Submerged Arc Welding (SAW/121) — Complete Technical Guide
Introduction and Scope
Submerged Arc Welding (SAW), designated 121 per ISO 4063, is a fusion welding process in which the electric arc forms between a continuous wire electrode and the workpiece, entirely covered by a layer of granular flux. The arc is not visible — it is literally submerged in the flux, which gives the process its unique characteristics.
SAW has the highest deposition rate of all arc welding processes (up to 25 kg/h in multi-wire configurations) and the highest thermal efficiency (η ≈ 0.99 per AWS Welding Handbook), making it ideal for long, straight, heavy-section welds in flat or horizontal position.
It is widely used in shipbuilding, pressure vessels, large-diameter pipelines, offshore structures, bridges and wind towers.
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Operating Principle
The SAW process follows this operational sequence:
| Step | Description |
|---|---|
| 1 | Granular flux is deposited ahead of the arc, covering the weld zone |
| 2 | The continuous wire is fed automatically into the joint |
| 3 | The arc melts the wire, base material and part of the flux, forming liquid slag |
| 4 | The slag solidifies over the weld bead, protecting it during cooling |
| 5 | Unfused flux is recovered by vacuum and reused |
Weld pool protection is provided exclusively by the flux — no external shielding gas is used. The slag formed serves metallurgical functions (deoxidation, alloying element addition) and protective functions (against atmospheric oxidation and cooling rate control).
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Process Variants per ISO 4063
ISO 4063 classifies several SAW variants:
| ISO 4063 No. | Designation | Electrode Type |
|---|---|---|
| 121 | SAW with solid wire | Solid wire (most common) |
| 122 | SAW with strip (strip cladding) | Metal strip for overlay |
| 124 | SAW with metal powder | Wire + powder addition to flux |
| 125 | SAW with flux-cored wire | Tubular wire with internal flux |
The most common industrial electrode configurations are:
| Configuration | Description | Typical Deposition Rate |
|---|---|---|
| Single wire | 1 electrode, 1 power source | 5–10 kg/h |
| Tandem (twin arc) | 2 electrodes in line, 2 power sources | 10–18 kg/h |
| Multi-wire | 3–5 electrodes, independent power sources | 15–25 kg/h |
| Strip | Wide strip for overlay | 8–15 kg/h (cladding) |
In the tandem configuration, the lead arc (DC+) ensures penetration while the trail arc (AC) fills, enabling welding speeds up to 3 m/min on thin sections.
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Flux Classification — EN ISO 14174
The flux is the most critical consumable in SAW. Standard EN ISO 14174 classifies fluxes by several criteria:
By manufacturing method
| Type | Manufacturing | Characteristics |
|---|---|---|
| Fused (F) | Raw materials melted in a furnace, solidified and crushed | Glassy, homogeneous grains; hygroscopic; good consistency; limited in deoxidant and ferroalloy addition |
| Agglomerated (B) | Powders mixed with binder (sodium/potassium silicate), granulated and dried at 400–800 °C | Can incorporate ferroalloys and deoxidants; less hygroscopic; lower current consumption |
| Sintered (S) | Similar to agglomerated but sintered at higher temperature | Higher grain mechanical strength; lower moisture absorption |
| Mixed (M) | Mechanical mixture of two or more types | Allows fine-tuning of properties |
By basicity index (BI)
The basicity index classifies the metallurgical behaviour of the flux:
| Classification | BI (Boniszewski) | Effect on Weld |
|---|---|---|
| Acid | BI < 1.0 | Good surface appearance; easy slag removal; lower toughness |
| Neutral | 1.0 ≤ BI ≤ 1.2 | Balance between appearance and mechanical properties |
| Basic | BI > 1.2 | Better impact toughness; lower oxygen content in weld; harder slag removal |
| Highly basic | BI > 2.0 | Maximum toughness; used in pressure vessels and cryogenic applications |
