Orbital welding is a mechanized version of the gas tungsten arc welding (GTAW) process. In manual GTAW, the welder moves the welding torch and controls the welding current. In orbital GTAW, the tungsten is installed in a weld head that clamps on the tube

or pipe. The tube remains in place while the weld head rotor revolves or orbits around the weld joint circumference to complete the weld.

The process may be used to produce autogenous welds (without the addition of filler materials), or filler may be added that becomes part of the finished weldment. The welding is done in an inert gas atmosphere to protect the metal from oxidation as it is heated to melting temperature. Gas metal arc welding (GMAW) may also be used for orbital welding.

Power Supplies. Orbital tube welding power supplies control weld parameters that typically include welding currents, background and pulse amperes (which determine the amount of heat input into the weld), travel speed (RPM), timers that control the amount of time at a particular setting, delay of rotation at the start of the weld, and a current downslope at the end of the weld.

A timed prepurge and postpurge are usually used to time the flow of inert gas into the weld head before arc initiation and to continue the purge for a timed period after the arc has been extinguished. This allows the weld to cool sufficiently to prevent oxidation before the weld head is opened to remove the welded tube.

Orbital tube welding is generally done autogenously, so additional controls used for wire feed are not required. Power supplies used for orbital tube welding generally supply 100to 150 amperes of welding current, direct current, electrode negative.

Modern orbital tube welding power supplies are microprocessor-based. This permits the storage of weld programs or schedules for a large number of tube sizes. The programs can be written, entered into the power supply, and modified by the operator based on welding results, and programs can be changed without loss of other programs. The power supply may be able to print out the weld schedule or to interface with a

computer for documentation of operational weld parameters.

Weld Heads. Weld heads for orbital tube welding typically span a range of sizes. For example, a weld head for tubing up to 38.1 mm (1-1/2 in.) outside diameter (OD) may also be able to weld tube measuring 6.4 mm (1/4 in.), 9.5 mm (3/8 in.), 12.7 mm

(1/2 in.), 19.1 mm (3/4 in.), and 25.4 mm (1 in.).

Autogenous (fusion) tube welds are practical in diameters from 3.2 rnm (0.125 in.) up to about 152 mm (6 in.) with wall thicknesses up to 4 rnm (0.154 in.). Standard orbital weld heads have tube clamp inserts on both sides of the weld to hold the tubes during welding, and the tungsten electrode is located in the rotor in the centerline of the head. To weld two tubes or fittings in a particular size weld head, the length of tubing or the straight section of the fitting must reach from the outside of the head to the electrode location in the weld head center.

 

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