Magnetic particle inspection (MPI)

Magnetic field induction (MPI) is a technique used to detect cracks and faults in ferromagnetic materials. If the material is structurally sound, the magnetic flux is concentrated inside the material, but if there’s a crack, the magnetic field is distorted, and the magnetic flux leaks out. Small iron particles cover the surface and demonstrate the leakage flux. Due to their low magnetic susceptibility, this technique works for ferritic steels and irons but not austenitic steels. Magnetization can be generated through various methods, such as applying a permanent or electro-magnet, passing a large current through the specimen, or placing the specimen inside a current-carrying coil. Residual magnetization is sometimes used, where iron particles are applied after magnetization, either as a dry powder or suspended in a liquid.

MPI Operating Methods:

1. Direct magnetization or indirect magnetization are both used to magnetize the test piece.

• Direct Magnetization: A magnetic field is produced by running an electric current through the test object.
• Indirect magnetization: An external magnetic field is applied.

2. Application of Magnetic Particles: After the magnetised component, magnetic particles coat it. These particles can be dry powder or moist, suspended in a liquid. To increase visibility, the particles are frequently coloured or fluorescent.
3. Inspection: Any surface or near-surface imperfections will cause the magnetic field to be interrupted and leak out as it passes from one pole of the material to the next. “Magnetic flux leakage” is the term used to describe this phenomenon. These leaking spots draw the magnetic particles, which create visible indicators on the test piece’s surface. These accumulation patterns can be used to determine faults’ kind, form, and orientation.

4. Demagnetization: If the test component is returning to service or undergoing additional processing, it is crucial to demagnetize it after inspection. Remaining magnetism may obstruct the component’s proper use or operation.
5. The indications created by the magnetic particles are then assessed, and if found to be significant, they are reported.

Read more: How Does Magnetic Particle Inspection Works?

Benefits of MPI:

• It is swift and reasonably priced.
• Able to find even minor surface fractures.
• Easily transportable, enabling on-site inspections.

Limitations:

• It is only applicable to ferromagnetic materials.
• Mostly, it finds surface and near-surface flaws.
• The right magnetization is essential to avoid missing flaws.
• The geometry and size of the component might affect the capacity to create the required magnetic field.

Safety considerations: As with many industrial operations, safety comes first. Electrical safety, careful handling and disposal of consumables (such as magnetic particles), and protection from ultraviolet radiation if fluorescent particles are employed are all crucial factors to consider when conducting MPI.