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 9. 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.
ASME requirements for certification of a person as an orbital welding operator requires the person to set up the weld head and program the welding machine and produce 6 consecutive samples that will pass bend and/or tensile testing. Professional instruction is typically obtained for a person to be able to make samples that pass the test.
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 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.
