Magnetic Particle Testing (MPT) is a method used to detect surface and near-surface flaws in various ferromagnetic materials. It is primarily used for detection of cracks. The specimen is magnetised either locally or overall. If, anyhow there is a flaw, the magnetic field gets distorted, causing leakage in the local magnetic flux. This leakage is displayed by covering the surface with very fine iron particles applied either dry or suspended in a liquid. The particles accumulate at the regions of flux leakage, producing a build-up which can be seen visually even when the crack opening is very narrow. Thus, the crack can be identified as a line of iron powder particles.
The method can be applied on almost all metals which can be strongly magnetized, like ferritic steels and irons. It is a type of inspection process included in the non destructive testing.
The method of magnetisation is to produce a magnetic field with lines of force at a large angle to the expected direction of the cracks. The application of magnetisation can be used more than once in different directions based on the requirement.
The magnetisation is generally produced by any of these following methods:
• Applying a permanent or electro-magnet to the surface (magnetic flow)
• Passing a large current through the specimen by means of current prods (current flow)
• Putting the specimen inside an electrified coil, or forming a coil around the specimen
• Making the specimen a secondary loop of a transformer (suitable for ring-shaped specimens)
• Placing an electrified coil or loop close to the specimen’s surface
• Threading an electrified bar through a hollow specimen.
The application of electricity
The electric current used can be of any waveform from DC to AC. The amount of electricity required to produce adequate magnetisation is dependent on the supply, process and material of the specimen. It is important to ensure that the applied electricity is appropriate for the specimen size and shape and also that the direction of the magnetic flux produced is suitable for the cracks expected.
The process of applying iron particles
Usually the iron particles, dry powder or liquid (magnetic ink) is applied while the magnetising current is still flowing. Residual magnetisation is also used at times, when the particles are applied after magnetisation. Some steels retain sufficient magnetic property for this method to be satisfactory, and in this case smaller, more portable, magnetising equipment can be used.
If you want to have an inspection done through this process, consider to call a professional inspecting team in Melbourne.