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Structural Steel Fabricator Turns to Sub Arc Technology to Boost Productivity
"No matter how hard you try to improve Sub Arc welding productivity, you're stuck in a rut if you only have one way to do it," states Jerry Brown, maintenance supervisor at St. George Steel Fabrication, Inc. "Now that we have Sub Arc power sources with a variable balance AC squarewave technology, we have multiple choices. As a result we can be more productive."
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Fig. 1 - Bead cross section. |
More productive is an understatement. By using the variable balance AC Sub Arc technology found exclusively in the Summit Arc 1000(tm) power source from Miller Electric Mfg. Co., St. George Steel reduced welding time by 60 percent and improved quality at the same time. They are no longer concerned about excess heat input and the warping and loss of ductility that can follow. Full penetration is achieved, and every weld has passed x-ray inspection without repair.
Heavy Weldments, Fast Delivery
St. George Steel specializes in large, heavy weldments. This includes wind towers, pressure vessels, dryers, stacks, petroleum and water tanks.
"We have been in business for more than 50 years, and we are an ASME code facility with U and R stamps. We supply our customers with quality products, competitively priced and in a timely manner," said J. Kay Flygare, general manager of St. George Steel's "north facility" in Murray.
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Fig. 2 - Balance Control |
The company's large-scale projects require plenty of square footage. In addition to the Murray plant near Salt Lake City, which employs about 75, the company's "south facility" in St. George (near the Arizona border), is more than twice the size and has a workforce of 150. With its fleet of flatbed tractor-trailers, it's not uncommon to see St. George Steel trucks criss-crossing the country with over-sized loads weighing 60,000 lbs. or more.
"We have a reputation for quality work on large projects, so when we learn about a method that will upgrade our product and make St. George Steel more productive for our customers - and more competitive in this industry - we're willing to listen," said Flygare.
Brown adds that, "It's our job to provide the foremen in the shop the opportunity to succeed. We find equipment that's going to give them the highest output, the best quality consistency and that's user-friendly."
Eliminating Compromise
Prior to using variable balance AC Sub Arc technology, St. George Steel always welded with DC electrode positive (DC EP) or DC electrode negative (DC EN). For single wire applications, this "conventional technology" used to be the only way to weld.
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Fig. 3 - Inside Flange Weld |
DC-only Sub Arc limits a fabricator's ability. They can emphasize just one of three major variables: high deposition rates, ideal penetration profiles or lower heat input.
DCEP, where the electricity flows from the electrode to the plate, preheats the plate more than the electrode. This provides deeper penetration and good wetting at the toes of the weld. However, it has a lower deposition rate and higher heat input. This can be an unfavorable combination for making long welds at higher speeds, for welding plate sensitive to warping ("oil canning") and welding heat treatable materials (e.g., 4130, 4140). In the latter, excess heat may cause base metal dilution and subsequent loss of alloy properties in the weld area, which makes the weld susceptible to cracking.
DCEN can reduce base metal dilution and warping problems. However, because the heat flows from the plate to the electrode, the weld bead doesn't penetrate as deeply. DC EN does offer higher deposition rates, but it does not run well at lower amperages without sacrificing productivity.
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Fig. 4 - Four Shells |
Seeing the Browns' frustration and process limitations, Mike Weaver, account manager at welding supply representative Airgas-Intermountain, Inc., introduced them to Miller's Summit Arc 1000. This "all process" Sub Arc power source has an AC/DC output, a 100 percent duty cycle rating of 1000 amps/44 volts and uses three-phase primary power for energy efficiency. Most importantly, it provides St. George Steel with a new problem-solving tool.
"Variable balance AC squarewave solves common DC Sub Arc problems because fabricators like St. George Steel can tailor how much heat they want directed into the plate and into the electrode by varying the balance control," says Weaver.
This allows emphasis on two of three primary attributes, not just one. Fabricators can achieve good penetration with increased deposition, good penetration with lower heat input, or increased deposition with lower heat input. "The most promising balance combinations are 66 to 40 percent EP/34 to 60 percent EN," says Weaver. "Airgas and Miller worked with St. George Steel to determine the best variables for their application."
