Double-Sided Submerged Arc Welding for Spiral Pipe
Double-sided submerged arc welding (DSAW) is the process used to produce spiral welded (SSAW) steel pipe. The pipe is formed continuously from steel strip into a helical cylinder, and the spiral seam is welded in two passes - one from the inside and one from the outside - under a blanket of granular flux. The result is a deep-penetration, high-purity weld that gives large-diameter pipe the integrity required for water, gas and structural service.
Why Two Weld Passes?
Internal pass:
welds the root from inside the pipe, reaching the full wall thickness from the bore side.
External pass:
completes and caps the weld from the outside, ensuring full penetration and a smooth external bead.
Benefit:
two-sided welding eliminates the root gap and lack-of-fusion defects that single-side welding leaves in thick walls.
The Submerged Arc Principle
In submerged arc welding, the arc burns beneath a layer of granular flux. The flux shields the molten pool from the atmosphere, preventing oxidation and porosity, and it alloys the weld metal for strength. The weld zone is not visible during welding - hence "submerged" - which allows very high current and deposition rates. This makes DSAW the most productive and consistent welding process for thick-walled large-diameter pipe.
Weld Quality and Process Control
| Parameter | Control Objective |
|---|---|
| Edge preparation | Clean, burr-free strip edges for defect-free fusion |
| Forming angle | Consistent helix angle for uniform weld geometry |
| Flux condition | Dried, recycled flux for stable arc and low hydrogen |
| Weld offset | Internal and external passes aligned to the seam centre |
Inspection and Testing
100% weld inspection:
automated ultrasonic testing (UT) or radiographic testing (RT) of the full spiral seam.
Hydrostatic testing:
every pipe is pressure tested to verify leak-tightness.
Mechanical verification:
tensile, bend and (for line pipe) Charpy impact tests on the weld and heat-affected zone.
Documentation:
results recorded on the mill test certificate for full traceability.
Frequently Asked Questions (FAQ)
Why is DSAW used for spiral pipe instead of ERW?
DSAW delivers the deep penetration and high deposition needed for thick walls and large diameters, while ERW is limited to thinner walls and smaller diameters.
What does the flux do?
It shields the molten weld pool from air (preventing porosity and oxidation), stabilizes the arc, and adds alloying elements to the weld metal.
Is the internal weld as important as the external?
Yes - the internal root pass ensures full penetration; without it, the weld would have a root gap prone to failure.
How is weld quality verified?
With 100% ultrasonic or radiographic inspection of the seam, hydrostatic testing, and mechanical tests on weld coupons.
Can DSAW spiral pipe be used for high-pressure gas?
Yes, when made to API 5L PSL2 with impact testing and full NDT; the spiral seam is a proven design for gas mains.
Does the spiral seam weaken the pipe?
No - the helical seam distributes stresses and, with proper welding and inspection, the pipe meets the same design factors as straight-seam pipe.
