Uses and limitations of autogenous orbital GTAWShawn Martin | November 25, 2019
Autogenous orbital gas tungsten arc welding (GTAW) is a controlled, precise welding technique developed to reduce the chance of operator error and improve weld quality when joining tubes or other static workpieces with a non-consumable electrode. The orbital welding head articulates a tungsten electrode through a full 360-degree rotation. Welding parameters are typically programmed through a graphical user interface and the weld is encapsulated in an inert gas chamber. Although a wire feed can be used to support continuous seam welding and achieve full penetration, autogenous welds complete the same tasks without a filler metal. It is predominantly used to join 316 SS pipes, tubes and fittings in aerospace, semiconductor, biopharmaceutical and other high-purity applications.
Uses and advantages
Autogenous orbital GTAW is a highly productive automated welding process. It is also tightly controlled and addresses regulation requirements for several industries where vital inspection logs and records are required, as is the case for pharmaceutical facilities as mandated by the U.S. Food and Drug Administration.
Autogenous orbital GTAW produces single pass, complete joint penetration on square edge butt joints. Multiple passes are not required, no filler metal is required and joint preparation is minimized, reducing fabrication costs. Advanced welding controllers also provide a memory function so that welding parameters inclusive of base metal, tube diameter, fit-up, shielding gas, arc length, arc voltage, electrode type and geometry can be saved and recalled, allowing accurate weld repeatability.
Other advantages result from the ability to complete welds in restricted spaces where it is increasingly difficult or even impossible to complete a manual weld. In addition, less skill is required to complete the job. GTAW requires a highly skilled operator but autogenous orbital GTAW has a much flatter learning curve. The operator is supplied with operating procedures that must be followed and is largely responsible for troubleshooting welding deficiencies early in the process.
While autogenous orbital GTAW presents several advantages over manual GTAW or orbital GTAW, where a filler metal is required, it also presents several limitations. It is most suitable for joining pipe or tube diameters of no more than 6 in. This tolerance is at the upper limit of operations as acute eccentricity in pipes, tubes or fittings becomes problematic due to larger variances of the arc gap distance. The weld head also tends to overheat when joining larger diameter work pieces, which can lead to a loss in productivity.
Autogenous orbital GTAW is not the preferred method of joining certain corrosion resistant alloys, such as duplex stainless steels, or super-austenitic alloys like AL-6XN. Both alloys provide greater corrosion resistance than 316 SS; however, welded seams suffer from reduced corrosion resistance and a filler metal is required. AL-6XN requires a filler metal over alloyed in molybdenum to preserve desirable characteristics. Duplex stainless steel requires a nickel-enriched filler metal; otherwise nitrogen must be introduced as an alloying element.
Autogenous orbital GTAW has been extensively used in semiconductor, biopharmaceutical and other high-purity applications. It is the preferred method of assembly for piping units composed of 316 SS up to 6 inches in diameter. To maintain productivity and weld quality alternative methods should be investigated when joining larger diameter pipes or when joining higher-alloyed corrosion resistance materials.