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Fig. 5 - Outside weld bead |
12-ft. Shells
The opportunity for St. George Steel to add four Summit Arc 1000 power sources came when a large project required increasing output. The project involved transforming 10-by-40 ft. sheets of 5/8-in. carbon steel into shells 12 ft. in diameter. Flat plate is cut to size on an X-Y plasma table, and then rolled on a pyramid roller to form a cylinder. The longitudinal seam is "sealed" with MIG welding so the shell can be managed. The shell goes to a station and the outside of the seam is Sub Arc welded from the top of the shell. The shell is rotated 180 degrees and the seam is welded on the inside for 100 percent penetration.
Currently, this Sub Arc welding is performed with a DC process. However, Brown and shop foreman Art Lopez plan to transition to the variable balance AC squarewave process because of increased travel speeds, which they learned about in the next stage of fabrication.
This stage involves welding a 2-1/2-in. flange to the bottom shell and smaller flanges on three additional shells. When joined, the 10-ft tall shell sections together to form a continuous 40-ft. long section.
Those welds must be made along the inside and outside of each cylinder around the circumference. To accomplish this work, St. George rigged a system in which the cylinders rest in a tube frame bed, with cam-follower rollers to roll the cylinders. This system maintains a consistent angle and provides repeatability between cylinders.
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Fig. 6 - Assembly |
So that one Summit Arc 1000 power source can weld both the inside and outside of the shell, Jerry Brown arranged a splitter that lets the welding output be shifted between two Miller HDC 1500 control systems, RAD wire drive assemblies, flux hoppers and OBT 1200 torch. One assembly is positioned at the top of a platform to weld the outside passes (the first and last pass) and one is positioned on the shop floor to weld the inside pass (see Fig. 6). Both assemblies swing, move in and out and up and down to allow precise positioning
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"The rollers, jig and wire drive assembly mounts provide consistency," says Brown. "We have a welding system that provides consistency, so I have to build something around it to maintain consistency. Also, this set-up eliminates downtime between inside and outside passes. We swing one mount over, drop it in place and go into weld production."
60 Percent Less Weld Time
Welding the flange to the shell requires welding on the inside and outside of the cylinder (after beveling the shell plate and preheating both parts to 200 F). With DC EP Sub Arc, this required five passes with an average travel speed of 15 inches per minute (IPM) using a 5/32 in. AWS EM12K wire. Using the same electrode and a variable balance AC squarewave process set at 66 percent EN/34 percent EP, the Summit Arc 1000 achieves an average travel speed of 22 IPM-and completes the joint in just three passes.
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Fig. 7 - Shell view |
Traveling around a cylinder with a 12-ft. diameter at 15 IPM takes 60 minutes. At a minimum, welding the shell took 300 minutes with the old DC EP process. However, to keep interpass temperatures from getting too high, the shell needs time to cool, so actual welding time is longer. Conversely, the variable balance AC squarewave process takes 123 minutes, and, because it lowers heat input and requires fewer passes, waiting between passes is not necessary. It uses less flux.
"Excess heat causes all kinds of problems, such as parts pulling when they cool," says Brown. "We have a tolerance of 1/4 in. on the shells. That sounds like a lot, but if you tried to weld a shell with the DC process without waiting for it to cool between passes, it would pull the flange so far out of tolerance that it would not be 90 degrees anymore and we'd never get it true again."
Tip of the Iceberg
Due to the time pressures of the shell project, the Browns and Lopez have not had the luxury to fully explore the capabilities of their new system. However, initial lab tests are very promising. For example, they can butt two 5/8-in. pieces of steel flush together, achieve 100 percent penetration with one pass on each side and maintain a weld face just 3/4-in. wide.
"This eliminates the time it would ordinarily take to bevel such a joint and prevents having a humongous weld face, which wastes filler metal and adds excess heat," says Brown. "We are now working with Miller experts, and we hope to achieve 100 percent penetration with just a single pass."
Another technology upgrade is the addition of Miller's new digital, programmable HDC controllers. This will allow pre-setting and locking parameters for 12 Sub Arc welding programs.
"Our customers look at us as the experts in steel fabrication. They want quality welds that are consistent throughout the job. We can pre-program the job's parameters, lock them in, and we know they will be consistent throughout the job. That is a relief for the customer. Coupled with the significant time savings, AC Sub Arc makes us more productive and greatly enhances our competitiveness."
