September 13, 2010
Advancements in SAW concentrate on faster deposition rates and narrow-groove welding. And today's controls, power supplies, and consumables are meeting industry's demand for process efficiency.
Thick metal helps power the world. From wind towers to enormous pressure vessels used in nuclear plants, massive plate—from a little more than an inch to more than a foot in thickness—helps build the vessels that are the bedrock of infrastructure. Joining a good portion of these plates is an old standby process, submerged arc welding (SAW).
Industry demand has pushed the process’s speed limits even for the most difficult applications. The high-pitched crackle of the arc under a mound of flux granules remains as it ever was, but now seriously thick weld joints are being created faster. Modern systems accomplish the feat by adding more wires to the setup and narrowing the weld joint geometry, so there’s less of a gap to fill.
In SAW the flux blanket reacts with elements on top of the weld pool, forming a slag barrier that protects the joint from the atmosphere and slows the cooling rate. But slag isn’t so helpful if it becomes trapped inside the weld. Here, specific weld bead geometries help matters. The ideal weld bead is slightly concave and wets against the sidewall, providing a smooth transition between the base and weld metal. When that transition at the weld toe isn’t so smooth, troubles arise. If the bead becomes convex, having an almost ice-cream-scoop-shaped cross section, slag can become trapped in that undercut portion, where the convex bead curves inward on the edges.
Over the years engineers have perfected SAW in several ways. They have developed flux and wire chemistries that promote slag separation, ensuring all slag remains on top of the weld bead and not under or inside it. Another has been to make wire positioning and feeding more precise, which becomes particularly important as bevel angles are tightened. The wire must be placed just close enough to the sidewall to create the desired wetting action, with that concave bead wetting smoothly to the base metal, but not so close as to melt away the sidewall to create undercut. That undercut is an area ripe for flux entrapment, creating slag inclusions.
All these process variables require precise control of travel speed, wire feed, and the arc’s electrical characteristics, control that becomes more challenging when speeding the process. But according to sources, today’s controls, power supplies, and consumables are meeting the challenge.
SAW has several process iterations that combine wires and power sources. The common ones are single-wire, twin-wire, and tandem-wire SAW. Single-wire SAW uses, as the name suggests, a single wire, and the process often runs in the direct-current electrode-positive (DCEP) polarity for maximum penetration. In twin-wire SAW, two wires emerge from the same welding head and use energy from the same power source.
In tandem SAW, two wires feed from separate welding heads and power sources into the same molten weld pool. The setup allows fabricators to use a combination of polarities. When it comes to current, DCEP usually provides the best penetration, but deposition rates may suffer. Direct current electrode negative (DCEN) provides high deposition, but in some circumstances penetration issues may arise. Alternating current (AC) provides a happy medium between the two.
In the tandem process, the leading wire may be set at DCEP for penetration, while the trailing wire may be set at AC to provide the fill. Two DC arcs running next to each other isn’t practical because of electrical interference, but two AC arcs in a tandem setup are becoming more popular to increase deposition, particularly with new power sources.
Tandem-twin SAW uses four wires: two fed from a leading welding head and two more from the trailing head (see Figure 1). Its principal benefit is high deposition rates. Process characteristics also have allowed some to narrow the bevel angle. For instance, a 60-degree included angle, common in wind tower fabrication, might be reduced to 50 degrees.
As a recent publication from ESAB explained, “The reduced joint volume accounts for another major productivity advantage over and above the extremely high deposition rate.” In fact, industry continues to strive for greater deposition rates by adding more wires. The same publication showed a lab setup of a six-wire SAW process. But integrating more wires and narrowing the bevel angle also require another look at fluxes and wire types.
According to the ESAB publication, “In terms of weldability, the flux-wire combination must accommodate the high deposition rates of tandem-twin and, more specifically, provide good slag detachability in narrow joints.” In addition, “The correct positioning of the head is crucial to the success of the process, especially in regards to slag detachability.”
Small-diameter wires also offer more penetration than larger-diameter wires at the same current. This can help especially when using tighter bevels. As Jack Schroeder, North American sales manager for ESAB Automation, explained, “The single-wire process usually uses the largest-diameter wire. Therefore, it will dead-short higher up inside the joint, and it may not give you the penetration you need. Twin-wire typically uses smaller-diameter wires and allows you to feed down near the bottom of the joint to produce the desired penetration.”
Tandem-twin SAW has become a standard process in Europe, Schroeder explained, particularly among wind tower and heavy-vessel manufacturers, often welding plate 1 in. to several inches thick. During one test, ESAB cited that four 2.5-mm-diameter wires in a tandem-twin configuration deposited a weld at 38 kilograms/hour.
Welding a joint several inches thick is one thing; welding one that’s more than a foot thick is something else, and that’s the challenge faced by those joining reactor vessels in the nuclear and refining industries. These plates can be 13 in. thick and can require huge welds of a hundred passes or more.
