Welding Defects and Imperfections — Complete Guide (ISO 6520)
Introduction
In welding terminology, an imperfection is any deviation from the ideal welded joint, while a defect is an imperfection that exceeds applicable acceptance limits. ISO 6520 systematically classifies and numbers all possible imperfections in welded joints.
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Classification per ISO 6520
| Group | Ref. No. | Type of Imperfection |
|---|---|---|
| 1 | 100 | Cracks |
| 2 | 200 | Cavities (porosity, shrinkage) |
| 3 | 300 | Solid inclusions (slag, tungsten) |
| 4 | 400 | Lack of fusion and penetration |
| 5 | 500 | Shape and dimension (undercut, excess weld metal) |
| 6 | 600 | Miscellaneous imperfections |
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Group 1 — Cracks (No. 100)
Hot cracking
Occurs during solidification. Causes: high sulphur/phosphorus content, inadequate W/D ratio, excessive parameters, poor joint preparation.
Cold cracking (Hydrogen-induced)
Occurs hours or days after welding. Causes: diffusible hydrogen, martensitic structure in HAZ, high residual stresses, insufficient preheat.
Prevention: Control carbon equivalent (CE), use basic (low-hydrogen) electrodes, apply adequate preheat.
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Group 2 — Porosity (No. 200)
Spherical or elongated cavities caused by trapped gases. Causes: grease/moisture/oxides, insufficient gas shielding, excessive arc length, excessive travel speed, damp electrodes.
Types: uniformly distributed, clustered, linear, wormhole.
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Group 3 — Solid Inclusions (No. 300)
- Slag inclusions (301): Non-metallic material trapped in the weld. Causes: poor interpass cleaning, incorrect electrode angle.
- Tungsten inclusions (3041): Tungsten particles in the weld pool. Specific to TIG (141).
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Group 4 — Lack of Fusion and Penetration (No. 400)
- Lack of fusion (401): No metallurgical bond between filler and base metal. Causes: low current, excessive speed, incorrect angle.
- Lack of penetration (402): Weld does not fully penetrate the root. Causes: insufficient root gap, excessive root face, low current.
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Group 5 — Shape and Dimension (No. 500)
- Undercut (5011-5012): Depression in the base metal along the fusion line
- Excess weld metal (502): Excessive reinforcement
- Misalignment (507): Offset between workpieces
- Incompletely filled groove (511): Insufficient weld metal
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Quality Levels — ISO 5817
| Level | Description | Typical Application |
|---|---|---|
| D | Moderate | Simple structures, static loads |
| C | Intermediate | General construction |
| B | Stringent | Pressure equipment, offshore, nuclear |
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Detection Methods
| Imperfection | Visual | DPI | MPI | UT | RT |
|---|---|---|---|---|---|
| Surface cracks | ✓ | ✓ | ✓ | ✓ | ✓ |
| Internal cracks | — | — | — | ✓ | ✓ |
| Porosity | — | — | — | ✓ | ✓ |
| Inclusions | — | — | — | ✓ | ✓ |
| Lack of fusion | — | — | — | ✓ | ✓ |
| Undercut | ✓ | ✓ | ✓ | — | — |
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Practical Summary
- Prepare the joint — clean surfaces, correct bevel dimensions
- Control parameters — current, voltage, travel speed, stick-out, gas flow
- Proper technique — angle, weaving, progression speed
- Correct consumables — dry electrodes, proper wire, gas per WPS
- Clean between passes — remove slag, brush, visually inspect
- Pre/postheat — for high CE steels or thick sections
- Qualification — welder qualified per ISO 9606
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*Article based on ISO 6520-1, ISO 5817 and AWS Welding Handbook training materials. Published on WeldFC Academy — the technical reference in metalworking.*