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Manufacturing Processes > Joining processes > Submerged Arc Welding(SAW)

 

Submerged Arc Welding(SAW)

 

Process description

A blanket of flux is fed from a hopper in advance of an electric arc created between a consumable electrode wire and the workpiece at the joint line. The arc melts the parent metal and the wire creates the weld as it is automatically fed downwards and traversed along the weld, or the work is moved under welding head. The flux shields the weld pool from the atmosphere preventing oxidation. Any flux that is not used is recycled.

Materials

  • Carbon, low alloy and stainless steels, and some nickel alloys.
  • Dissimilar metals are difficult to weld.
SWA

Process variations

  • Self-contained, mainly automated a.c. or d.c. systems with up to three welding heads.
  • Can have portable traversing welding unit using a wheeled buggy (for long welds on ship’s deck plates for example), self-propelled traversing unit on a gantry or moving head type (for shorter weld lengths) and fixed head where the work rotates under the welding unit (for pressure vessels).
  • Copper-coated electrode wire can be solid or tubular. Tubular is used to supply the weld with additional alloying elements. Wire sizes range from Ø0.8 to Ø9.5 mm.
  • Can use a strip electrode for surfacing to improve corrosion resistance (pressure vessels) or for hardfacing parts subject to wear (bulk materials handling chute).
  • Fluxes available in powdered or granulated form, either neutral or basic. Neutral fluxes used for low carbon steel and basic fluxes for higher carbon steels.
  • Bulk welding: uses an iron powder placed in the joint gap in advance of the flux and electrode to increase deposition rates.
  • For thin sections can use a flux-coated electrode wire.

Economic considerations

  • Highest weld deposition rate of all arc welding processes.
  • Speeds ranging 0.1 to 5 m/min.
  • Economic for straight, continuous welds on thick plate using single or multiple runs.
  • High power consumption offset by high productivity.
  • Economical for low production runs. Can be used for one-offs.
  • Tooling costs low to moderate. Need for jigs and fixtures important for accurate joint alignment.
  • Equipment costs moderate to high.
  • Direct labor costs low to moderate. Skill level required low to moderate.
  • Flux handling costs can be high.
  • Finishing costs moderate to high. Slag produced at the weld area needs to be removed.

Typical applications

  • Ships
  • Bridges
  • Pressure vessels
  • Structural steelwork
  • Pipework

Design aspects

  • Design complexity limited.
  • Typical joint designs possible using SAW: butt and fillet in heavier sections (see Appendix B – Weld Joint Configurations).
  • Suitable for horizontal welding, but can perform vertical welding with special copper side plates to retain flux and mold the weld pool.
  • Welds should be designed with straight runs.
  • Minimum sheet thickness =5mm (6mm for nickel alloys).
  • Maximum sheet thickness, commonly:
    • Carbon, low alloy and stainless steels =300mm
    • Nickel alloys =20 mm.
  • Multiple weld runs required on sheet thicknesses ≥40 mm.
  • Unequal thicknesses very difficult.

Quality issues

  • High quality welds can be produced with low levels of distortion due to fast welding rates.
  • Good weld uniformity and properties, although on large deposit welds a coarse grain structure is formed giving inferior weld toughness.
  • Access for weld inspection important, e.g. NDT.
  • Large weld beads can cause cracking. Weld penetration can be controlled by using a backing strip when using high currents.
  • Joint edge and surface preparation important. Contaminates must be removed from the weld area to avoid porosity and inclusions on each pass.
  • A heat affected zone always present. Some stress relieving may be required for restoration of materials original physical properties.
  • Can alter composition of weld by addition of alloying elements in the electrode.
  • Flux must be clean and free from moisture to prevent weld contamination.
  • Weld ideally left to cool to room temperature to allow the slag to peel off.
  • Welding variables automatically controlled. Monitoring of welding voltage is used to control arc length through varying the wire feed rate, and thereby improving weld quality.
  • Pre-heating of workpiece can reduce porosity and hydrogen cracking, especially on high carbon steels.
  • ‘Weldability’ of the material important and combines many of the basic properties that govern the ease with which a material can be welded and the quality of the finished weld, i.e. porosity and cracking. Material composition (alloying elements, grain structure and impurities) and physical properties (thermal conductivity, specific heat and thermal expansion) are some important attributes which determine weldability.
  • Surface finish of weld good.
  • Fabrication tolerances typically ±2 mm.

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