Narrowing the groove in these joints can reduce the number of passes and weld metal used, but there’s a problem, as explained by Jason Williams, national sales manager at Advanced Manufacturing Engineering Technologies (AMET), a welding systems manufacturer based in Rexburg, Idaho. “If you’ve got such a narrow gap, it’s ideal to have the wire angled toward the sidewall. But with that [near vertical] sidewall, you run the risk of just cutting into it, which opens the door to slag inclusions.”
Enter narrow-gap tandem SAW, a process iteration that has been used on joints that are more than a foot deep (see Figure 2 and Figure 3). With narrow joints that deep, it can be difficult or impossible to angle the entire weld head to achieve the desired wire angle without interference issues. There is simply not enough room to position the weld head correctly.
To position the wire correctly, narrow-gap tandem SAW does something unusual: Servomotors manipulate the wire angle at the contact tips, so they can be placed just at the right angle, not too close to the sidewall and not too far away. Because this is a tandem process, each wire has its own welding head and power source, so the contact tip positions may be manipulated independently (see Figure 4).
As Don Schwemmer, president of AMET, explained, “You may want to have more tilt on the lead arc relative to the trail arc, because you may want specific fill rates for that lead and trail wire, which may have different optimal torch angles. And as more people try to [perform the tandem process] using an AC-AC configuration, they may want to program the wire angles to have more fill on that trailing arc.”
A tandem configuration with both welding heads running AC provides higher deposition. In fact, the penetration capabilities of AC are actually moving a little closer to those of DCEP. It’s a sign, Schwemmer said, of how far SAW has come since the advent of the digital signal processors inside today’s inverter-based welding machines.
AMET’s narrow-gap system uses a Lincoln Electric Power Wave® AC/DC 1000® SD, which can be digitally controlled. The Arc Link digital control on this multiprocess power source allows fabricators to change wave forms between weld passes; for instance, from DCEP for penetration to AC for maximum fill.
Many power sources now offer square-wave AC, which has done a lot toward perfecting the AC arc in SAW, sources said. AC in the familiar sine wave can cause arc inconsistencies, with current sloping down and diminishing to zero as it switches polarities. The square wave, however, minimizes the time it takes to switch from positive to negative. Voltage and current remain consistent before dropping almost instantaneously through zero to a consistent value in the negative polarity. The duration of those square waves also can be tweaked to match the application—a slightly longer duration on the positive end of the square wave, for instance, to help penetration. All this, Schwemmer explained, has made AC more attractive even in some extreme welding applications that in the past couldn’t have been done without the penetration (and relatively low deposition rate) of DCEP.
Narrow-gap SAW is a fully automated process. AMET uses a laser scanning system developed by Meta Vision Systems, which has offices in the U.K. and Canada. Placed ahead of the arcs, the laser measures the joint profile, which can be difficult to detect manually in narrow joints 12 in. deep. The laser communicates joint profile information to a central control, which in turn communicates with the welding power source and the servomotor-controlled contact tips. The process often works with circumferential joints, so servo control is also provided for the turning rolls that rotate the workpiece.
All this helps the system adapt to changing weld conditions. The control knows when the vessel has been turned 360 degrees, so it can instruct the contact tips to change their angle after every pass. At this point, the system knows it has turned the workpiece completely around and instructs the weld heads to adjust their height for the next pass.
The controller also can tell the welding power sources to change polarity or ignite the trailing arc. For instance, the leading wire may run alone in DCEP for maximum penetration during the root pass. Then for the fill passes, the trailing wire may be lit, and the entire operation may switch to AC-AC for good penetration and weld deposition.
As the weld beads are made and the torch height continues to increase, the “effective” weld diameter changes and, hence, the overall weld length. “Since the integrated system knows the height change, it can adjust the workpiece RPM [that is, the turning roll travel rate] to maintain a constant inch-per-minute surface speed [weld deposition rate],” Williams explained.
If the laser detects something unexpected, the system can adjust the travel rate to maintain a more consistent fill. “If, say, the laser sensor detects a higher-than-normal section, it will increase the travel rate during the higher section, then return to the programmed speed. The system also will slow the travel speed when it senses a lower-than-nominal height,” Williams said.
If a weld section is too low, the system slows to deposit more metal; if a section is too high, the system speeds up to deposit less weld metal. Over multiple sections, this evens out those sections so they match the rest of the weld.
“It’s all about removing variation,” Williams explained.
Many narrow-gap SAW applications, current and potential, are about as critical as you can get. After all, what could be more critical than a nuclear reactor vessel? So if welds are so critical, why go through all the trouble speeding the welding process with multiple wires and narrow gaps, particularly when repair costs are so high? Why not stick by the tried-and-true, albeit slow, methods?
According to sources, it’s because the potential cost savings are so high. Besides, all the advancements—from power sources to consumables to joint tracking and real-time monitoring—can make certain applications more robust and actually lower defect rates.
“We’re talking about the largest weld joints here,” Williams said. Such a weld may have a bevel with a wide included angle and could require literally hundreds of passes to complete. What if hundreds of weld passes could be reduced to, say, 60?
This, sources said, is what makes greater weld deposition and narrower grooves so attractive.
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