BI = (CaO + MgO + BaO + SrO + Na₂O + K₂O + Li₂O + CaF₂ + 0.5(MnO + FeO)) / (SiO₂ + 0.5(Al₂O₃ + TiO₂ + ZrO₂))
Flux handling requirements
| Parameter | Requirement |
|---|---|
| Storage | Dry location, temperature >10 °C above ambient, RH < 50% |
| Baking (agglomerated) | 250–350 °C for 2–4 h before use (if exposed to moisture) |
| Baking (fused) | Generally not required (glassy, low absorption) |
| Reuse | Recovered flux must be sieved and blended with new flux (max. 50/50) |
| Contamination | Never mix fluxes from different brands/types |
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Wire-Flux Combination Classification — EN ISO 14171
Standard EN ISO 14171 classifies solid wire + flux combinations for non-alloy and fine-grain steels. The classification follows the format:
ISO 14171-A (tensile strength-based system):
Example: S 50 4 AB S2
| Position | Meaning | Example |
|---|---|---|
| S | Solid wire for submerged arc | S |
| 50 | Minimum tensile strength in MPa (×10) | 500 MPa |
| 4 | Absorbed energy 47 J at −40 °C | −40 °C |
| AB | Flux type (A=acid, B=basic, AB=neutral) | Neutral |
| S2 | Solid wire classification | S2 |
Most common solid wires
| Designation | Typical composition (%) | Application |
|---|---|---|
| S1 | 0.06–0.14 C; 0.80–1.25 Mn | General construction steels |
| S2 | 0.06–0.14 C; 1.40–1.85 Mn; 0.15–0.35 Si | Structural steels, pressure vessels |
| S3 | 0.06–0.14 C; 1.40–1.85 Mn; 0.45–0.75 Si | Higher deoxidation requirements |
| S4 | 0.06–0.14 C; 1.65–2.20 Mn; ≤ 0.50 Si; 0.30–0.60 Mo | High-strength steels, elevated temperature service |
Typical wire diameters: 2.0 mm; 2.4 mm; 3.2 mm; 4.0 mm; 5.0 mm.
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Welding Parameters by Thickness
Parameters vary significantly with material thickness and joint preparation:
Butt joints — single wire, DC+
| Thickness (mm) | Wire Ø (mm) | Current (A) | Voltage (V) | Travel Speed (cm/min) | No. of Passes |
|---|---|---|---|---|---|
| 6 | 2.4 | 350–400 | 28–30 | 50–70 | 1 |
| 8 | 3.2 | 400–450 | 28–31 | 45–60 | 1 |
| 10 | 3.2 | 450–500 | 30–32 | 40–55 | 1–2 |
| 12 | 4.0 | 500–550 | 30–33 | 35–50 | 1–2 |
| 16 | 4.0 | 550–600 | 31–34 | 30–45 | 2 |
| 20 | 4.0 | 550–650 | 32–35 | 25–40 | 2–3 |
| 25 | 4.0–5.0 | 600–700 | 33–36 | 25–35 | 3–4 |
| 30–40 | 4.0–5.0 | 600–750 | 33–38 | 20–35 | Multi-pass |
Fillet joints — single wire
| Leg size (mm) | Wire Ø (mm) | Current (A) | Voltage (V) | Travel Speed (cm/min) |
|---|---|---|---|---|
| 5 | 2.4 | 350–400 | 28–30 | 60–80 |
| 6 | 3.2 | 400–450 | 29–31 | 50–65 |
| 8 | 3.2 | 450–550 | 30–33 | 40–55 |
| 10 | 4.0 | 550–650 | 32–35 | 30–45 |
| 12 | 4.0 | 600–700 | 33–36 | 25–40 |
Essential variables and their influence
| Parameter | Effect when increased |
|---|---|
| Current (A) | Increases penetration and deposition rate; narrower bead |
| Voltage (V) | Increases bead width; flatter bead; higher flux consumption |
| Travel speed | Decreases penetration and width; risk of undercut at excessive speed |
| Wire diameter | Larger diameter = lower penetration for same current; wider bead |
| Electrode extension (stick-out) | Increases deposition rate; decreases penetration (Joule heating in wire) |
| DC+ polarity | Maximum penetration (70% of heat in workpiece) |
| DC− polarity | Higher deposition rate, lower penetration (70% of heat in electrode) |
| AC current | Compromise between penetration and deposition rate; eliminates arc blow |
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Joint Preparation for SAW
SAW normally operates in PA (flat) or PB (horizontal fillet) position. Joint preparation is critical:
| Joint Type | Thickness (mm) | Groove Angle | Root Gap | Root Face |
|---|---|---|---|---|
| I (square) | ≤ 12 (1 side), ≤ 16 (2 sides) | 0° | 0–1 mm | N/A |
| V | 12–30 | 50–60° | 0–2 mm | 2–3 mm |
| X (double V) | 20–60 | 50–60° | 0 mm | 2–4 mm |
| U | > 30 | 10–20° | 0–2 mm | 3–5 mm |
| K (double U) | > 40 | 10–20° | 0 mm | 3–5 mm |
For square-butt joints up to 12 mm, SAW can weld from one side using ceramic backing or flux backing support, eliminating the need for a manual root pass.
