Fabricating Recommendations in the Welding of Two

8/3/2019 Fabricating Recommendations in the Welding of Two

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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

Fabricating Recommendations in the Welding of Two

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