Inspection method · MT
Magnetic particle testing (MT)
A sensitive and practical method for finding cracks at or just below the surface of ferromagnetic components.
01How it works
The component is magnetised with a yoke, prods, coils or current flow. Discontinuities disturb the magnetic field and create leakage fields that attract finely divided ferromagnetic particles, dry or in suspension, visible or fluorescent. Indications form at cracks transverse to the field direction, so testing is done in two directions.
Field strength is verified with indicators, and surface preparation (removing paint, scale and grease) is essential.
02Typical industrial applications
- Weld toes and fillet welds
- Forgings and shafts
- Castings of carbon and low alloy steel
- Machined components and gears
- In-service crack detection
03Benefits and what it detects
- High sensitivity for surface cracks
- Fast and economical
- Detects slightly subsurface flaws
- Portable equipment
04Limitations
- Only ferromagnetic materials, not austenitic stainless or aluminium
- Requires two directions of magnetisation
- Surface coatings reduce sensitivity
- Demagnetisation may be required
- Limited depth
05Specifying and reviewing the method
Specify method standard, technique (yoke, fluorescent or colour contrast), extent and acceptance levels. ISO 17638 with acceptance levels in ISO 23278 are common for welds.
Other references: ISO 17638, ISO 23278, ISO 9934