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| Companies thinking of making a change to the SAW process should consider the benefits it offers, as well as some factors that make it a good fit for a specific application, to help get the most from the investment. (Photo courtesy of Miller Electric Mfg. Co.) |
The submerged arc welding (SAW) process has the potential to substantially improve deposition rates and productivity, and to provide repeatable weld quality. However, it is better suited for some applications than others. When considering SAW, there are numerous factors that can affect process success. Material thickness, along with joint design, fit-up and length all need to be assessed.
Also, be aware that achieving maximum success with SAW requires some homework and an investment in equipment up front — but it can yield a significant and quick return in many cases.
How does SAW work?
SAW is a wire-fed process, like gas metal arc welding (GMAW, or MIG). Wire is fed through a torch that typically moves along the weld joint by mechanization. Understanding and controlling SAW is not significantly different than understanding and controlling GMAW. Setting the machine is similar, and many welding variables remain the same: Voltage still influences bead width, amperage still influences penetration, and increasing wire feed speed still increases amperage and deposition (assuming constant contact-to-work distance and use of a CV power supply).
Unlike GMAW, SAW relies on granular flux to protect the arc from the atmosphere. The arc is buried — submerged — in the flux and is not visible during normal operation. As the arc melts the wire, flux and base material to form the weld pool, the molten flux performs important functions such as deoxidizing, alloying, shaping and generating a protective atmosphere for the weld deposit.
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| Single-wire SAW applications can achieve deposition rates of up to 40 pounds per hour, depending on wire size, type and polarity. (Photo courtesy of Miller Electric Mfg. Co.) |
What can be gained with SAW?
An optimized SAW process can provide gains in throughput, time savings, weld quality and consistency, as well as an improved environment for the operator.
Single-wire SAW applications can achieve deposition rates of up to 18 kilograms per hour, depending on wire size, type and polarity. While achieving this value is not typical for most applications, it can be quite easy to use SAW to improve deposition rates over a current GMAW, flux-cored arc welding (FCAW) or stick metal arc welding (SMAW) process. Welding equipment and filler metal manufacturers can assist in determining starting parameters and provide insight on improvement potential.
In addition to productivity gains, SAW can provide repeatable weld quality. SAW is almost exclusively a mechanized process. The arc and/or work-motion machinery maintains consistent travel speeds and torch positioning, so operators with less hands-on welding experience can easily oversee it. Companies can then allocate their most skilled personnel in the most demanding areas of the operation.
SAW also offers an improved working environment because it has low fume generation and no visible arc. This minimizes UV exposure, so operators do not need to wear a helmet or welding jacket, and it’s easier for other tasks to occur near the welding operation in progress.
Lastly, excellent mechanical properties of the finished weld are another benefit of SAW. Many medium- to high-basicity wire/flux combinations can easily obtain high toughness — even at or below -60 degrees Celsius, which can be difficult even for well-designed, rutile-based FCAW wires. Certain SAW wires and fluxes can also help maintain properties at high-heat inputs, further optimizing potential deposition rates.
What equipment is needed for effective SAW?
SAW can offer substantial productivity gains in certain applications, but achieving those results requires investing in the proper equipment, in addition to the power supply and wire feeder. Therefore, SAW typically has a higher capital investment than other processes.
To help optimize SAW mechanization — and to provide varying levels of flexibility depending on application needs — there are numerous accessories available.
In some applications, the torch is kept stationary and the workpiece is moved using positioning equipment. When arc motion is required, there are several options:
- SAW welding tractors offer portability and flexibility for bringing welding to jobs located throughout the shop or perhaps inside a vessel.
- Side beams or gantry setupsare not portable but instead are a fixed installation, requiring work to be brought to the weld cell. This reduces time spent on setup/changeover, but also reduces flexibility.
- An integrator can help design a custom system, such as girth welding for storage vessels or circular welders for attaching nozzles. Some solutions can be integrated with positioning equipment to weld more complex geometry such as pipe saddles.
Compared to robotic welding, SAW mechanization is much more accessible. It’s typically simpler to implement and become familiar with. Although operator attention is required with SAW, it’s often easier to adjust during welding compared to a robotic welding operation. In addition, SAW equipment is generally designed for ruggedness and reliability.
However, keep in mind that SAW is limited to flat and horizontal position welding, which allows the use of high-current/high-deposition welding parameters. Using SAW for entire weldments with multiple welds may require large positioning equipment; several options include drop-tilt, head and tailstock. Sometimes this positioning equipment can be cost-prohibitive, but in other cases the return on investment can quickly justify it and the SAW process compared to welding out of position with another process.
