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FABRICATING RECOMMENDATIONS IN THE WELDING OF TWO-SIDED GROOVE WELDS
Figure 1. One advantage of two-sided butt joints over their single-sided complements is that joint
economy improves above as the plate thickness increases due to the joint geometry. This improvement
in joint economy by moving from a single-V weld to a double-V weld is displayed here graphically.
Figure 2. Besides the joint geometry creating a 50 percent reduction, the superior joint economy of two-
sided groove welds is attributed to the smaller opening of the joint at the surface of the plate. Because
this distance is smaller, there is less weld reinforcement in two-sided joints compared to their single-
sided counterparts. This concept of reinforcement is displayed here.
Figure 3. A schematic of a back-gouged two sided butt joint. By gouging to sound weld metal on the
back-side of the root pass, chances for a clean X-ray are virtually assured.
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Figure 4. A successful weld joint profile using passes conducted with AC Square Wave polarity, a 5/32 in
diameter electrode, 750 amps, 32 volts, 1.25 in CTWD and a 20 ipm travel speed.
Regis Geisler, Lincoln Electric
A common misconception is that the deeper the penetration of the weld bead, the better. But the only
reason for greater than 115 percent penetration would be insurance against the wire possibly tracking
away from the center of the joint. If we ensure that the weld beads meet in the middle, only 105
percent penetration is required. So to accommodate either wire or joint wander we can increase the
width of the weld bead by increasing the output voltage and achieve the same effect of insuring against
joint tracking issues.
By Regis Geisler
Here are some real-world considerations and practices to improve quality and productivity during the
welding of two-sided butt joints that are frequently used in pressure vessels and wind tower
construction.
As mentioned in last months column (Choosing A Butt Weld Joint Preparation, Welding Tips, May
2011), fabricators have traditionally tended to prefer the use of two-sided butt joints to achieve
complete penetration in groove welds when the plates to be welded are greater than one inch in
thickness. There are a few distinct advantages to two sided groove welds that makes them a very
attractive joint design for use in industries such as pressure vessel fabrication and wind tower
construction.
The first advantage to two-sided butt joints over their single-sided complements is that joint economy
improves above as the plate thickness increases due to the joint geometry. This improvement in joint
economy by moving from a single-V weld to a double-V weld is displayed graphically in Figure 1. In
addition to the joint geometry creating a 50 percent reduction, an additional component of the superior
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joint economy of two-sided groove welds is attributed to the smaller opening of the joint at the surface
of the plate. Because this distance is smaller, there is less weld reinforcement in two-sided joints
compared to their single-sided counterparts. This concept of reinforcement is displayed in Figure 2.
The second advantage is that the residual stresses can be forced to be more balanced in a two-sided
groove weld, provided that the appropriate pass sequence is utilized. By proper pass sequence, I am
referring to an alternating pass deposition progression, where a weld layer is deposited on one side
followed by a weld layer on the second side of the joint.
However, two-sided groove welds are not a panacea. It was also mentioned last month that two-sided
complete joint penetration groove welds require that two root passes be deposited. The inherent fear
regarding root passes is that incomplete penetration and trapped slag can easily occur and these
conditions may not be detected until after the joint has been fully welded. Without a doubt, thefabricator will want to minimize the risk that this will occur in order to prevent a costly weld repair.
To pre-empt the possibility of these weld defects as a potential problem, most fabricators preferentially
choose to perform the air-carbon arc gouging process on the un-welded second side of the joint. By
gouging to sound weld metal on the back-side of the root pass, chances for a clean X-ray are virtually
assured. A schematic of a back-gouged two sided butt joint is shown in Figure 3. However, back-gouging
is quite often unnecessary, as other less-costly fabrication techniques can be employed that will ensure
complete penetration through the root without weld defects.
Consider an example where a pressure vessel is being fabricated using 1.5 in thick sections. These
segments are being butted together and welded in a double-V joint configuration. Both plates have a
3/16 in root face (land) in the middle of the through-thickness, a symmetrical 30 deg bevel on the first
side and a 30 deg bevel on the second side (in other words, a 60 deg included angle for both Vs). This
3/16 in root face is actually quite common, as this typically ranges from 3/16 in to in.
The next step would naturally be to place a quick root pass on the first side. This root pass can bedeposited with any of a variety of welding processes. For example, either the gas-shielded flux core arc
welding (FCAW-G), gas metal arc welding (GMAW) or submerged arc welding (SAW) processes can all be
used to deposit this pass. When the fit-up is not perfect in some places say greater than 0.100 in it is
required that the welder be wary that the weld could burn through and leave a hole in the root of the
joint. That is why, in these situations, it is very common for the FCAW-G or GMAW processes to be used
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for the quick root pass, because these two processes operate at lower output power and can be more
forgiving when large gaps are encountered.
