# Magnetic particle inspection (MT / MPI)

> Magnetic particle inspection (MT, also MPI) magnetises a ferromagnetic part and applies fine iron particles; flux leaking at a surface or near-surface discontinuity attracts the particles into a visible indication.

- Source: https://amps.global/glossary/magnetic-particle-inspection/
- Type: Glossary
- Published: 2026-09-05
- Tags: NDT, magnetic particle inspection, surface methods

## Key facts

- Ferromagnetic materials only (carbon and low-alloy steels, not austenitic stainless or aluminium)
- Magnetisation by electromagnetic yoke, prods, coils or central conductor; particles dry or in suspension, visible or fluorescent under UV-A
- Detects surface and slightly subsurface flaws; parts may need demagnetising afterwards
- Codes: ASME BPVC Section V Article 7, ISO 17638 (welds), acceptance in ISO 23278

Also known as: MT, MPI, magnetic particle testing, magnetic particle examination.

## How it works

A magnetic field is induced in the part. Where a crack cuts the flux lines, the field leaks out of the surface and holds the particles applied to it, drawing an indication that is wider than the crack itself and easy to see, especially with fluorescent particles under UV light. Field strength and direction are verified with indicators such as a pie gauge or shims.

## Where it is used

Welds in structural steel, pressure equipment and pipelines, machined components, lifting equipment, castings and forgings. It is fast, cheap and works through thin coatings, which makes it the default surface method on steel; liquid penetrant is used instead on non-magnetic materials.

## MPI in AMPS

The AMPS AI report assistant is illustrated on an MPI report: the note "MPI on 2 welds, they comply" was rewritten as "Magnetic Particle Inspection conducted on two circumferential welds confirms compliance with ASME B31.3 standards". In AMPS, reports are produced from your own Microsoft Word templates through the template engine: job data such as the client, purchase order number, equipment serial numbers, location and dates is filled in automatically, an AI assistant can tidy the wording, the report can be signed in AMPS, and the finished Word file is saved in the job's report folder in your OneDrive, SharePoint or Google Drive.

## Related pages

- [Liquid penetrant testing (PT)](https://amps.global/glossary/liquid-penetrant-testing/)
- [Using the AMPS reporting system](https://amps.global/case-studies/04-reporting-system/)
- [AWS D1.1](https://amps.global/glossary/aws-d1-1/)

## Sources

- [Magnetic particle inspection on Wikipedia](https://en.wikipedia.org/wiki/Magnetic_particle_inspection)
- [ASME Boiler and Pressure Vessel Code](https://www.asme.org/codes-standards/bpvc-standards)
- [ISO - International Organization for Standardization](https://www.iso.org/)