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Key takeaways
- Visual examination comes first and is required on every weld.
- PT and MT find surface-breaking defects; MT works only on magnetic materials.
- RT and UT look through the weld for internal defects. UT, especially phased array, avoids radiation and is increasingly used instead of RT.
Visual examination (VT)
Every weld is examined visually for surface defects: cracks, undercut, porosity, lack of fusion at the surface, excessive reinforcement, misalignment and arc strikes. VT is cheap, and a good visual inspector prevents many later problems.
Liquid penetrant testing (PT)
A coloured or fluorescent liquid is applied to the surface, drawn into open defects, wiped off and revealed with a developer. PT finds surface-breaking cracks and porosity on any non-porous material, including stainless steel, duplex and non-magnetic alloys.
Magnetic particle testing (MT)
The part is magnetised and fine iron particles are applied. Particles gather at surface and slightly sub-surface defects that disturb the magnetic field. MT is faster and more sensitive than PT on carbon and low-alloy steel, but doesn’t work on austenitic stainless steel or other non-magnetic materials.
Radiographic testing (RT)
X-rays or gamma rays pass through the weld onto film or a digital detector. Defects show as density changes. RT is good at finding volumetric defects such as porosity and slag, and gives a permanent record. Its drawbacks are radiation safety (exclusion zones, work at night), and it is less reliable at finding planar defects such as lack of fusion that aren’t aligned with the beam.
Ultrasonic testing (UT)
Sound waves are sent into the weld and reflections from defects are recorded. Phased array UT (PAUT) and time-of-flight diffraction (TOFD) produce images and records similar to RT. UT is good at finding planar defects and measuring their depth, has no radiation hazard, and is now widely accepted by codes as an alternative to RT for butt welds above a minimum thickness.
Related checks
- Hardness testing: for sour service and PWHT verification.
- PMI (positive material identification): XRF or OES to confirm alloy composition.
- Ferrite measurement: for duplex welds.
| Method | Finds | Works on |
|---|---|---|
| VT | Surface defects | All materials |
| PT | Surface-breaking defects | Non-porous materials |
| MT | Surface and near-surface defects | Ferromagnetic materials only |
| RT | Volumetric internal defects | All materials |
| UT / PAUT | Planar and volumetric internal defects | Most materials (coarse-grained ones are harder) |
Code reference
ASME B31.3, para. 341 Examination requirements and extent by fluid service.
ASME BPVC Section V Nondestructive examination methods.
What to take back to your desk
On an inspection and test plan, look at which method is assigned to which weld type. Branch connections and fillet welds can’t be radiographed well, so they usually get MT or PT instead. Make sure that is written down.
Educational content. Always design to your project specification.
