Welding Inspection and Testing
This module tests whether you can select the right inspection method for a given defect, material and geometry, and interpret the acceptance criteria correctly.
Method families and what they find
- Visual (VT) first, always: profile, undercut, alignment, spatter, root condition.
- Surface methods: PT for any non-porous material, MT only for ferromagnetic materials.
- Volumetric: RT (sensitive to volumetric defects and to planar defects transverse to the beam) and UT (best for planar defects, needs one accessible surface and couplant).
- Destructive: tensile, bend, Charpy impact, hardness, macrosection - normally on procedure test pieces.
Standards
The method-selection matrix is ISO 17635, applied against ISO 5817 quality levels B (stringent), C (intermediate) and D (moderate). Technique standards are ISO 17636 for RT and ISO 17640 for UT; acceptance levels in NDT standards are not the same as ISO 5817 levels.
Worked example: surface crack in austenitic stainless
A 10 mm austenitic stainless butt weld shows a suspected toe crack. MT is invalid - the material is non-ferromagnetic. PT will detect and size the surface-breaking portion. RT can confirm whether the crack extends internally, but UT is heavily limited by grain scattering and anisotropy; low-frequency dual-element probes or TOFD may be needed. So the correct answer is PT plus RT, with UT only as a qualified supplementary technique.
Traps
- Specifying MT on austenitic, duplex or aluminium joints.
- Relying on RT alone to find lack-of-fusion that lies parallel to the beam.
- Confusing ISO 5817 quality level B with NDT acceptance level 2 - different documents, different criteria.
- Forgetting that UT sensitivity depends on probe frequency, surface condition and operator competence.