Advances in SAW attack structural welding challenges
May 10, 2011
SAW offers a cost-effective and efficient means for automated welding of plates, beams, flanges, and profiles found in structrual steel environments. Technology advancements in flux combinations and SAW equipment have helped boost arc-on times, which have elevated productivity levels within the process.
Union Carbide introduced the submerged arc welding (SAW) process to the welding industry in 1936. Originally developed to weld longitudinal seams on pipe, SAW offered numerous advantages that were quickly recognized by other industries.
Some of these advantages of the SAW process are improved weld quality, high deposition rates, deep penetration, relatively fast welding on thin steel sheets, and the low welding fumes. In the 75 years since its introduction, SAW has undergone many advances to make it one of the most productive methods available today for high-volume welding.
SAW works by creating an arc between an electrode and the work surface, using a solid or tubular wire under a granular flux covering. The heat from the arc melts the electrode, the work surface, and the flux, creating the weld puddle. The flux contributes to the mechanical properties of the weld, deoxidizes the base metal, and protects the molten weld metal from atmospheric contaminants. When the weld is complete, it is covered by an easily removed layer of slag. While it is possible to perform this process with a hand-held torch, most SAW today is done with some form of automation.
SAW can be done with either direct-current (DC) or alternating-current (AC) power supplies. DC is more common because it is easiest to control and provides the best arc starting and stability. DC electrode positive (DCEP) is used most often and yields the deepest penetration. DC electrode negative (DCEN) produces up to 25 percent less penetration but offers the highest deposition rates. It is useful in overlay applications or applications with poor joint fit-up.
Amperage, voltage, travel speed, wire size, and electrical stick-out all play a role in the shape, size, and quality of the weld deposit. Amperage is directly related to deposition rate and depth of penetration, so an increase in amperage will increase both variables. Arc voltage is a measure of arc length and has an inverse relationship to penetration depth and a direct relationship to bead width. Travel speed or feed rate is inversely related to bead size and penetration.
Wire size affects deposition rate and penetration. A small-diameter wire has a smaller cross-sectional area, so it will provide a faster burnoff and, consequently, a higher deposition rate and deeper penetration compared with a larger-diameter wire at the same current. As a rule of thumb, electrode stick-out should be about eight times the wire diameter. For example, the stick-out for a 1⁄8-in.-dia. wire should be approximately 1 in. Using a longer stick-out will result in reduced penetration.
The challenge in today's marketplace is to maximize the productivity of the process by increasing the operator factor, also known as arc-on time or cycle time and making the process more efficient to use. Following are a few of the ways SAW equipment manufacturers are meeting these challenges.
Improving Electrode, Flux Combinations. Subarc fluxes generally are categorized as neutral or active. Some fluxes add silicon and manganese alloys to the weld metal, while others burn off these elements. The intensity of this chemical reaction depends on the amount of flux interacting with the wire. An increase in voltage or arc length will speed up the alloying or burnoff of these elements.
Active fluxes add a significant amount of manganese and silicon, which acts as a deoxidizer. This enhances resistance to porosity and improves bead appearance and toughness in high-dilution applications. Active fluxes are used primarily for single-pass or multilayer welding, with three to five layers normally the maximum. Neutral fluxes are used in multilayer welding of unlimited plate thickness. The alloying of elements, especially silicon and manganese, is carefully controlled by the choice of wire and flux.
You can raise productivity levels by increasing your travel speed or decreasing the number of passes necessary to complete a weld. Keep in mind, however, that higher travel speeds require more amperage. Reducing the number of passes often results in more heat input per weld pass. Both of these variables place significant demands on the flux.
If the process exceeds the heat and current capacity of the flux, the weld bead will become ropey or riddled with slag inclusions. Gouging and grinding to repair these problems adds significantly to production time and slows throughput. Also, welding at high-heat input tends to degrade the mechanical properties of the weld metal and the heat-affected zone (HAZ). If you choose to weld at a high-heat input, be sure that the mechanical properties of your flux exceed those of the base metal HAZ.
Using Multiple Wires. The simplest form of SAW uses a single wire and a DC power source. A new trend in recent years is twin-wire, or parallel-wire, SAW, which uses two relatively small-diameter wires with a single power source and feeder. Using two wires yields deposition rates that are 20 percent or more higher than single-wire DC SAW without significantly increasing heat input. The higher deposition results from the greater current density that is achieved by pushing a similar current over a smaller cross-sectional area of wire. Other benefits of twin-wire welding are excellent penetration and fast travel speed.
DCEN can provide a 20 to 30 percent deposition rate increase, but the penetration depth is so low that you actually risk a lack of fusion. Welding with AC provides a middle ground with an increased deposition rate over positive-polarity welding yet a greater penetration than negative-polarity welding. Square-wave AC output provides a more stable arc than conventional sine-wave AC welding because the current switches much more rapidly from peak-positive current to peak-negative current, with almost no time near zero voltage.
Another option is tandem welding—a process that uses two wires, each with its own power source and wire feeder. The lead arc is typically DCEP and the trail arc is AC. Both wires feed into the same molten puddle, allowing for deposition rates that are more than double what is achievable with single-wire SAW.
The most recent advancement combines twin and tandem welding. Known as tandem-twin SAW, this process uses two sets of twin wires, a combination of DCEP/AC or AC/AC, to achieve the highest possible deposition rate. The result can amount to a productivity gain of up to 40 percent compared with standard twin-wire welding.
Advancing Material Handling Technology. A majority of production time typically is lost to material handling. In other words, the more you need to turn or position the material before welding, the lower your operator factor will be. To address this problem, welding equipment manufacturers have introduced specific equipment for various applications.
For the structural steel industry in particular, machines are now available to create I-, T-, or L-beam configurations. The beam configurations may be wide flange, tapered, or nonsymmetrical. The beam machines can be placed individually or in assembly-line configuration (see Figure 1). The beams and resultant profiles can be welded with the web in the vertical position or with the web placed in a horizontal position.
One of the main advantages besides high production capacity is that the welding operation takes place when the flange and the web have been pressed together under pressure to completely eliminate the gap between the surfaces. This ensures the highest weld quality. These machines also can weld both sides of a beam simultaneously. When I-beams are manufactured, the web plate is welded to the first flange, the welded T is turned 180 degrees, and the second flange is welded at the next pass. Gas or induction heating equipment may be used to achieve straight beams. This is essential when welding T-profiles.
For flat plates, new equipment is available to weld both sides of the plate in a single pass. Using a tandem-wire configuration with a lead DC electrode followed by either an AC solid or tubular electrode, this method produces single-pass, full-penetration weld bead profiles at welding speeds up to 47 IPM.