NDE only tells you what the method can find, where it was applied. A specification saying “NDE per applicable standards” controls nothing.
The methods
UT - ultrasonic testing. Finds internal defects: inclusions, laminations, shrinkage cavities. Works through thickness, so it is the method for volumetric examination of forgings. Applied before machining, while there is still enough material to reject the part economically and before the geometry becomes too complex to scan.
RT - radiographic testing. Also volumetric. Better than UT at characterising what a defect actually is; slower, more expensive, and constrained by access and geometry. Used mainly on castings, where porosity distribution matters and the geometry suits it.
MT - magnetic particle testing. Finds surface and near-surface defects on ferromagnetic materials. Fast, sensitive to cracks. Applied to finished machined surfaces on carbon and alloy steel.
PT - dye penetrant testing. Finds surface-breaking defects on any material. The method for austenitic stainless steels, which cannot be examined by MT, and for hardfaced overlays.
When each is applied
| Stage | Method | Finds |
|---|---|---|
| Forging, before machining | UT | Internal defects in the raw section |
| Casting, after cleaning | RT | Porosity, shrinkage, inclusions |
| After machining | MT (ferritic) or PT (austenitic) | Surface cracks, laps, seams |
| After hardfacing | PT | Overlay cracking, lack of fusion at the surface |
| After PWHT | Hardness survey (not NDE, but part of the same acceptance set) | Incorrect heat treatment |
| After welding repair | MT or PT, plus UT or RT if the repair is deep | Repair defects |
How to specify the extent
The failure mode is a specification that names methods without saying where or how much. Better is to state, per requirement:
Forgings for pressure-containing parts shall be ultrasonically examined in accordance with [standard], acceptance level [level], prior to machining. Records shall be submitted for review.
All finished machined surfaces of pressure-containing parts shall be examined by magnetic particle method to [standard], acceptance criteria [reference].
Hardfaced surfaces shall be examined by liquid penetrant to [standard] after final machining.
Personnel performing NDE shall be qualified to [SNT-TC-1A / ISO 9712] Level II minimum, and qualification records shall be available for review.
Four sentences that an inspector can check against, instead of one that they cannot.
The one to insist on
UT on the forging before machining. If it was not done, it cannot be done retrospectively - the material has been machined away and the geometry has changed. A supplier who cannot produce this report either did not perform it, or did not do the forging themselves and did not receive the report from whoever did.
This is the single most informative document to ask for on a forged valve order.
Repairs
Weld repair of defects found by NDE is normal and acceptable when it is controlled. What matters:
- The repair is authorised before it is made, not reported afterwards
- The welding procedure is qualified for the material and the repair depth
- PWHT is applied per the qualified procedure, particularly on creep grades
- The repaired area is re-examined by the same method that found the defect
- The repair is recorded on the document package with its location
A supplier who reports repairs is behaving correctly. One whose valves never need any is either extraordinarily lucky or not reporting them.
Operator qualification
NDE results are only as good as the person who performed the examination. Qualification to SNT-TC-1A or ISO 9712, at the appropriate level for the method, with current certification. This is easy to ask for and rarely asked for.
Pending verification. The NDE equipment available in house, which examinations are performed by an external agency, and the qualification records of the personnel involved must be documented on this page before it goes live.