Weld defects compromise structural integrity and must be identified, classified, and addressed systematically. EN ISO 6520-1 provides the standard classification system, while EN ISO 5817 defines quality levels (B, C, D) and acceptance criteria.
Classification of Weld Imperfections
EN ISO 6520-1 groups weld imperfections into six categories: cracks (Group 1), cavities (Group 2), solid inclusions (Group 3), lack of fusion and penetration (Group 4), imperfect shape and dimensions (Group 5), and miscellaneous imperfections (Group 6).
Cracks — The Most Critical Defect
Cracks are never acceptable in any quality level per EN ISO 5817. They are classified by orientation (longitudinal, transverse), location (weld metal, HAZ, base metal), and mechanism (hot cracking, cold cracking, lamellar tearing).
**Hot cracks** form during solidification when low-melting-point films at grain boundaries are pulled apart by shrinkage stresses. Prevention: control sulphur and phosphorus content, ensure adequate Mn/S ratio, avoid excessive restraint, and use filler metals with controlled chemistry.
**Cold cracks** (hydrogen-induced cracking) are the most common type in structural steel welding. They occur hours or days after welding when hydrogen trapped in the weld diffuses to areas of high stress concentration. Prevention: use low-hydrogen consumables (H5/H10 per EN ISO 2560), apply preheat per EN 1011-2, and control interpass temperature.
Porosity and Cavities
Porosity results from gas entrapment during solidification. Common causes include: contaminated base metal or filler wire, insufficient shielding gas coverage, excessive moisture, and incorrect welding parameters.
Scattered porosity up to defined limits may be acceptable at quality level C and D per EN ISO 5817, but clustered porosity and wormholes are typically cause for rejection.
Non-Destructive Testing Methods
Detection method selection depends on the defect type and location:
- **Visual testing (VT)**: EN ISO 17637 — surface defects, mandatory 100% for all quality levels\n- **Magnetic particle testing (MT)**: EN ISO 17638 — surface and near-surface defects in ferromagnetic materials\n- **Penetrant testing (PT)**: EN ISO 3452-1 — surface-breaking defects in all materials\n- **Ultrasonic testing (UT)**: EN ISO 17640 — internal defects, volumetric inspection\n- **Radiographic testing (RT)**: EN ISO 17636 — internal defects, provides permanent record
EN 1090-2 specifies minimum inspection percentages based on execution class: EXC1 (visual only), EXC2 (10–20% NDT), EXC3 (20–50% NDT), EXC4 (100% NDT for full-penetration welds).
Prevention Through Process Control
The most effective defect prevention strategy is rigorous adherence to qualified Welding Procedure Specifications (WPS) per EN ISO 15614-1, performed by welders certified per EN ISO 9606-1, under welding coordination per EN ISO 14731.
A quality management system meeting EN ISO 3834 (parts 2, 3, or 4 depending on requirements) ensures systematic control of all welding variables from material procurement through to final inspection.