Weld defects directly affect the mechanical strength, corrosion resistance and fatigue life of the joint. In modern weld quality management, defects are classified per EN ISO 6520-1 and acceptance criteria are set by EN ISO 5817.
Defect Classification (EN ISO 6520-1)
| Group | Defect Type | Typical Example |
|---|---|---|
| 1 | Cracks | Cold crack, hot crack, lamellar tearing |
| 2 | Cavities | Porosity, gas pores |
| 3 | Solid inclusions | Slag inclusion, oxide inclusion |
| 4 | Lack of fusion/penetration | Incomplete penetration, lack of side wall fusion |
| 5 | Shape imperfections | Undercut, insufficient fill, overlap |
| 6 | Miscellaneous | Spatter, arc strike |
EN ISO 5817 Quality Levels
- Level B (high): Fatigue-loaded structures, pressure vessels, defence industry.
- Level C (medium): Standard structural steel, EN 1090 EXC2.
- Level D (low): Low-load, static general fabrication.
1. Cold Cracking
Cold cracking is a hydrogen-induced delayed crack that forms hours or even days after welding. Three conditions must coexist:
- Diffusible hydrogen (inadequate drying, humid environment)
- High tensile stress (thick section, restrained joint)
- Susceptible microstructure (martensite, high carbon equivalent)
Prevention: Preheat + basic electrode + H5 consumable + slow cooling. Calculate per EN ISO 1011-2 Method B.
2. Hot Cracking
Forms at high temperature during solidification. Typical in stainless steels, aluminium and nickel alloys. Cause: segregation of low-melting constituents (sulphur, phosphorus) at grain boundaries.
3. Porosity
Gas voids trapped in the weld metal. Main causes:
- Surface contamination (oil, rust, paint, moisture)
- Insufficient shielding gas flow or wrong gas selection
- Incorrect electrode angle or travel speed
- Wind effect (field welding)
4. Lack of Penetration
Weld metal fails to reach the root region. Most critical defect in thick-section welds because it significantly reduces the load-carrying cross-section. Detection: UT or RT is mandatory.
5. Lack of Side Wall Fusion
Insufficient fusion between weld metal and base metal. Usually caused by low heat input, excessive travel speed or incorrect electrode angle. Detected by UT and RT.
NDT Detection Methods
| Method | Defects Detected | Standard |
|---|---|---|
| VT (Visual) | Surface crack, porosity, undercut | EN ISO 17637 |
| PT (Penetrant) | Open surface crack, porosity | EN ISO 3452 |
| MT (Magnetic) | Surface + near-surface cracks (ferromagnetic) | EN ISO 17638 |
| UT (Ultrasonic) | Internal defects, lack of penetration, lack of fusion | EN ISO 17640 |
| RT (Radiographic) | Internal defects, porosity, slag, cracks | EN ISO 17636 |
Repair Strategy
- Define defect extent by NDT.
- Remove: carbon arc gouging, grinding or milling.
- Verify removal by PT/MT.
- Repair weld: same or lower heat input, preheat + basic electrode.
- Final NDT: full VT + UT/RT.
Practical rule: After two unsuccessful repair attempts in the same area, the component is rejected or completely re-manufactured.
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