Submerged arc welding (SAW) is a common welding process used widely in structural and pressure vessel construction. It uses a blanket of granular flux to keep the arc and the molten weld metal away from the atmosphere. That blanket does four things at once:
- Protects the weld from environmental contamination — oxidation, reduction, and the pick-up of other contaminants
- Stabilises the arc during welding
- Prevents spatter and sparks from flying
- Suppresses the radiation and fumes typical of shielded metal arc welding (SMAW)
In submerged arc welding the arc and the weld zone are buried under that layer of flux. The process needs a continuously fed consumable electrode — solid wire or tubular — and can be fully automatic or semi-automatic. The arc is flat and is maintained between the end of a bare wire electrode and the work. As the electrode is fed in and melted, the granular flux provides a protective cover beneath which welding takes place. Part of the flux becomes molten and forms the blanket; this fusible flux may contain lime, silica, manganese oxide, calcium fluoride and other compounds. In the molten state the flux becomes conductive, which lets it carry a constant current between the electrode and the work. The rest of the flux is recovered and reused, unless it has been contaminated.
SAW is preferred over other methods for its inherent qualities: process variables are easy to control, quality is high, penetration is deep, the finish is smooth, thick sections can be welded, and the weld pool is shielded from the atmosphere. The flux earns its place several times over during a weld — it enhances the arc, produces shielding gas, protects the weld from the atmosphere, assists bead shape, deoxidises the puddle, and in some cases adds alloy to the weld metal.
How SAW flux is manufactured
Flux is made in one of three ways, and the manufacturing route decides how it behaves.
Fused flux
All the raw materials are combined, melted down in a high-temperature furnace, cooled to a solid, then ground to small particles. The particle size has to fall within a specified range for the flux to perform consistently.
A fused flux is very stable at high welding currents, non-hygroscopic so it will not absorb moisture, and chemically homogeneous, which produces more consistent welds. It also has high grain strength, so it does not break down as easily during flux recycling.
Agglomerated or bonded flux
The raw materials are combined into a dry mixture, bonded with a liquid binder such as potassium silicate or sodium silicate, baked at low temperature and sieved to a specified particle size.
Bonded fluxes may contain metallic deoxidisers, which makes them a good option for welding over rust and mill scale, and they may carry added alloys to improve weld metal properties. They prevent porosity better than fused flux and they peel cleanly. The trade-off is moisture: bonded flux is more likely to absorb it from the environment, so it has to be dried in an oven again before use.
Mechanically mixed flux
Simply a combination of ingredients dry-mixed with no binder. The ingredients may include one or more fused or bonded fluxes.
Active, neutral and alloying flux
The effect a flux has on the alloy content of the weld is described by three terms.
Active flux. The manganese and silicon levels in the deposit vary significantly with a change in welding voltage. Active fluxes typically carry additional Mn and Si, which helps deoxidise the puddle, clean impurities and improve wetting. They beat neutral fluxes for welding over rust and mill scale and can take higher travel speeds. They are used where there are minimal or no impact requirements, and are generally limited to single-pass welds — they are not used on heavy-walled pressure vessels, because the extra manganese paired with high voltage and increased flux consumption raises the risk of weld cracking.
Neutral flux. As the name suggests, it will not alter the weld metal chemistry, so it is permitted in all welding applications and for an unlimited number of passes. It gives good mechanical properties and good cold-weather impact values. It does not perform well over heavy rust and mill scale, though: base metal impurities increase the odds of poor bead appearance, inconsistent weld toes and porosity, so pre-weld joint preparation may be necessary.
Alloying flux. Contributes elements other than Mn and Si to the deposit — chromium, molybdenum and nickel among them.
Choosing between the three is a question of what the joint has to survive, not of which flux is best in the abstract.