But if the fabricators manufacturing process provides a tight fit-up along the entire length of the joint
then the SAW process, accompanied by its generally higher output amperage and voltage, can be
employed. With the joint configuration dimensions listed above, a tried-and-true root pass procedure
that has been developed using the single-arc AC Square Wave SAW process can be used. The wire
diameter that is (or should) most often be selected is 5/32 in diameter, as the top end of the output
range of this diameter electrode aligns with the top amperage rating of 1000 amp welding power
sources. A proven procedure in this situation is 625 amps, 28 volts, a contact-tip-to-work distance
(CTWD) of 1.5 in, and a travel speed of 30 ipm.
Note that we have chosen to use AC Square Wave polarity over DC+ polarity for this pass since we willachieve a higher melt-off rate per amp of output, which will provide us with the same deposit layer
thickness at a faster travel speed. This, in turn, will reduce the level of penetration on this root pass. This
higher melt-off rate with AC polarity (compared to DC+ polarity) is a consequence of a higher proportion
of the welding heat being transferred to the electrode in AC polarity due to the back-and-forth flow of
electrons.
However, if there is more heat in the electrode with AC polarity, then there is less heat in the plate. This
may necessitate the use of DC+ polarity (and hence a larger root face) for the first several passes to get
better bead wetting on extremely thick plate (say 4 in or greater). After approximately in of the joint
has been welded, the AC Square Wave polarity can be implemented to achieve higher melt-off rates.
The root pass on the first side should be followed immediately by a second or hot pass on the same
side. In staying with the same train of logic that is, the maximization of productivity we will continue
our use of the SAW process. A good starting procedure with AC Square Wave polarity and a 5/32 in
diameter electrode would be 750 amps, 32 volts, 1.25 in CTWD and a 20 ipm travel speed.
At this point, you may wonder why we would want to continue welding on the same side of the joint.
After all, it was stated above that we would want to minimize the build-up of residual stresses by
alternating the deposition of weld layers between sides. This would be true for all of the weld layers
except for this one. What we are trying to accomplish here is a combined thickness of the root and hot
pass that will provide enough depth to prevent burn-through for the first pass deposited on the SECOND
side.
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This leads us to the next step in the process of constructing of our pressure vessel, which may be new to
some fabricators. Our stated desire is to avoid the common practice of back-gouging the second side
before depositing weld metal. As mentioned earlier, this quality measure is performed due to fears of
weld defects in the root. However, with the SAW process, a properly designed weld joint and second-side, root-pass welding procedure will allow the arc to punch into the weld bead(s) that have
deposited on the first side. Note that no additional preparations are required to be performed on the
second side before this pass is to be deposited. Rather, all that is needed is simply experience and belief
in the axiom that amperage is proportional to the depth of penetration.
Exactly what amperage is necessary to achieve this complete joint penetration? This, of course, depends
upon the thickness of the land and the bevel angle. But successful welding of the second root pass on
our two-sided joint example has been conducted with the following procedures: AC Square Wave
polarity and a 5/32 in diameter electrode, 750 amps, 32 volts, 1.25 in CTWD and a 20 ipm travel speed.
A similar procedure for the remaining passes can be used, resulting in a weld joint profile similar to that
shown in the macro photograph in Figure 4.
It is noteworthy that this welding pass procedure is identical to what was used for the hot pass on the
opposite side of the plate. While we do want the root pass welds to meet in the middle, the weld is
not actually any stronger with 115 percent or greater penetration. There is a common misconception
that the deeper the penetration of the weld bead, the better. For this reason welders often say lets
crank up the amperage. But the only logical rationale for greater than 115 percent penetration wouldbe insurance against the possibility of the wire tracking away from the center of the joint. So if we
could ensure that the weld beads meet in the middle, then all that is really required is 105 percent
penetration. Therefore, in order to accommodate either wire or joint wander we can increase the width
of the weld bead by increasing the output voltage. This will have the same effect of insuring against joint
tracking issues.
Not coincidentally, there are actually two distinct benefits to having just over 100 percent penetration
versus having 115 percent penetration. The first is that width-to-depth cracking issues are mitigated,
because the weld bead will be wider due to a lower output amperage and higher output voltage. Thisrelatively wider bead creates a weld microstructure in which the grains will be less likely to be aligned
perpendicular to the centerline of the weld. This reduces the chances of centerline cracking in the weld.
Furthermore, when the microstructure aligns itself in this fashion, it can have the effect of improving
weld metal toughness.
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An additional, incremental improvement to weld metal toughness can be attained due to the fact that
when there is 105 percent penetration, there is more re-heated weld metal present in a Charpy V-notch
sample taken from the center of the weld joint. With 115 percent penetration, the majority of the
Charpy V-notch sample is as-cast weld metal, which tends to have lower impact toughness.
The above discussion is just a small sample of some real-world considerations and practices that can be
implemented to improve quality and productivity during the welding of two-sided butt joints. This type
of joint design is frequently used in applications such as the fabrication of pressure vessels and wind
tower construction
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