Why K-TIG Compares to Plasma Arc Welding

K-TIG and Plasma Arc Welding (PAW) are both arc welding processes which allow full penetration welding. Both create what is referred to as a ‘keyhole’ through the material being welded, and both use higher currents that conventional GTAW welding. But that’s where the similarities end.

Plasma welding, developed in the 1960’s has traditionally been used to fill the large gap which has existed between conventional arc welding processes such as TIG and MIG, and high energy density processes such as laser and electron beam welding. 

The need for a new, ultra-high productivity welding technology has been obvious for many years but until recently this has remained an elusive objective.

Much more recently, there’s been a seismic shift in arc welding technology that provides up to 100x productivity gains over GTAW, with twice the speed and double the penetration of plasma welding. The process is a new GTAW variant, is simple to operate and comes at a modest capital cost. The process is called Keyhole TIG or Keyhole GTAW (K-TIG).

What is Keyhole TIG?

 Developed by the Australian Government’s Commonwealth Science & Industrial Research Organisation (CSIRO), Keyhole TIG is a high-speed, high-productivity variant of GTAW/TIG.  Like GTAW, it utilises a non-consumable tungsten and an inert shielding gas.  All gas mixes suitable for GTAW can be used with K-TIG.

K-TIG has 8x the penetration of GTAW, allowing it to perform x-ray quality welds in materials up to 5/8 inch (16mm) thick in a single pass, without the need for edge bevelling. The resulting welds are performed at up to 100x the speed of conventional TIG/GTAW. Gas consumption is reduced by more than 90%, and wire consumption is reduced by in excess of 90% or eliminated entirely.

The technology is being used widely in ASME vessels & tanks, cryogenics, oil & gas, power generation, water treatment, renewable energy, nuclear and defence fabrication, tube mills, pipe spooling and a wide variety of general fabrication applications.

The defining feature of K-TIG welding is the concentration of the arc to create a ‘keyhole’ which fully penetrates the joint, and allows for single-pass welding in materials up to 16mm in thickness. The process does not require wire or edge bevelling. Relative to conventional GTAW/TIG, productivity gains and overall savings of 70% to 90% are typical. Relative to Plasma Arc Welding (PAW), productivity gains of more than 50% are typical.

K-TIG inherits all of the intrinsic weld quality attributes of GTAW to produces welds of outstanding quality and strength. K-TIG is ideally suited to corrosion resistant and reactive metals including stainless steel, duplex, super duplex, hastelloy, inconel, titanium and zirconium. High-quality, low-sulphur carbon steels are also suitable. K-TIG is a GTAW process as defined by ASME.

Advantages

  • 70% to 90% productivity gains and costs savings relative to GTAW
  • Single-pass, full penetration up to 16mm
  • Does not require wire or edge bevelling
  • Extremely clean, high-quality, ripple-free welds

Disadvantages

  • Not suitable for orbital (5G/6G) applications
  • Not suitable for copper or aluminium

K-TIG can:

  • Weld in 1G and 2G positions
  • Weld pipe diameters down to approximately 70mm (3 inches), with no upper diameter limit
  • Weld titanium up to 5/8 inch (16mm) in thickness
  • Weld stainless steel up to 1/2 inch (13mm) in thickness.
  • Weld carbon steel up to 11/32 (9mm) in thickness

If the material to be welded exceeds K-TIG’s upper limit, the material can be prepped with a thick landing, with a full penetration root pass performed followed by a filler pass.

What is Plasma Welding?

Plasma welding developed in the 1960’s has traditionally been used to fill the large gap which has existed between conventional arc welding processes such as TIG and MIG, and high energy density processes such as laser and electron beam welding.

Plasma welding is widely regarded as the most complex of the arc welding processes. The constricted, high energy density plasma jet is produced by maintaining a critical balance between plasma gas flow rate, shielding gas flow rate, current, orifice diameter and the alignment between the electrode and the orifice. This requires meticulous set-up and frequent maintenance.

Plasma welding’s limited speed, penetration, propensity to entrap gas voids, difficulty in managing tie-ins, high complexity, numerous critical parameters, minimal fitup tolerance, high cost of consumables and rapid process drift (typically 20-30 minutes) due to erosion of the nozzles and orifice have contributed to limiting its adoption.

Much of the installed base of plasma welding equipment are legacy systems and are no longer being produced or supported by a manufacturer.

PAW uses a special torch which fully envelopes a non-consumable electrode within a water-cooled nozzle. A high current is passed through the electrode to form a plasma jet, and this plasma jet is then forced through a very narrow aperture orifice. The orifice constricts the arc, and the plasma jet then exits the orifice at a high velocity and high temperature. The process requires the management of two inert gases: one which is used to form the arc plasma; and the second to shield the arc plasma.

PAW can be used autogenously in thicknesses up to 8mm, however parameter settings become critical as material thickness increases. High-quality welds can be achieved with the process, however high operator skill and frequent maintenance is required in order to maintain all required parameters within tolerance.

Advantages

  • Full penetration welding possible in materials up to 8mm
  • Fast welding speeds
  • Well suited to 1G (vertical down) welding applications
  • Reduced requirements for edge bevelling, typically square butt joint presentation

Disadvantages

  • Complex process requiring high operator skill
  • Multiple welding gases and numerous critical parameters
  • High capital and maintenance costs
  • Minimal tolerance to mismatches and gaps
  • High heat input can result in significant distortion and loss of mechanical properties

How does K-TIG overcome the issues of Plasma Welding?

Plasma welding is characterised by the need for highly accurate electrode alignment, frequent maintenance, the need for both plasma and shield gases to form the jet and protect the orifice, very accurate determination and maintenance of flow rates, low keyhole stability, highly precise fit-up, and a high degree of operator skill to set up, operate and maintain the equipment.

By contrast, K-TIG is very simple to operate. The arc structure and keyhole develop spontaneously and are maintained automatically by the controller throughout the weld. There is no plasma nozzle or orifice, no precise electrode alignment is required, only one welding gas is used, flow rate is not critical, and the torches are very robust.

The welding speed, penetration and productivity of K-TIG is typically twice that of PAW. K-TIG delivers consistent, x-ray quality welds and handles tie-ins easily in circumferential welds, overcoming PAW’s well known difficulties with ties-in and tendency to entrap gas voids. K-TIG’s high-density arc creates a consistent and smooth keyhole through the joint and its ultra long-life electrodes make process drift negligible.

K-TIG is a much simpler, more robust process than plasma arc welding. I can’t think of a material, thickness or weld geometry where plasma arc welding would be chosen over K-TIG~ Attila Szabo, Principal Joining Engineer, GE

Who should use K-TIG?

K-TIG is ideally suited to fabricators who currently operate plasma arc systems. Indeed, the majority of K-TIG users globally have retired plasma welding system in favour of K-TIG. Many of these are fabricators involved in pressure vessels, suction vessels, cryogenics, tanks and similar applications which require a highly robust process capable of performing both longitudinal and circumferential welds to ASME IX standards. The technology is being used widely in oil & gas, power generation, water treatment, renewable energy, nuclear and defence fabrication, tube mills, pipe spooling and a wide variety of general fabrication applications.

Switching to K-TIG is straightforward. It’s likely that you already operate using a positioner, rotator, seamer, column and boom or robot. If so, these are all compatible with K-TIG, and may continue to be used.

For a detailed comparison of K-TIG and Plasma Arc Welding, please click here.

If you’re interested in learning more about how K-TIG can assist you to increase productivity, reduce costs and improve quality, feel free to contact us or use the free K-TIG savings assessment service to quantify the savings on your specific applications or project.

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