Non-Destructive Testing in Welding: VT and PT
What are Non-Destructive Tests (NDT)?
Non-Destructive Tests (NDT) evaluate the integrity of a component without damaging it. In welding, NDT is fundamental to detect imperfections that may compromise safety.
Applicable European standards: EN ISO 17637 (VT) and EN ISO 3452 (PT).
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Visual Examination (VT)
Standard: EN ISO 17637
When is it performed?
- Before welding: check groove, cleanliness, fit-up, gaps
- During welding: check each pass
- After welding: final check
What is detected?
| Imperfection | Detectable by VT? |
|---|---|
| Surface lack of fusion | ✅ |
| Undercut | ✅ |
| Excess penetration | ✅ |
| Surface porosity | ✅ |
| Surface cracking | ✅ (good lighting) |
| Internal imperfections | ❌ |
Minimum requirements (EN ISO 17637)
- Illuminance: 500 lux minimum
- Viewing distance: ≤ 600 mm
- Viewing angle: ≥ 30° to the surface
- Examiner qualified to VT (EN ISO 9712)
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Penetrant Testing (PT)
Standard: EN ISO 3452-1
Sequence (6 steps)
- Pre-cleaning
- Penetrant application — 5–60 min dwell
- Excess removal
- Developer application
- Development time — 10–30 min
- Examination and interpretation
System types
| System | Penetrant | Developer |
|---|---|---|
| Type I — Fluorescent | UV | Dry or wet |
| Type II — Visible | Red | White spray |
⚠️ PT does not detect internal imperfections.
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VT vs PT comparison
| Criterion | VT | PT |
|---|---|---|
| Surface imperfections | ✅ | ✅✅ |
| Internal imperfections | ❌ | ❌ |
| Speed | Very fast | 30–90 min |
| Cost | Very low | Low |
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Personnel qualification (EN ISO 9712)
- Level 1: Under supervision
- Level 2: Performs and interprets
- Level 3: Approves procedures
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