Also, because operators cannot see the arc’s position during welding, joint tracking equipment may be needed. Options range from simple, such as a laser that indicates the future position of the welding arc, to more complex, such as a tactile probe that can automatically adjust torch position.
Consult with an integrator or equipment manufacturer to determine the combination of equipment to maximize the potential and determine the ROI of an SAW operation.
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| SAW welding tractors offer portability and flexibility for bringing welding to jobs located throughout the shop or perhaps inside a vessel. (Photo courtesy of Miller Electric Mfg. Co.) |
What parts can be effectively welded using SAW?
There are several factors that make a part right for SAW. Material type and thickness are two important considerations.
SAW is best suited for carbon and low-alloy steels, but it can be used for stainless steel and nickel-based alloys as well. And while thick materials are the most common, it is a misconception that SAW can only be used on thick materials.
SAW is used successfully on thin materials in many applications, such as propane tanks and water heaters. Although high amperages are used, the travel speed increases significantly in these cases so that the resulting heat input is low. For example, single-torch SAW can be used to weld 6.5-millimeter material in a single pass at 800 amps with a travel speed of 76.2 centimeters per minute (or more, depending on joint design). Note that welding thinner materials also requires greater attention to the “smoothness” of the mechanization, joint tracking and consistency of joint preparation. Joint backing using copper and/or welding flux is a popular solution for improved repeatability.
Regardless of material thickness, key part considerations for successful SAW implementation include the following:
- Joint and part geometries: SAW is suited to straight-line joints, since parts with jogs in the weld require more complex and expensive mechanization to handle repeatedly. And while SAW is well-suited for high-volume components, that doesn’t mean it can only be used for the exact same part over and over. Even job shops can take advantage of SAW. Parts don’t need to be identical, but they should have similar geometries to maximize the process. For example, it’s common for SAW and its equipment to easily weld both 3.7-meter-diameter and 3-meter-diameter pressure vessels since the geometries are similar. The idea is to find parts that can utilize the same arc/work motion equipment and equipment placement to minimize changeover and, therefore, downtime.
- Long weld joints: A disadvantage of SAW is the required interpass cleaning. For this reason, it’s better suited to long weld joints (often 1.2 meters or longer), which can be cleaned during welding. With shorter welds, the total amount of time spent cleaning is greater because multi-tasking is more difficult, and the ratio of arc-on time versus time spent repositioning/readjusting equipment becomes smaller. As a side note, it is also important to consider investing in flux recovery and reconditioning equipment (a vacuum and oven) to minimize consumable costs.
- Circumferential welds larger than 200 millimeters in diameter: SAW is a popular choice in pressure vessel and pipe applications, because the vessel or pipe can be rotated on positioners. But below this diameter, flux containment becomes more difficult because the flux waterfalls off the pipe. Because the weld cooling rate in SAW is slower than other processes, using it on smaller-diameter pipe can also result in an unacceptable bead profile.
- Parts with good access: SAW equipment is bulky, which make space and part access key considerations. A system may need to be custom-designed for use in smaller spaces, but wire feeding may become an issue. The large diameters simply aren’t as flexible as the small diameters used on a robotic GMAW arm.
Joint design considerations
Good part fit-up is necessary for successful SAW — otherwise there could be a problem with burn-through. These issues must be compensated for prior to the welding process, and may require mechanical fixturing and special attention to part preparation.
“Seal beads” made using GMAW, FCAW or SMAW can be used to help compensate for less-than-ideal fit-up. These quick extra weld passes add time to the operation, but are often less time-consuming than if the entire joint was welded with a process other than SAW.
Potential problems can also be solved by reconsidering the joint. The deep penetration of the SAW process may allow root faces to be increased — or joint preparation to be eliminated entirely.
It may still be necessary to perform multi-pass welding, depending on material thickness and/or mechanical properties desired for the application. This approach can be better than significantly increasing heat to complete a weld in a single pass. Even though high amperages lead to higher deposition rates, SAW is not infinitely tolerant of heat input (a common misconception).
Considering the ROI of SAW
The SAW process can provide significant advantages for productivity and quality in the right application. However, it’s important to have a good understanding of what the process involves — and make sure your specific application is well-suited to SAW — before making the investment.
Integrators and equipment manufacturers can offer help in design and implementation of an optimized SAW process, or advise when SAW may not be the right process. In certain applications, the impact on the bottom line can be significant.
source: http://www.hobartbrothers.com
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