Welding process
Electroslag welding (ESW)
A vertical joint is filled in a single continuous pass as wire melts in a bath of electrically resistive molten slag held between water-cooled shoes, with no arc burning after start-up.
How it works
The joint is set up vertically with a gap between the members and water-cooled copper shoes clamped over both faces to contain the molten metal. An arc is struck initially to melt a starting layer of flux, but once enough molten slag has formed the arc extinguishes and the process becomes non-arc for the rest of the weld.
Current then passes from the wire through the electrically resistive molten slag bath to the workpiece. The resistance of the slag generates the heat directly, keeping the bath molten and melting both the continuously fed wire and the joint faces. Molten metal, being denser, sinks through the slag and collects below it as the weld pool.
The whole assembly moves upward as the joint fills, with the shoes travelling with the pool and the slag bath riding on top of the metal. Because the joint is filled in one continuous operation, extremely thick sections can be welded in a single pass, but the very slow cooling produces a coarse cast grain structure with correspondingly low toughness.
Strengths
- Fills extremely thick joints in one continuous pass, with no interpass cleaning at all
- Very high deposition rate and metal utilisation on heavy vertical seams
- Little joint preparation is needed, since square-edge members with a gap are sufficient
- Distortion is low because heating is uniform and symmetrical about the joint
Limitations
- The coarse, slow-cooled cast structure gives poor notch toughness unless the joint is normalised afterwards
- Restricted to the vertical position, since the pool must be contained by shoes and gravity
- Setup is elaborate and an interrupted weld is very difficult to restart soundly
- Not permitted for many fracture-critical applications because of the toughness concern
Key variables
- Slag bath depth
- Too shallow lets the arc reignite and destabilise the process; too deep reduces penetration into the joint faces and slows filling.
- Voltage
- Sets the resistance heating in the slag bath and therefore how deeply the joint faces are melted at the edges of the pool.
- Wire feed speed
- Controls fill rate and how fast the assembly must travel upward to keep the pool at a constant level under the shoes.
- Oscillation and wire spacing
- On wide joints the wire oscillates or multiple wires are used so heat reaches both joint faces evenly.
- Cooling water flow
- Keeps the shoes intact and shapes the weld face; loss of cooling melts the shoe and floods the joint.
What an inspector watches for
- Confirm the shoes seat tightly against the members, because leakage of molten metal ruins the weld and is a serious burn hazard
- Watch for any interruption in the pass, since a restart leaves an unfused plane across the entire joint thickness
- Verify whether post-weld normalising is required, as the as-welded coarse grain structure is the process's defining weakness
- Check joint gap and alignment carefully before starting, since nothing can be corrected once the weld is running