Shielded Metal Arc Welding (SMAW)

Overview

In SMAW, current from a power source flows through the electrode to the workpiece, creating an arc that melts the base metal and the electrode tip. The electrode’s flux coating decomposes under heat, generating shielding gases and forming a slag layer over the weld bead to protect it during solidification. Operators control arc length, travel speed, and electrode angle, making SMAW highly adaptable but dependent on welder skill. The process is suitable for ferrous and some non‑ferrous metals across a wide range of thicknesses and positions.

Apparatus and Working

Apparatus

• Power source: AC/DC welding machine with adjustable current output (transformer/inverter).
• Electrode holder: Insulated clamp to hold and conduct current to the SMAW electrode.
• Work lead and clamp: Return cable connecting the workpiece to the power source.
• Electrodes (consumable): Flux‑coated rods (e.g., E6010, E6013, E7018) matched to material and service.
• Personal protective equipment (PPE): Welding helmet (auto‑darkening), gloves, jacket, safety boots.
• Ancillary tools: Chipping hammer, wire brush, grinder for slag removal and joint preparation.
• Measurement and prep: Weld gauges, angle grinder, clamps, and cleaning agents.

Working steps

1. Joint preparation: Clean base metal; bevel edges if required; ensure proper fit‑up and tack welds.
2. Parameter selection: Choose electrode type/diameter; set current per rod specifications and position.
3. Arc initiation: Strike the arc (scratch/tap), establish stable arc length (~equal to electrode core diameter).
4. Welding: Maintain travel speed and correct electrode angle (typically 10–15° push/drag depending on rod).
5. Slag management: Allow bead to cool; remove slag with chipping hammer and brush before subsequent passes.
6. Multi‑pass sequencing: Apply root, fill, and cap passes as per WPS; control interpass temperature.
7. Inspection and finishing: Clean, visually inspect, and grind/correct as needed; verify dimensions/profile.

Principle

SMAW relies on electric arc heating to melt the base metal and consumable electrode. The electrode’s flux coating stabilizes the arc, provides deoxidizers and scavengers, forms shielding gases, and creates a slag cover that protects the solidifying weld metal from oxygen and nitrogen in the atmosphere. Metallurgical reactions in the weld pool and flux determine the weld’s microstructure, mechanical properties, and defect susceptibility. Proper current, arc length, and technique minimize spatter, undercut, and inclusions, yielding sound welds.

Advantages and Disadvantages

Advantages

• Highly portable and simple equipment; ideal for field work and repairs.
• Works well in outdoor and windy environments due to slag protection.
• Versatile for various materials and thicknesses, including all positions.
• Wide electrode selection allows tuning for penetration, toughness, and deposition.
• Lower equipment cost compared to advanced processes (GMAW/GTAW).

Disadvantages

• Lower deposition efficiency due to slag; requires frequent electrode changes.
• Higher dependence on welder skill; inconsistent results with poor technique.
• Slag removal between passes increases cycle time.
• More spatter and fumes compared to some processes; requires robust PPE and ventilation.
• Limited productivity for long welds versus semi‑automatic processes.

Applications

• Structural steel fabrication: Beams, columns, braces, and on‑site erection.
• Pipelines and pressure parts: Tie‑ins, repairs, and maintenance welding.
• Shipbuilding and offshore: Hull repairs, deck structures, and support frames.
• Heavy equipment and mining: Repair of buckets, frames, and wear components.
• General maintenance: Farm machinery, rail, and utility infrastructure.
• Cast iron and low alloy steels: With specialized electrodes and procedures.

Electrodes and technique

• Cellulosic (e.g., E6010/E6011): Deep penetration, fast‑freeze slag, ideal for root passes and out‑of‑position welds.
• Rutile (e.g., E6013): Smooth bead appearance, easy arc starting, moderate penetration; good for thin sections.
• Low hydrogen (e.g., E7018): Improved toughness and crack resistance; requires dry storage and higher skill.
• Stainless electrodes (e.g., E308/E309): Corrosion‑resistant welds for stainless and dissimilar joints.
• Hardfacing electrodes: Wear‑resistant overlays for maintenance and refurbishment.

Technique essentials include stable arc length, correct travel speed to balance penetration and bead profile, and appropriate manipulation (weave/straight stringer) per WPS and electrode type. Maintain rod angle to control puddle and avoid undercut. For low hydrogen rods, keep electrodes dry in heated quivers to prevent moisture absorption and hydrogen cracking.

Welding defects commonly associated with SMAW

• Lack of fusion: Poor sidewall fusion due to low heat input, excessive travel speed, or incorrect angle.
• Incomplete penetration: Root not fully fused from improper joint prep or inadequate current.
• Porosity: Gas pockets from contaminated base metal, damp electrodes, or long arc length.
• Slag inclusions: Entrapped slag from improper cleaning between passes or incorrect bead overlap.
• Cracking (hot/cold): Hydrogen‑induced or solidification cracks from high restraint or poor preheat.
• Undercut: Groove at toe from high current or excessive travel speed/angle.
• Overlap and excessive reinforcement: Poor bead shape and lack of fusion at edges.
• Arc strikes: Unintended arc contact causing localized hard spots/crack initiation.

Procedure and quality control

• WPS/PQR compliance: Follow qualified procedures for base metal, filler, position, and parameters.
• Preheat/interpass control: Reduce hydrogen cracking and manage heat input for toughness.
• Slag removal and cleaning: Between passes to prevent inclusions and ensure fusion.
• Post‑weld inspection: VT, MT/PT (surface), UT/RT (volumetric) per code and service criticality.
• Storage of electrodes: Keep low hydrogen rods dry; re‑bake per manufacturer if required.