NDT for Steel: VT, MT, PT, UT and RT Explained
Non-destructive testing methods for steel and welds explained: visual (VT), magnetic particle (MT), dye penetrant (PT), ultrasonic (UT) and radiography (RT) — what each finds and when to use it.
Saudi Steel Work Editorial Team · · 3 min read
Short answer: Non-destructive testing (NDT) finds defects without damaging the part. Visual testing (VT) checks surface quality of every weld; magnetic particle (MT) and dye penetrant (PT) find surface-breaking cracks; ultrasonic testing (UT) finds internal defects and measures thickness; radiography (RT) records internal defects on film or digital images. The specification and code set which methods and extent apply.
NDT verifies weld and material quality. Specifications usually define methods and the percentage of welds tested.
Methods
| Method | Detects | Works on | Notes |
|---|---|---|---|
| VT — Visual | Surface defects, profile, size | All | First and most important; 100% of welds |
| MT — Magnetic particle | Surface and near-surface cracks | Ferromagnetic steel | Fast, low cost |
| PT — Penetrant | Surface-breaking defects | All non-porous metals | Used on stainless, aluminium |
| UT — Ultrasonic | Internal defects, laminations, thickness | Most metals | Needs skilled operator; PAUT gives imaging |
| RT — Radiography | Internal volumetric defects | All | Permanent record; radiation safety controls |
Typical uses in steel fabrication
- Fillet welds: VT, plus MT on a percentage
- Full-penetration butt welds: UT or RT on a specified percentage or 100%
- Plates for T-joints in thick sections: UT for laminations
- Pipe welds: RT or UT per piping code
- Tank shells: RT/UT per API 650; vacuum box on bottoms
Personnel qualification
NDT operators are typically certified under schemes such as ISO 9712 or ASNT SNT-TC-1A (Levels 1–3).
Acceptance criteria
The code defines what is acceptable: AWS D1.1 for structures, ASME for pressure equipment, API 1104 for pipelines, ISO 5817 quality levels in European practice.
Planning NDT
- Define methods and extent in the ITP (quality inspection checklist)
- Schedule NDT after required cooling or hold times (delayed cracking)
- Allow time for repairs and re-testing
See WPS and PQR and our inspection support.