Types of Magnetic Particle Inspection

Magnetic particle inspection is a non-destructive weld inspection technique that we here at Gammax pride ourselves on delivering. Magnetic particle inspections, also known as MPI, are used to find surface and near-surface discontinuities.

Conducting magnetic particle inspection is rather quick and simple because it does not require the level of surface preparation that other nondestructive test methods do. It is now one of the NDE techniques used most frequently as a result.

Below, we will go into more depth about the Magnetic Particle Inspection procedure.

Magnetic Particle Inspection in Detail

Wet Magnetic Particle Testing (WMPT) and Dry Magnetic Particle Testing (DMPT) are two versions of the relatively detailed MPT procedure.

The method starts in either case by passing a magnetic current through the component. Any flaws in the material will obstruct the flow of current and result in the spread of magnetism from them. A “flux leakage field” will be produced at the damaged area.

The component is covered with metal particles in the second process. The flux leakage field will pull the particles to the area of damage if there are any faults on or near the surface. This gives a clear picture of the flaw’s general size and shape.

Magnetic particle inspection has several advantages over other non-destructive weld test techniques. It is very portable, typically affordable, and doesn’t require a thorough pre-cleaning procedure. MPT is one of the best methods for finding small, shallow surface cracks. It moves quickly and easily and penetrates thin coatings. The size and shape of test specimens are also subject to some restrictions.

The approach has limitations despite its benefits. The substance needs to be ferromagnetic, and the magnetic field’s direction and strength are equally important. Only surface and close-to-surface flaws are detected using the procedure. Those lower down need different approaches.

This method occasionally calls for large currents, which makes the “burning” of test parts possible. Additionally, the component needs to be demagnetised after MPT, which can occasionally be challenging.

Don’t worry, in any case. We have a lot of expertise in these situations and handle things accordingly!

How is Magnetic Particle Inspection carried out?

If the material is sound, the specimen is either locally or generally magnetised, and the magnetic flux is primarily internal to the material. But if a surface-breaking fault exists, the magnetic field is perturbed, leading to localised magnetic flux leakage in the vicinity of the flaw.

Applying very small iron particles to the surface, either dry or suspended in a liquid, will show the leakage flux. Even when the crack opening is very small, the particles build up at the areas of flux leakage, causing a build-up that may be observed visually. As a result, a line of iron powder particles on the surface represents a fissure.

This technique can be used with ferritic steels, irons, and any metal that can be highly magnetised, but typically not with austenitic steels.

It is typical to apply the magnetisation multiple times in various directions because the magnetisation method must create a magnetic field with lines of force at a significant angle to the anticipated direction of the cracks to be discovered.

However, there are ways to swing the field direction. At the same time, magnetisation occurs, for instance, in two directions that are mutually at right angles.

How is the magnetisation produced in Magnetic Particle Inspection?

We may use any of the following techniques to create the magnetisation:

a) applying a permanent magnet or electromagnet to the surface (magnetic flow);

b) passing a large current through the specimen or locally using current prods (current flow);

c) placing the specimen inside a current-carrying coil or forming a coil around the specimen;

d) using the specimen as the secondary loop of a transformer (induced current) – appropriate for ring-shaped specimens;

e) positioning a current-carrying coil or loop close to the specimen:

f) passing a current-carrying rod through a hollow specimen.

However, the amount of current needed to achieve sufficient magnetisation varies depending on the waveform of the supply, the technique for magnetisation, and the material of the specimen.

The electric current used can be either DC or AC with any waveform. It is crucial to check that the current being used is appropriate for the specimen’s size and shape and that the magnetic flux’s direction matches the anticipated cracks.

How are cracks detected when using Magnetic Particle Inspection?

A swinging or rotating magnetic flux can be created by combining two magnetic fields; this flux will detect a crack in any orientation.

It is customary to apply the iron particles, whether in the form of dry powder or liquid (for magnetic ink). At the same time, the magnetising current is still in motion. However, residual magnetisation is occasionally used when the particles are applied after magnetisation.

Several types of steel still have enough magnetism to make this approach effective. In these cases, smaller, more portable magnetising equipment can be utilised.

More frequently than dry granules, magnetic inks (particles suspended in a liquid) are utilised. They are sprayed using a low-pressure spray, dipping, or brush.

Using a lot of ink and giving the particles enough time to spread throughout the surface and move into any crevices is crucial. This might add a very small layer of white paint to provide a better contrast on dark surfaces.

There are other coloured particles available, as well as fluorescent particles that need UV-A illumination. The signs of cracks can be maintained through photography or clear peel-off sticky film.

While rough and soiled surfaces can be used with MPI methods, fault sensitivity may suffer. There have been created Magnetic techniques for underwater applications. MPI can only find sub-surface defects under very specific circumstances.

How can we help you?

By applying a direct or indirect magnetic field to a component, our technicians at Gammax can identify faults in ferromagnetic materials. Continually improving our client experience helps us maintain our reputation for excellence. Our trained NDT personnel offer high-quality MT services to various sectors across the UK. Don’t hesitate to get in touch with us to discuss any services that you may require.

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