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Equipment
SAW equipment comprises:
| Component | Function |
|---|---|
| Power source | DC or AC; 600–1500 A units; constant voltage (CV) characteristic |
| Wire feeder | Controlled speed, synchronised with current |
| Welding head | Guides wire and contact tip; joint alignment |
| Flux hopper | Flux reservoir with flow control valve |
| Recovery system | Vacuum unit to collect unfused flux for reuse |
| Travel system | Tractor, column and boom, or gantry (application dependent) |
In tandem configuration, each electrode has its own power source. Typically the lead arc operates on DC+ (penetration) and the trail arc on AC (fill and arc blow reduction).
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Advantages and Limitations
Advantages
| Advantage | Detail |
|---|---|
| Maximum thermal efficiency | η ≈ 0.99 — virtually all energy goes into the weld |
| High deposition rate | 5–25 kg/h depending on configuration |
| Metallurgical quality | Excellent pool protection; low hydrogen (< 5 ml/100g with basic flux) |
| No visible UV radiation | Arc covered by flux — no specific face protection required for operator |
| No spatter | Flux contains all projections |
| Full automation | Fully mechanised process; excellent repeatability |
| Deep penetration | Can weld thicknesses up to 16 mm without a groove (I-joint, 2 sides) |
Limitations
| Limitation | Detail |
|---|---|
| Welding position | Limited to PA (flat) and PB (horizontal fillet) |
| Accessibility | Heavy equipment; not applicable in many field situations |
| Minimum thickness | Generally ≥ 5 mm (high currents on thin sections cause burn-through) |
| Zero visibility | Operator cannot see the arc or pool — alignment and parameters must be correct before starting |
| Equipment cost | High initial investment compared to SMAW or GMAW |
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Typical Defects and Prevention
| Defect (ISO 6520 No.) | Main Cause | Prevention |
|---|---|---|
| Porosity (2011/2012) | Moisture in flux; joint contamination (rust, oil, paint) | Bake flux; clean joint; sieve reused flux |
| Slag inclusions (301) | Insufficient cleaning between passes; wrong flux; excessive speed | Fully remove slag between passes; adjust speed |
| Lack of fusion (401) | Insufficient current; wire misalignment; insufficient root opening | Check alignment; increase current; correct preparation |
| Lack of penetration (402) | Low current; high speed; insufficient root gap | Increase current; reduce speed; correct gap |
| Undercut (5011/5012) | Excessive voltage; excessive speed | Reduce voltage; reduce speed |
| Hot cracking (100) | High S and P in base metal; inadequate W/D ratio | Control chemical composition; adjust parameters for W/D > 1.2 |
| Cold cracking (H₂) | Diffusible hydrogen; hardenable steel; residual stresses | Use dry basic flux; preheat; control heat input |
| Asymmetric bead | Head misalignment; incorrect positioning | Check alignment before starting; use seam tracking systems |
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Industrial Applications
| Sector | Typical Application | Main Advantage |
|---|---|---|
| Shipbuilding | Hull panels, decks, bulkheads | High productivity on long plates |
| Pressure vessels | Longitudinal and circumferential seams | Metallurgical quality; low H₂ |
| Pipelines (Oil & Gas) | Longitudinal pipe welding (spiral and UOE) | Speed (tandem up to 3 m/min) |
| Offshore structures | Tubular nodes, platform legs | Deep penetration; reliability |
| Wind towers | Cans and flanges | Continuous circumferential welding |
| Steel bridges | Plate girders, orthotropic decks | Full automation; repeatability |
| Cladding | Stainless steel overlay on carbon steel | Strip cladding (No. 122) with strip |
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Reference Standards
| Standard | Content |
|---|---|
| ISO 4063 | Designation of welding processes |
| EN ISO 14171 | Classification of wire-flux combinations for non-alloy and fine-grain steels |
| EN ISO 14174 | Classification of fluxes for submerged arc welding |
| ISO 9692-2 | Joint preparation for submerged arc welding |
| ISO 15614-1 | Welding procedure qualification |
| ISO 9606-1 | Welder qualification (SAW partly mechanised: 121, 125) |
| EN ISO 14731 | Welding coordination |
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*Technical article based on EN ISO 14174, EN ISO 14171, ISO 4063, ISO 9692-2, AWS Welding Handbook — Welding Metallurgy and Lincoln Electric — The Procedure Handbook of Arc Welding. Published on WeldFC Academy.*