In the literature of welding the Gas Tungsten Arc Welding pro­cess is usually shortened to GTAW. It is also sometimes called TIG (Tungsten Inert Gas) welding.

When root beads of exceptional quality must be made, the GTAW process is very frequently used. Entire welds are seldom made by this process except in situations where unusually stringent requirements must be met, such as in the case of space vehicles.

Usually, only the root bead is welded by the GTAW process. However, sometimes the second pass is also made by this process because GTAW welded root beads tend to be somewhat thin. Stainless steel and high-alloy steel pipes, as well as mild steel pipes, are welded by the GTAW process, especially for high-pressure pipe joints that require high-quality welds.

The outstanding features of the GTAW process are:

1. Welds of exceptional quality can be made in almost all metals used by industry

2. Practically no post-weld cleaning is required

3. The arc and the pool of molten metal are readily visible by the welder

4. No filler metal is transported across the arc stream; thus there is no spatter

5. Welding is possible in all positions

6. There is no slag which might be trapped in the weld.

When weld deposits made by the Shielded Metal-Arc (Consum­able Electrode process) and the GTAW processes are compared, the GTAW deposit is cleaner because there is no slag deposit, inciden­tally eliminating any chance of weld defects caused by slag inclu­sions. Moreover, the outside surface of the weld (Fig. 6-1 left), and, on root beads, the inside surface as well, are very smooth as shown in Fig. 6-І (right). This eliminates the necessity for deslagging, grinding, or chipping after the root bead has been completed. The

inside smoothness of the weld significantly reduces turbulence in any flowing substance inside of the piping system. In many cases this is a very important factor, such as in nuclear power plant piping sys­tems.

A root bead made by the GTAW process is metallurgically sound throughout By choosing the correct rod to deposit the filler metal, it is possible to obtain a wetd having the same chemical, metallurgical, and physical properties as the base metal in the pipe. Defects such as oxidation are eliminated because the blanket of inert gas covers the weld and whipping is not used to deposit the bead.

The GTAW Process

Gas tungsten arc welding is a process whereby the base metal and the filler metal are melted by the intense heat of an arc that is maintained between the work and a non-melting tungsten electrode. An inert gas, or mixture of inert gases, is used to shield the hot metal

from the effects of the atmosphere. Filler metai, supplied by a rod, is used to supplement the base metal except when welding very thin sheets of metal, for which no filler metal is used.

This process is shown in Fig. 6-2. The tungsten electrode is held within a welding torch, which also supplies the inert gas that is expelled from the end of the gas nozzle at a rate of 15 to 30 cubic feet per hour to form a shield over the hot metal. Right-handed welders hold the GTAW welding torch in the right hand while the rod that supplies the filler metai is held in the left hand. For left-handed welders these positions are usually reversed. In either case, both hands are used for GTAW welding.

Equipment

The major components of the GTAW process are;

1. The welding machine

2. The shielding gas and the gas controls

3. The GTAW welding torch

4. The tungsten electrode.

Welding Machine. The welding machines used for the GTAW process are specially designed for this purpose. Those welding machines designed for consumable arc welding, either AC or DC, can also be used if they are equipped with a special high-frequency attachment; however, the best welds are obtained by using machines designed specifically for the GTAW process.

GTAW machines are available in the form of AC/DC rectifiers or as DC generators driven by an electric motor or an engine. Either straight or reversed polarity can be used with direct current - High - frequency current is used only for starting the welding arc when using DC current; it is always used with AC current. The welding current is turned on by a foot or a hand control.

The current characteristic used depends upon the type of metal to be welded. Specific recommendations are given in Table 6-І.

The Shielding Gas. To prevent oxygen and nitrogen in the air from contaminating the weld, either argon or helium, or a mixture of both, is used as a shielding gas. Argon is more widely used since it is easier to obtain and because it is a heavier gas, thus providing better protection, or shielding, at a lower flow rate. A gas flow of 15 to 30 cubic feet per hour (CFH) is normally used.

The gas is stored in a cylinder, Fig. 6-3, and control of its flow is similar to that used to control the flow of oxygen by a gas gage.

Fig. 6-4. A. The GTAW welding torch. B. Components of the GTAW welding

torch.

Beyond this gage, the gas to the torch and the weld is controlled either by a switch mounted on the torch or by a foot pedal. When the gas leaves the tank it is fed through an electrical control valve that is actuated by the switch which allows the gas to flow only when the welding current is turned on. The gas can be made to flow contin­uously by means of manual control but this can be very costly.

GTAW Torch. The GTAW torch, Fig. 6-4A, holds the tungsten electrode and directs the welding current to the arc. Torches can be water-cooled or air-cooled, depending on the welding current amperage. For root bead welding, which is normally deposited using a current range of 75 to 200 amps, water-cooling is not required.

The parts of a torch are shown in Fig. 6-4В. A nozzle surrounds the electrode, which is held in place by a collet chuck. Different collets, ranging in size from.020 to.250 inch, are available to accommodate the same range of electrode diameters. To insert an electrode, the cap is removed and the correct-size collet is inserted in the torch. Insert the electrode in the torch and push it about l/2 inch beyond the end of the nozzle, using the wrench furnished for this purpose to adjust the collet. Attach the cap to the torch and tighten it lightly. Then adjust the electrode so that it extends beyond the nozzle the correct distance, which is usually about 1 y2 to 2 times the electrode diameter, and finger tighten the cap. The torch is then ready to be used.

Two different types of nozzles are available. One type is made of ceramic, which is not transparent. Another is of glass, which offers better visibility of the pool of molten metal when welding.

Electrodes. The electrodes used with the GTAW process are made from tungsten alloys. They have a very high melting point (6900F) and are practically nonconsumable. When properly positioned, the electrode is located over the puddle and the intense heat of the arc keeps the puddle liquid. The electrode, which must be kept clean at all times, must never touch the molten metal to avoid the possibility of contaminating its tip with metal from the puddle. Should it become contaminated, the electrode must be removed from the torch and the contaminant removed either by grinding or by break­ing off the end of the electrode to remove the contaminated portion.