Magnetic particle inspection (MT or MPI) is a versatile non-destructive testing method for finding surface and near-surface defects in ferromagnetic materials, used both in the field and in the factory. The part is magnetised with a magnet or dedicated magnetising equipment. Where there is a discontinuity, the magnetic field is disrupted and a leakage field forms at the surface. Fine iron particles applied to the surface are attracted to this leakage field and gather directly over the defect, forming a visible indication. FKT NDT carries out magnetic particle testing on site.
Where It Is Used
- Welds and heat-affected zones in carbon and low-alloy steel
- Steel structures, bridges, shipyard and railway work
- Fabrication and periodic checks of pressure vessels, boilers and pipelines
- Castings, forgings, shafts, gears and lifting equipment
- Monitoring fatigue cracks in in-service equipment
The method only works on ferromagnetic materials. Austenitic stainless steel, aluminium and copper alloys are tested with liquid penetrant instead.
What Defects It Detects
- Surface cracks (weld, grinding, heat-treatment and fatigue cracks)
- Near-surface cracks and inclusions
- Lack of fusion and overlap breaking the surface
- Laps and seams in forgings
A discontinuity shows best when it lies at right angles to the magnetic field, so each area is normally magnetised in two perpendicular directions. Detection depth is limited; deeper internal defects require ultrasonic testing.
Magnetisation Techniques
The magnetic field can be introduced in several ways: with a permanent magnet or an electromagnetic yoke, by passing current directly through the part, or by passing current through a coil around the part or a central conductor through it. Two field types can be produced:
- Longitudinal field: runs parallel to the long axis of the part; produced with a coil or a yoke.
- Circular field: runs around the circumference; produced by passing current through the part or through a central conductor.
Particles can be applied dry or as a wet suspension, and can be visible (coloured) or fluorescent, viewed under UV-A light. With coloured particles, a thin white contrast paint is applied first to improve visibility.
Standards
Magnetic particle testing is commonly applied according to:
- EN ISO 9934-1 – Magnetic particle testing, general principles
- EN ISO 17638 – Magnetic particle testing of welds
- EN ISO 23278 – Magnetic particle testing of welds, acceptance levels
- ASME Section V, Article 7 – Magnetic particle examination
- ISO 9712 – Personnel certification
How the Inspection Works
- Preparation: the surface must be clean and dry. Oil, grease and loose scale hinder particle movement and affect the result.
- Magnetisation and particle application: particles are applied while the part is magnetised; each area is checked in two perpendicular directions.
- Evaluation: indications are examined under suitable lighting and assessed against the acceptance criteria.
- Reporting and post-treatment: the report covers the part, magnetisation technique, particle type, standard, findings and result. Where needed, the part is demagnetised and cleaned.
Advantages
- Detects both surface and near-surface indications
- Irregularly shaped parts are easy to inspect
- Surface preparation is less critical than for some methods, such as penetrant testing
- Fast, with indications visible directly on the part
- Considerably lower cost than many other NDT methods
- Highly portable, especially with battery-powered equipment
FAQ
Can magnetic particle testing be used on stainless steel?
Not on austenitic stainless steel, which is non-magnetic; penetrant testing is used there. Martensitic and ferritic stainless steels can be tested.
Can MT be done through paint?
Through thin coatings to a limited extent, but coating thickness reduces sensitivity. The acceptable thickness is set by the applicable standard and procedure.
How deep can MT find defects?
MT is essentially a surface and very-near-surface method. Deeper internal defects need ultrasonic or radiographic testing.
Is demagnetisation needed after testing?
It is done when residual magnetism would affect later welding or machining, or the service of the part.
For a magnetic particle testing quote, please contact us through our contact page.