Orbital welding is a specialized area of arc welding whereby the arc is rotated mechanically through 360° (180° in double up welding) around a static workpiece, such as a pipe, in a continuous process. This method and technology was developed to address the issue of operator error in manual gas tungsten arc welding (GTAW) applications requiring precision tube and pipe welding. To ensure high-quality repeatable welds a more stringent weld criteria was set by the ASME. In orbital welding, an automated computer-controlled process runs with little intervention from the operator.
History Rodrick Rohrberg of North American Aviation invented the orbital welding process to address fuel and hydraulic fluids leaking in and around the plumbing of the X-15 Rocket Research plane.
Equipment The main components of the typical orbital welding system are the power supply with integrated computer control, the welding head, and either a wire feed mechanism or a presharpened tungsten electrode. Welding of certain sizes and material types will also require a water/coolant system. There are a large number of factors that influence the welding result. These aspects include the arc length, magnitude, and pulse frequency of the welding current, welding speed, inert shielding gas, parent material, filler material, weld preparation, and thermal conductivity. Ultimately, a high-quality weld is achieved through detailed knowledge of how to precisely adjust all these parameters for each individual welding task.
Application
The welding process It is very difficult to achieve the highest standards of quality and safety using manual welding. This is due to certain welding positions, overhead and down-hand welds for example, often leading to faulty welds due to restricted access the user has in these welding positions. In order to have complete control over the weld pool, a perfect balance must be maintained between gravitational force and surface tension at every position of the torch. By using mechanised variants of the technique, certain parts of the welding process are handled by mechanical components. Note that a welding operator is always monitoring and controlling the process. In an ideal situation, all welding parameters would be fully programmed before welding is started. In practice, however, the presence of variable constraints means that it is often necessary for the welder to make corrective interventions and create custom programs for the application. Orbital tube welding can use open weld heads and enclosed weld heads. Each style of weld head uses the fusion process described in ASME Section IX. No filler metal is added. A successful orbital weld is 100% automatic and repeatable as long as the operator monitors variables and performs periodic inspections of the weld seam for complete penetration of the weld. Noticing that a variable has changed during the welding process is a necessary skill for orbital welding technicians and operators that can be easily missed. Training and experience are required for an operator to be successful at consistently producing acceptable, repeatable, high-precision welds that meet today's standards. Several critical variables can affect the success of an orbital weld. These include using the proper weld program in the orbital welding power supply or controller to match the settings to the pipe size, wall thickness, and tube material. Other key factors include the pipe preparation and face, the correct positioning of the weld head on the tube, and the amount of oxygen present during the welding process. Maintenance of the weld head often becomes a factor in the repeatability of successful welds. Weld head internals can become charred from improper use. The charring is carbon deposits that can conduct electricity and short-circuit the current flow from the tungsten. Orbital weld heads contain a system of precision planetary gears that can wear out over time. Proper cleaning and maintenance is required. Successful orbital welding is also dependent upon using high quality tubing material. Typically only 316L stainless steel tubing (not pipe) and fittings are used for automatic orbital welding and are obtained from a number of specialty manufacturers. The weld quality depends upon having a reasonably clean source of Argon for backing and shielding gas. Minimum purity would be 99.995% for typical industrial applications. For some applications it is necessary to use ultra high purity argon, 99.9998% purity and such applications requires the use of all high purity purge equipment (valves, regulators and flow control). Typically, no rubber components can be used for purge gas apparatus since the rubber absorbs and releases moisture and oxygen into the argon stream. Moisture and oxygen (in Argon) are contaminants detrimental to a successful automatic orbital weld. Weld coupons, pieces of metal used to test a welders' skill, are typically prepared at the beginning of a welding shift, any time any variable is adjusted or changed and at the end of the shift (and more frequently as required by an inspector). Each coupon must be examined internally and externally to verify full penetration, proper bead width and other criteria. With smaller diameter pipe or tubing, it is usually necessary to section open the coupon to examine the weld bead. All coupons must exhibit complete penetration and consistent bead width. Variations in consistency are an indicator of a problem that must be resolved before continuing. Orbital welding is more commonly performed on tubing than on pipe for several reasons, most important being that the production of tubing yields very consistent outside diameters which is critical to proper fit up in the weld head. Automatic Orbital GTA welding has become the standard joining method for high integrity gas and liquid systems used in the Semiconductor and Pharmaceutical manufacturing industries. These systems are rated for extreme purity and leak tight integrity. An entire specialty industry supplying valves, fittings, regulators, gauges and other components for orbital welding and use in high purity applications has developed since the mid 1980s. For tube welding in high purity applications only a fully enclosed weld head may be used.
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