Orbital welding is a specialized area of welding whereby the arc is rotated mechanically through 360° (180 degrees in double up welding) around a static workpiece, an object such as a pipe, in a continuous process. The process was developed to addresses the issue of operator error in gas tungsten arc welding processes (GTAW). In orbital welding, computer-controlled process runs with little intervention from the operator. The process is used specifically for high quality repeatable welding.
Equipment of Orbital Welding Machine
The main components of every orbital welding system are the power source and controller, the welding head and, where required, a wire feed mechanism. Welding of certain sizes and material types will also require the use of a water/coolant system. There are a large number of factors that can have an influence on 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.
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 welder will always be 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.
Orbital Welding of Tubing using totally enclosed weld heads is a fusion process under ASME Section IX. No filler metal is added.
A successful automatic orbital GTA weld is 100% repeatable as long as the operator monitors variables and performs periodic samples or coupons which are inspected for complete penetration. Noticing that a variable has changed is a primary skill and can be easily missed. Training and experience are required for an operator to be successful at consistently producing acceptable welds.
The successful automatic orbital GTA weld is very dependent upon refinement of several critical variables that involve programming the welding machine and set-up of the "weld head”.
Maintenance of the weld head often becomes a factor in repeatability of successful welds. Weld head internals can become charred from improper use. The charring is carbon deposits which can conduct electricity and short circuit the flow of current from the tungsten. 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 GTA welding and are obtained from a number of specialty manufacturers.
Successful orbital welding is also dependent 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 diameters 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 typically only performed on TUBING and not 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 suppling 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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