Welding Defects: Causes and Remedies
Introduction
EN ISO 6520-1 classifies welding imperfections into 6 main groups. For a welder or inspector, identifying the defect, understanding its cause and knowing the remedy is a fundamental competence. Acceptance or rejection is decided by the applicable product standard — typically EN ISO 5817 for steels.
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Group 1 — Cracks
Cold Cracking (HAZ Cracking)
- Cause: diffusible hydrogen + martensite + residual stresses (cracking triangle)
- Location: HAZ, typically 24–72 h after welding
- Remedy: preheat, low-hydrogen consumables (H5/H4), PWHT
Hot Cracking
- Cause: segregation of impurities (S, P) at grain boundaries during solidification
- Remedy: low S and P consumables, increase bead width/depth ratio (>1), reduce heat input
Reheat Cracking
- Cause: carbide precipitation during PWHT in low-alloy steels with Cr, Mo, V
- Remedy: slow heating ramp, avoid susceptible steels
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Group 2 — Porosity
| Type | Cause | Remedy |
|---|---|---|
| Distributed porosity | Moisture in consumable or base metal | Dry consumables, clean base metal |
| Wormhole porosity | Gases trapped by rapid solidification | Reduce travel speed |
| Surface porosity | Contamination by oil, paint, moisture | Clean and degrease before welding |
| Crater pipe | Abrupt arc extinction | Use crater-fill function |
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Group 3 — Solid Inclusions
- Slag inclusions: previous passes not cleaned; remedy: thorough brushing between passes, increase bevel angle
- Tungsten inclusions (TIG): electrode contact with pool; remedy: maintain electrode-to-work distance
- Oxide inclusions: insufficient cleaning on aluminium; remedy: brush with dedicated stainless brush immediately before welding
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Group 4 — Lack of Fusion and Penetration
| Imperfection | Cause | Remedy |
|---|---|---|
| Lack of sidewall fusion | Low current, excessive electrode angle | Increase current, correct angle |
| Lack of root penetration | Insufficient root gap, low current | Increase gap, adjust parameters |
| Inter-run lack of fusion | Previous pass not cleaned | Clean between passes |
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Group 5 — Shape Imperfections
Undercut
- Cause: excessive current, wrong electrode angle, high travel speed
- Remedy: reduce current, correct angle, reduce travel speed
- EN ISO 5817 criterion: level B — depth ≤ 0.5 mm; level C — ≤ 1 mm
Overlap
- Cause: low current, excessively low travel speed
- Remedy: increase current or travel speed
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Group 6 — Miscellaneous Imperfections
- Spatter: voltage too low (short-circuit) or too high (unstable spray); remedy: adjust voltage and inductance
- Angular distortion: asymmetric cooling; remedy: alternating weld sequence, fixturing
- Burn-through: excessive heat input on thin sections; remedy: reduce current or use backing
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Reference Standards
- EN ISO 6520-1:2007 — Welding and allied processes. Classification of geometric imperfections in metallic materials. Part 1: Fusion welding
- EN ISO 5817:2014 — Welding. Fusion-welded joints in steel, nickel, titanium and their alloys. Quality levels for imperfections
- EN ISO 17637:2016 — Non-destructive testing of welds. Visual testing of fusion-welded joints
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