Technology Gets to the Root of Pipe Welding
Open root welds on pipes can
be made three to four times faster than GTAW by using the Surface
Tension Transfer® process. When integrated with an internal
spacer clamp into a new automatic orbital pipe welding system,
even faster production is possible, with no lack of fusion.
Pipe welding codes, whether for
applications in the field or in the plant, require high-quality root
pass welding. To ensure that the joints will not leak,
especially for steam or pressurized applications, a weld must
penetrate completely through the pipe.
In the past, pipe welding was
done by one of three methods, each of which has its advantages
and disadvantages. These are the methods that have been used.
Gas tungsten arc welding
(GTAW) is popularly known as TIG. Travel speeds are slow, heat
input is usually high, and it requires high operator skill level.
Gas metal arc welding
(GMAW) - also known as MIG - is a much faster process than GTAW.
However, because operator skill level is hight and heat input difficult
to control, fusion may not always be 100 percent.
Shielded metal arc welding
(SMAW), also known as stick, can be cost effective in terms of
equipment but requires high operator skill. Frequent starts
and stops are another potential problem.
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Smoke and Spatter are reduced when pipe
joints are welded by means of the Surface Tension
Transfer (STT®) process.
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By contrast, the Surface Tension Transfer (STT)
process makes it possible to complete open root welds three or
four times faster than GTAW, with low heat input and no lack
of fusion. The STT process uses high frequency inverter
technology with advanced waveform control to produce a
high-quality weld with less spatter and smoke. For pipe welding, the
process also makes it easier to perform open gap root pass
welding, with better back beads and edge fusion. It is easier
to operate than other processes, yet produces consistent, X-ray
quality welds. The STT process results in a complete back bead
without shrinkage from the 12 to 6 o'clock weld positions.
Also, because current control is independent of wire feed
speed, the process allows greater flexibility under all conditions.
Controlling Spatter and Smoke
STT is a proprietary Lincoln Electric
process that makes use of Wave Form Control Technology™ to
control current precisely and rapidly during the entire welding
cycle. It is unique in that it is neither constant current
(CC) nor constant voltage (CV). Instead, the power source adjusts
current automatically to the instantaneous heat requirements of the
arc.
Spatter and smoke are reduced with this
process, whether the arc shielding gas is 100 percent CO2,
blends of argon and CO2 or helium mixtures for use with
stainless steel. Reducing spatter minimizes final weld surface
preparation and allows the operator more welding time before the gun
nozzle must be cleaned of accumulated spatter.
In open root pass pipe welding, the STT process
controls the wave form of the welidng current for
excellent penetration control, fusion, and back bead. |
Reduced spatter also translates into
significant cost savings because more of the electrode is
applied to the weld joint, not as spatter on the pipe and
surrounding fixtures. Further cost savings are realized because
larger diameter wire can be used.
At the start of the cycle, when the
electrode shorts, the current is reduced immediately,
eliminating the incipient short. This low-level current is
maintained for a short time so that the surface tension forces can
begin transferring the drop to the puddle, forming a solid
mechanical bridge. A high level of pinch current is then
applied to accelerate the transfer of the drop. The necking
down or squeezing of the shorted electrode is monitored. When a
specific value is reached, the pinch current is reduced
quickly to a low value before the fuse separates. When a short breaks,
it does so at a low current, which produces very little spatter.
Next, the arc is reestablished and a high current known as peak
current is applied. This momentary pulse of current establishes
the arc length and causes the arc to broaden and melt a wide
surface area, which eliminates cold lapping and promotes good
fusion.
Spacer clamp and welding shed on-site and ready for set up. |
Better Pipe Welding Results
The constant voltage GMAW process
normally used for pipe welding does not control the current
directly. Instead it controls the average voltage. This can
cause the weld puddle temperature or fluidity to be too high,
and the internal bead may be flat or shrink back into the root. This is
known as "suck back." Also, when using conventional short arc
GMAW, the operator must concentrate the arc on the lip or
leading edge of the puddle to ensure proper penetration and
fusion. If the arc is too far back on the puddle, penetration
will be incomplete. If the arc is too far ahead, the electrode
shoots through the gap and causes whiskers to form inside the
pipe.
Because Surface Tension Transfer
controls the welding current independently of wire feed speed,
the process makes it easy to control the temperature or
fluidity of the puddle to ensure proper penetration and fusion. This
is what makes it so attractive for open root pipe welding
applications. In the 5G position, the operator simply has to
stay in the puddle. Experienced pipe welders almost always find
the process a welcome improvement, both in ease of welding and
comfort. They particularly appreciate the reduction in spatter
when welding in the 6 o'clock position.
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As the decision process
evolves, the vendor and the fabricator will continue working together
to determine the appropriate system accessories, including safety
devices, the optimal layout for the robotic cell, manpower and
training requirements, and service and maintenance
requirements (internal vs. outside vendor support).
The STT process is gaining
acceptance in pipe welding and similar applications, which require
precise control of heat input as well as smoke and spatter
reduction. Since the heat is controlled directly, the internal
backbead profile is also controlled. Welders find that not only
are open root welds easier to make, but their mechanical and
metallurgical properties are excellent. Superior welding bead
profiles can be attained with improved properties in the heat
affected zone. Moreover, open root welds are made without the
use of ceramic or copper internal backup. In the case of copper,
corrosion is thus eliminated by avoiding the possibility of
copper inclusions.
The process is effective for
welding mild and high-strength steels, as well as stainless steel
and related alloys. On steel, it offers the advantages of low
hydrogen and 100 percent CO2 shielding with low spatter. When
welding duplex stainless, critical pitting temperature is
significantly better with STT than with GTAW, and travel speeds
three or four times that of GTAW can be obtained, with much
less skill.