01Classification
ISO 6520-1 groups imperfections into cracks, cavities, solid inclusions, lack of fusion and penetration, imperfect shape and dimensions, and miscellaneous imperfections. Acceptance limits for each are given by quality level in ISO 5817.
02Main imperfections at a glance
| Imperfection | Typical causes | Best detected by |
|---|---|---|
| Hydrogen cracking | Moisture in consumables, high hardness HAZ, restraint, insufficient preheat | MT, PT (surface), UT |
| Solidification cracking | Unfavourable bead shape, impurities, high dilution | VT, PT, MT, UT |
| Porosity | Contamination, damp flux, inadequate gas shielding, draughts | RT, VT for surface pores |
| Lack of fusion | Low heat input, incorrect torch angle, poor access | UT |
| Incomplete root penetration | Small root gap, low current, poor preparation | VT (root side), RT, UT |
| Undercut | Excessive current or travel speed, wrong angle | VT with gauge |
| Excess reinforcement, overlap | Technique, parameters | VT with gauge |
| Slag inclusions | Poor interpass cleaning | RT, UT |
03Prevention first
Most imperfections are prevented by qualified procedures, correct consumable handling, joint preparation and fit-up, preheat and heat input control, and skilled welders. That is why welding inspection starts before the arc is struck.
04Choosing quality levels
Level B is stringent, C intermediate and D moderate. Fatigue loaded structures may need additional requirements. The design code or product standard determines the level; the inspector applies it.
Published 2026-11-08 · QUALITAS TR technical team