Submerged Arc Welding (SAW)

Submerged Arc Welding (SAW) is a high‑productivity arc welding process in which the arc, molten weld pool, and filler wire are “submerged” under a blanket of granular flux. The flux shields the arc from the atmosphere, stabilizes the process, and forms slag that protects the solidifying weld metal. SAW delivers deep penetration, high deposition rates, and consistent quality for long, straight welds on medium to thick sections.

Overview

SAW employs a continuously fed bare wire (or strip) electrode powered by DC or AC, while flux is delivered ahead of the arc. The arc melts the wire and base metal; flux decomposes to provide shielding gases and forms a slag cover. Because the arc is hidden beneath the flux, spatter and fume are reduced, visibility is limited, and process monitoring relies on parameters and bead appearance post‑weld. SAW is commonly automated or mechanized for long seams in plate, pipe, and structural components.

Apparatus and Working

Apparatus

  • Power source: AC/DC constant voltage or constant current machines; single or multi‑arc systems.
  • Wire feeder and torch: Continuous feeding of bare wire (1.6–6 mm) and current transfer to the arc.
  • Flux delivery system: Hoppers/nozzles to dispense granular flux ahead of the arc; flux recovery unit.
  • Travel carriage: Automated tractor or gantry with speed control for straight, consistent motion.
  • Consumables: SAW wire (e.g., EM12K, EA2, stainless or low alloy) and compatible flux (neutral/active).
  • Ancillary tools: Seam tracking, guides, preheat equipment, and slag removal tools.
  • PPE and safety: Eye/face protection, gloves, ventilation for fumes, and handling for hot slag.

Working steps

  1. Joint preparation: Bevel/fit‑up per WPS; clean mill scale, rust, coatings; set root gap/land.
  2. Parameter setup: Select wire diameter, polarity (DC+/DC‑/AC), current, voltage, and travel speed.
  3. Flux application: Lay flux ahead of torch; ensure uniform coverage and correct layer thickness.
  4. Arc initiation: Start wire feed and arc under flux; verify steady burn and stable bead formation.
  5. Welding: Maintain constant travel speed and wire stick‑out; monitor amperage/voltage for stability.
  6. Slag management: Allow solidification; remove slag between passes; reclaim and sieve flux if reused.
  7. Inspection: Clean and visually assess bead profile; perform NDT as required by code/service.

Principle

SAW uses the heat of an electric arc between a continuously fed wire and the workpiece to melt and join metals. Granular flux covers the arc, decomposing to produce protective gases and forming a slag that shapes and shields the weld. Polarity and current density control penetration and bead geometry. Metallurgical reactions between wire, flux, and base metal determine weld chemistry, cleanliness, and mechanical properties.

Advantages and Disadvantages

Advantages

  • Very high deposition rates and productivity; ideal for long seams.
  • Deep penetration and excellent fusion with proper parameters.
  • Low spatter and reduced fume due to submerged arc and slag cover.
  • Consistent quality with mechanized/automated operation.
  • Good mechanical properties and low hydrogen potential with dry flux.

Disadvantages

  • Limited to flat/near flat positions; not suited for complex geometries.
  • Requires precise joint prep and alignment; equipment is less portable.
  • Flux handling, drying, and recovery add process complexity.
  • Arc is invisible; real‑time visual monitoring is restricted.
  • Risk of slag inclusions and porosity if parameters/cleanliness are poor.

Applications

  • Shipbuilding: Longitudinal and circumferential seams on hull and deck plates.
  • Pressure vessels and boilers: Shell seams, longitudinal welds, and nozzle attachments.
  • Pipelines and spiral pipe: SAW in pipe mills for high‑productivity seam welding.
  • Structural fabrication: Plate girders, beams, and bridge components.
  • Heavy equipment: Thick plate assemblies and wear‑resistant overlays (strip cladding).

Process variables and consumable selection

  • Current and voltage: Control heat input, bead shape, and penetration; higher current increases deposition.
  • Travel speed: Balances penetration vs. reinforcement; too fast risks lack of fusion, too slow excess heat.
  • Wire diameter and stick‑out: Larger wires raise deposition; stick‑out affects preheating and current density.
  • Polarity: DC+ favors penetration; DC‑ increases deposition; AC helps reduce arc blow in multi‑wire setups.
  • Flux type: Neutral flux maintains weld chemistry; active flux modifies Mn/Si pickup for desired properties.
  • Multi‑wire/strip variants: Twin, tandem, or multi‑arc SAW for even higher productivity or cladding.

Consumable pairing (wire+flux) must meet required mechanical properties and impact toughness. Flux should be dry and stored per manufacturer guidance; reclaimed flux must be sieved and free of slag fines to prevent inclusions.

Welding defects commonly associated with SAW

  • Slag inclusions: Entrapped slag from poor cleaning, incorrect bead overlap, or low heat input.
  • Lack of fusion: Inadequate sidewall fusion due to high travel speed or low amperage/voltage.
  • Porosity: Gas entrapment from damp flux, contamination, or excessive flux depth.
  • Undercut/overlap: Improper parameter balance causing groove at toe or cold lap at edges.
  • Cracking: Hydrogen or solidification cracks from high restraint, improper preheat, or chemistry.
  • Arc blow: Magnetic interference with DC; mitigated by AC or altered ground placement.
  • Excess reinforcement/mismatch: Poor alignment or parameter control affecting profile and fit.

Procedure and quality control

  • WPS/PQR compliance: Define joint design, parameters, consumables, and positions; qualify per code.
  • Flux handling: Drying, storage, and recovery procedures to maintain low moisture and cleanliness.
  • Preheat/interpass: Control to avoid cracking and meet toughness requirements.
  • Pass sequencing: Plan root/fill/cap passes; remove slag between passes; verify bead overlap.
  • Inspection: VT for profile and defects; MT/PT for surface; UT/RT for volumetric integrity.
  • Documentation: Record parameters, consumable batch numbers, and inspection results for traceability.