Enhance the benefits of SAW
Based on its ability to weld at high currents and provide high deposition rates, submerged arc welding (SAW) can offer companies greater productivity — and potentially a competitive edge. Even performing SAW using a single solid-wire in direct electrode positive (DCEP) mode, the simplest configuration, can afford marked productivity improvements over many semi-automatic welding processes.
Another strength of the SAW process is its flexibility. Companies can balance their production needs with the desired mechanical properties and weld quality by altering procedures, equipment, wires and fluxes — solutions exist for a wide range of capital investment.
There are several methods to increase welding productivity. Often, these gains come from:
- Reduced arc-on time: Increasing travel speeds, reducing the number of passes or a combination of both makes it is easier to complete more welds in less time. Increased deposition rates provide the same advantage.
- Reduction or elimination of arc-off activities: Addressing the causes of distortion can lessen the need for weldment pre-cambering or post-weld straightening. Reducing the risk of burn-through minimizes time spent on rework. Minimizing time spent to remove light scale/rust before welding and chipping slag between weld passes is also ideal.
Reconsidering the existing weld joint, equipment setup, and filler metal and flux selection can help to reduce welding cycle time and eliminate arc-off activities to further enhance the benefits of the SAW process.
Joint design directly impacts the number of weld passes that are required in a SAW application and, therefore, welding duration. Reducing the cross section of the joint typically reduces the required arc-on time.
In semi-automatic welding, it is often necessary to use wider joints to ensure complete joint fusion. This is often not necessary for the SAW process; it is capable of greater penetration and better side wall fusion. Some companies may opt for narrow groove welding for SAW. However, even small reductions in the included angle of the joint can offer sizable productivity advantages.
In applications where it is possible to access both sides of the joint (particularly on thick materials), a double-sided groove weld provides a smaller total cross section than a single sided joint with the same included angle. The deep joint penetration of the SAW process often makes it possible to eliminate back gouging while still gaining proper fusion. Some welding codes may require requalification of the welding procedure to exclude back gouging; however, the savings can make it worthwhile. Double-sided welding can also balance some of the shrinkage stresses encountered during welding to help minimize overall distortion
Lastly, the use of flux in the submerged arc projects can be exploited when welding thin materials from one side using a square groove joint. Using a copper backing packed with welding flux or specially-designed backing flux can provide complete joint penetration and a consistent, attractive backside bead profile, even when using high amperages and travel speeds.
Like other wire-fed processes, DCEP polarity during the SAW process provides optimal penetration, while direct current electrode negative (DCEN) increases deposition rates at the expense of penetration. Employing a modern variable-balance square-wave AC (alternating current) power source allows for adjustments between these two extremes (and via some equipment, without interrupting welding). Instead of the 50/50 split of conventional AC, these power sources can be adjusted so that varying amounts of the AC cycle can be spent in DCEP or DCEN — for example, 80 percent DCEP and 20 percent DCEN — and still maintain arc stability due to the quick polarity changeover of square-wave AC. This capability is particularly advantageous for companies welding both thick and thin materials, since it offers the flexibility to balance both penetration and deposition rate needs.
However, the use of variable balance square wave AC needs to be combined with the right joint design. For instance, a thick square groove weld, which requires deep penetration but minimal deposition, may not be able to take advantage of AC.
Twin-wire torch configurations utilize a single power source and a single specialized torch that simultaneously feed two wires into the same weld pool. This configuration requires a relatively low capital investment for the specialized torch, wire-straightener and contact tips. The higher current density of smaller wires used here (up to 3/32 inch, typically) often increases deposition over a single wire at matching current, helping to keep heat input and the resulting welds consistent.
Conversely, tandem torch configurations demand a higher capital investment because they require two power sources, at least one of which must offer AC capabilities. Compared to twin-wire torch configurations, the ability to use even higher currents (from larger wires) improves deposition rates and provides faster travel speeds, while the ability to adjust each wire’s angle, current and voltage offers greater overall flexibility. While using a DC lead arc/AC trail arc is the most common configuration, the use of an AC/AC configuration improves deposition when the penetration of the DC lead arc is not necessary.
