Welding process
Submerged arc welding (SAW)
A continuously fed bare wire burns under a blanket of granular flux, so the arc is completely buried and enormous currents can be used without arc flash, spatter, or fume escaping.
How it works
Granular flux is laid ahead of the joint and a bare wire is fed down into it. The arc strikes beneath the flux blanket and burns inside a cavity of vaporised flux, entirely hidden from view. Because the arc is confined and insulated by the surrounding flux, radiation and spatter cannot escape and very high currents become practical.
The flux melts alongside the wire, forming a molten slag that floats on the pool, protects it from air, and refines its chemistry. Some flux formulations also transfer alloying elements into the deposit. Unmelted flux is recovered and reused, and the fused slag crust lifts off the finished bead, often peeling on its own as the weld contracts.
The heat that would normally radiate away is trapped in the joint, so thermal efficiency is high and deposition rates are very large. The same trapped heat gives a wide heat-affected zone and slow cooling, which improves toughness in many steels but makes the process unsuitable for thin material that cannot absorb the heat.
Strengths
- Very high deposition rates and deep penetration, allowing thick joints in few passes
- No visible arc, no arc flash, and very little fume or spatter escaping the flux blanket
- Excellent, repeatable bead consistency because the process is mechanised rather than hand-held
- Flux can be recovered and reused, and slag usually detaches with little effort
Limitations
- Restricted to the flat and horizontal positions, because the flux blanket and pool must be supported
- The arc cannot be seen, so the operator has no visual feedback and relies entirely on settings and tracking
- High heat input widens the heat-affected zone and can reduce toughness in sensitive steels
- Equipment is bulky and needs a manipulator, positioner, or track, so it is a shop process rather than a field one
Key variables
- Current
- The dominant control on penetration and deposition. Excessive current on a thin section burns through or produces an unacceptably wide heat-affected zone.
- Voltage
- Sets arc length and bead width, and controls how much flux is melted, which in turn affects the deposit's alloy pickup from the flux.
- Travel speed
- Balances against current to set heat input. Too fast gives a narrow, crack-prone bead; too slow gives a wide, shallow, low-toughness deposit.
- Flux depth
- Must be deep enough to bury the arc completely yet shallow enough for gas to escape and for the bead to form smoothly.
- Wire and flux combination
- The two act as a system, and the deposit's chemistry and toughness depend on the pairing rather than on the wire alone.
What an inspector watches for
- Verify flux type, condition, and dryness, since damp flux is a direct route to porosity in a joint nobody can watch being welded
- Check that flux depth is adequate but not excessive, because too shallow a blanket lets the arc break out and too deep a layer gives a ropey bead
- Confirm wire tracking and joint alignment, as the operator cannot see the arc and a drifting seam produces sidewall fusion problems
- Watch heat input against the procedure limits, because the process reaches very high currents easily and can overheat the heat-affected zone