By Tim Heston
Gas tungsten arc welding (GTAW) has a long history as a process capable of producing high-quality joints. If fabricators needed
greater speed and penetration, they turned to
processes like plasma arc welding (PAW) as well
as electron beam and laser beam welding. These
high-energy-density joining methods can force a
hole—a keyhole—through the material being welded. Molten metal fills in behind the keyhole to form
the weld bead, creating a joint in thick material in
only one pass.
Today fabricators have another option: a high-energy-density variant of GTAW. Dubbed K-TIG, the
process is well-suited for stainless steel; titanium;
nickel alloys like MONEL®, INCONEL®, and HASTEL-LOY; cobalt alloys; and superalloys. It’s capable of
welding square butt joints of up to 0.5-in. stainless
steel and up to 5/8-in. thick in titanium.
Although the process can work with high-quality,
low-sulfur carbon steels, it’s not well suited to low-quality carbon steels. Nor does it work on copper
and aluminum, which have thermal conductive
properties that aren’t suited for the process.
Welding mainly in the 1G and 2G positions (not 5G
or orbital), K-TIG is designed to be used in an automated setup. The torch is mounted on a slide, with
the head of the torch dropping into a supporting
carriage. As the height of the material changes during welding, the carriage raises and lowers the torch
to maintain consistent voltage.
The process reportedly can complete single-pass
welds at 10 inches per minute (IPM) in 0.5-in.-thick
material and almost 20 IPM in 0.236-in.-thick material. These speeds, along with the fact that the process operates at relatively low voltage (with a small
electrode gap between the tungsten and the workpiece), keep heat inputs within normal ranges.
Gas usage, be it a shielding or a backing/purging
gas, resembles gas usage in conventional GTAW.
Gas combinations like argon/hydrogen, argon/ni-trogen, and argon/helium can be used, depending
on the materials, and straight argon can work in all
The “K” in K-TIG stands for keyhole. The K-TIG
keyhole develops spontaneously using a single conventional welding gas. The keyhole is created by the
GTAW variant’s high energy density arc transferred
by a tungsten electrode that resembles those used
in conventional GTAW, only thicker at 0.25 in.
It’s a high-current process compared to conventional GTAW. Typical currents range from 320 to 600
amps, though the process’s power source has 1,000
amps available at 100 percent duty cycle, mainly to
account for extreme applications like continuous
welding in tube mills.
The fundamentals behind the technology were
developed by engineers at the Commonwealth
Scientific and Industrial Research Organisation, an
industrial research division of the Australian gov-
ernment. The chief scientist involved with creating
the process acquired the rights for the technology,
and those rights were ultimately rolled into K-TIG,
a company based in Adelaide, the capital city of
So how exactly does K-TIG create the high energy
density needed for such penetration? According to
Neil Le Quesne, K-TIG CEO, “There are many elements
that need to be accomplished correctly to balance
the arc forces and create a stable keyhole. There’s
quite a lot of IP [intellectual property] involved in it.
But achieving that penetration is just half the story.
We then must control it. We do that by managing the
surface tension of the molten weld pool.”
K-TIG produces what Adam Poole, director of
sales and commissioning, calls a “self-correcting
keyhole,” which makes the process forgiving when
it comes to workpiece fit-up. The keyhole cross sec-
tion looks more like a martini glass than a cylinder.
“The keyhole narrows at the root, creating a shape
that naturally supports the molten weld pool. This
creates a very stable keyhole that is tolerant of im-
perfections in weld joint fit-up.”
Le Quesne described this phenomenon using a
soap bubble analogy. Picture a bubble held in place
by two rings above and below. “Move the rings, and
the bubble still will maintain its shape, even if the
rings are out of alignment,” he said, adding that
the moving rings represent changes in joint fit-up.
“This happens continuously as you move over gaps
and have joint alignment and mismatch issues.
Throughout, the integrity of the keyhole isn’t lost.”
According to the company website, tolerances
vary with the application. But on average, mismatch
between base metals can be up to 20 percent of the
joint thickness, while gaps can be as large as 10 per-
cent of the joint thickness.
The process is most commonly applied to square
butt joint geometries (no edge prep), but it can also
work with V-preps, U-preps, and single bevel joints,
especially in materials thicker than the process’s
“When material thickness exceeds the limit of the
process, our customers use what we call a Y preparation,” Le Quesne said, and described a hypothetical
setup in 1-in.-thick plate. “As opposed to a typical V
prep where you’d see a 1/16-in. root face and a 1/16-in.
root gap, we’d be using a 5/16- to 3/8-in. root face and
a zero root gap. Almost half of the welding is done
in the first pass.” He added that in these multipass
applications, the process starts in keyhole welding
mode for the root pass and then switches to melt-in
mode for the subsequent passes.
Sources said that although the process is well
suited for autogenous welding, a wire feeder can be
used to create a heavier cap or root, filling in preps
(such as the Y-prep described), or to change the
metallurgical properties of the weld.
The process still is relatively new; it’s been marketed significantly only for the past several years,
and it’s now being considered an option alongside
other processes known for their speed and penetration characteristics. The process can now be found
in 18 countries, including the U.S. According to
sources, the technology is now being used in vessel
and tank as well as tube and pipe fabrication, and in
various applications in industries like nuclear, water
treatment, oil and gas, and defense.
Images courtesy of K- TIG, 61-8-7324-6800,
Gas tungsten arc welding goes deep
GTAW process variant boosts penetration, productivity