For both options, it is important that the arc and/or work motion systems can handle an increase in travel speeds in order to gain the full productivity benefits. Also be aware that incorporating additional wires introduces complexity into the application that must be controlled.
Flux is the defining component of the SAW process and choosing the proper one is critical. Flux accomplishes many more tasks than simply shielding the weld from the atmosphere; in many cases, improved productivity is a primary goal during formulation.
Flux formulation influences the fluxes’ current-carrying capacity — the maximum current where high-quality weld profiles and the highest possible deposition rates can be obtained. Flux formulation also impacts slag release in that some are better suited for a given joint design (e.g. narrow groove welding). For example, fluxes that release well at low travel speeds (e.g. 16 inches per minute) may not release well at high travel speeds.
Fluxes for the SAW process are available in active and neutral types. Unlike neutral fluxes, active fluxes contribute significantly to the manganese and silicon composition of the weld. These elements help to clean the weld, and can help maintain tensile strength when welding at higher input heats or high-dilution. Active fluxes help the welds wet out smoothly and provide good slag release when welding at higher travel speeds or when the base metal is rusty, scaly, or dirty, reducing the risk of poor quality welds and time consuming pre- and post-weld cleaning. However, active fluxes should typically only be used for single or two-run welding, as excessive alloying that would occur in large multi-pass welds can contribute to brittle, crack-sensitive welds. Instead, neutral fluxes are used in these applications to obtain good weld quality.
As a rule of thumb for neutral fluxes: select a flux with the lowest basicity index that provides acceptable mechanical properties. Low-basicity fluxes tend to improve overall operating characteristics, especially when using high-productivity (hot and fast) parameters. High-basicity fluxes tend to offer improved toughness under most conditions, but not always at high heat inputs. Consult with a flux manufacturer to help make a suitable choice for a given application.
The SAW wire market is quite diverse. Each alloy/classification has strengths and weaknesses. As with flux selection, selecting the most productive wire can be a balancing act.
It is important to understand the impact that wire chemistry and heat input has on the mechanical properties of the weld in order to gain the best quality, avoid downtime for rework and achieve optimal productivity. Compared to other processes, SAW can generally operate and maintain the mechanical properties of the weld at higher heat inputs, but there are limits. The alloy content of some wires make them better suited to high-heat welding. Likewise, the manganese and silicon content of the wire can assist the flux with weld cleaning when the base metal preparation is less than ideal. Always know the proper welding parameters for the wire and flux being used.
Metal-cored wires are an option to improve productivity. The design of these wires can provide higher deposition rates and faster travels speeds, along with a wider, shallower penetration compared to solid wire welded at the same current.
These wires’ ability to achieve high travel speeds can be used to reduce heat input and minimizing weld distortion. The lower heat input can also, in some instances, improve mechanical properties. Likewise, these wires can help minimize the risk of burn-through and subsequent rework, and can sometimes eliminate the need to MIG weld a hot pass. In many cases, metal-cored wires can weld on relatively thin materials or joints with less than ideal fit-up while still using high-current, productive welding parameters.
Note, metal-cored wires may not be suitable for thick square or narrow groove joints, nor do they provide maximum benefits using AC.
Lastly, utilizing drum packaging (typically 600-plus pounds) instead of coils (e.g. 55 to 60 pounds) can reduce changeover frequency, and overall time and allow for greater production in the long term.
A more effective SAW operation can also yield greater cost savings. Labor is the largest cost of any welding operation. Even a small reduction in labor during the production of a quality weld can positively affect the bottom line, as well as productivity. When making productivity enhancements in the SAW process, however, it is important that the rest of the operation is equally efficient to achieve the greatest results. The manufacturing operations before and after the weld cell need to be streamlined — smooth workflow is key. For example, if the SAW process becomes faster and the paint booth is already over capacity, then improvement efforts may be in vain. Plan accordingly.