive process at higher depths. Since divers are found to be physically and mentally less capable at depths beyond 200 metres, GTAW can be designed to be controlled from surface particularly for pipe welding whose geometry of butt weld is extremely simple.

A track is clamped around the pipe, on which a crawler vehicle is installed carrying the welding torch and associated manipulator system which is capable of controlling torch workpiece distance and lateral weaving system of the welding head. A tractor carries one or two consumable feed systems, each capable of delivering wire to a feed nozzle attached to the welding torch.

Two consumable feed systems one on each side of the torch, permit welding by the preferred vertical up technique on both sides of the pipe without dismounting the crawler from the track. (Refer Fig. 9.52 which shows the simplified representation of typical orbital GTAW (system).

A television camera system, usually in combination with a coherent fibre optic bundle to allow the camera to be mounted away from the welding heat are used for remote viewing. It is customary to use cameras employing a solid state detector as an imaging element as there systems have the maximum available resistance to damage by the high levels of light from the arc. It is normal practice to use two cameras or a split bundle, to enable viewing of the front and rear of the weld pool.

The hyperbaric equipment should use corrosion resistant materials and be appropriately waterproofed. The welding station is at the surface, and is connected to underwater equipment by an umbilical which may be as much as 500 m long.

The welding power source and manipulator servo amplifiers are located close to the welding system to reduced cable losses. A great deal of effort is required to develop an effective welding procedure as the welding position changes continuously and also due to effects of the pressure.

At the limit of the weave, close to the side wall, the arc current is increased and the consumable feed rate reduced to maximise fusion. As the arc traverses the centre of weld, welding current is reduced and consumable feed rate increased in order to cool the weld pool and control the deposited bead shape.

Characteristics of Gas Metal Arc Welding (GMAW):

GMAW has also been used for hyperbaric conditions. On surface, dip transfer welding or short arc welding technique is used to transfer metal across the arc so that it is not influenced by gravity. In dip transfer technique, the wire is allowed to enter the molten weld pools short circuiting the arc.

A rapid increase in current heats and eventually ruptures the feed wire, the pool stabilises itself by surface tension forces and the wire is fed forward to repeat the process. However this technique, being low heat input process, is not suited under hyperbaric conditions due to fusion problem for welding of thick section.

Spray transfer technique which uses electromagnetic forces to project molten droplets across the arc, has also not been found suitable for hyperbaric conditions as it much depended on the welder skill.

Presently flux cored wire arc with controlled transfer pulse (CTP) GMA also called ‘Synergic’ welding is used. With advances in electronic technique, it is possible to vary the static and dynamic characteristics of the power supply to suit welding parameters. For a defined consumable with a specific shielding gas, metal transfer is principally controlled by the magnitude and duration of pulse of current applied to the arc.

As the electric field strength of the arc is dependent on the pressure, increased arc voltages and adjustment to the power supply characteristics are demanded for hyperbaric welding to ensure stable operation. The complex relationship between power supply characteristics, wire feed speed control strategy, and variation in welding torch/workpiece distance need to be analysed to make this system success.

Characteristics of Diverless Welding System:

Due to limitations of divers to work at depths beyond 200 metres, diverless welding systems are being developed. Unmanned or remotely operated vehicles (ROVs) are used. Three welding techniques appear to be suitable for diverless welding viz. high energy bonding (explosive welding), friction welding, and mechanical joining.