VT and MT: What Our Weld Inspection Actually Covers
Key takeaways
- Visual testing (VT) to ISO 17637 covers 100% of welds; magnetic particle testing (MT) to ISO 9934-1 covers 100% of load-bearing welds.
- VT finds surface-breaking geometry problems; MT finds fine surface cracks VT cannot reliably see.
- Dimensional checks run alongside: fillet leg size tolerance 0 to +2 mm.
- Weld precautions start before the arc — material certificates, groove condition and cleanliness are all checked first.
Inspection is a sequence, not one step at the end
Weld quality is not checked at the end; it is controlled at three points. Before welding: are we holding the right material and the right groove? During: are the parameters and interpass temperatures actually being followed? After: does the finished weld meet the acceptance criteria? Checking only the third is how most poor-quality fabrications pass.
Before welding
| Check | Requirement |
|---|---|
| Material certificate and grade | Certificate present and matching the physical marking |
| Groove dimensions, fit-up gap, cleanliness | Per joint design; no rust, oil or water within 20 mm of the groove |
During welding
First piece gets full inspection, and each unit receives at least two patrol checks during production, with actual values recorded. The checks cover parameters, interpass temperature and slag removal between passes.
After welding
| Method | Extent | Standard | Requirement |
|---|---|---|---|
| Visual testing VT | 100% | ISO 17637 | Per ISO 5817 level C acceptance |
| Magnetic particle testing MT | 100% of load-bearing welds | ISO 9934-1 | No relevant indications |
| Fillet leg size | Dimensional | — | Tolerance 0 to +2 mm |
| Hardness (arbitration) | As required | — | ≤ 380 HV10 in weld and HAZ |
Why both methods
VT is the base layer — it catches undercut, poor tie-in, excessive reinforcement, surface porosity and spatter, and it is applied to every weld. MT adds sensitivity: on ferromagnetic steel, magnetic particle testing will reveal fine surface-breaking cracks that visual examination can miss entirely.
That distinction matters most where it counts. In our procedure the load-bearing welds — the shell seam, the cutting edge joint, the lug group and the adapter joints — get MT at 100% plus additional acceptance restrictions: no cracks, no lack of fusion, no surface porosity and no undercut deeper than 0.5 mm. Everything else still gets VT at 100%.
When something fails
Imperfections beyond the limit are investigated for cause rather than simply ground out. The imperfection is fully removed by gouging or grinding, MT-verified, and repaired per procedure. Any single location may be repaired at most twice; beyond that it needs engineering approval. Each weld is stamped or logged to the welder, so traceability exists.
Personnel performing visual and magnetic particle testing are required to hold VT/MT qualification to ISO 9712 or equivalent.
FAQ
Can we have the inspection records? Yes — ask before production and we will prepare them with the shipment. It costs nothing extra; asking afterwards costs us both time.
Can our inspector attend? Yes, either your own or an appointed third party. We would rather have it looked at here than argued about later.
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