Magnetic particles: what they are and how the test is performed

Magnetism is a phenomenon in which forces of attraction or repulsion are exerted.

A little history of magnetism

The discovery of magnetism dates back more than two thousand years in the history of mankind. When a black stone capable of attracting iron was discovered in a Turkish city called Magnesia. This rock was given the name lodestone, more commonly known as lodestone.

The properties of lodestone were already known to the Greeks, Romans, and Chinese. When a lodestone is rubbed against a piece of iron, this piece is magnetized and is capable of attracting others. In this way it was discovered that the magnets generated in this way are polarized. Each end of the magnet was given the name “north pole” and “south pole”. These always exist, no matter how many times the magnet is broken.

It was the British scientist William Gilbert who established the physical law that “opposite poles attract, like poles repel”. Gilbert dedicated himself to investigating magnetism, electricity, and the relationship that could exist between the two. This is how he came to the conclusion that the earth behaves like a giant magnet. And that magnetism is a natural force.

The field of action of magnetic particles

Known as a magnetic field, it is the scene of action of magnetic particles. It is represented by “lines of force”, which establish the direction of the magnetic field. Also, they define its intensity as inversely proportional to the space between the lines of force.

Relationship between magnetism and electricity

Magnetic particles could not act unless electricity intervened with them. Magnetism is the result of the movement of electrons in atoms. Thus it is defined as a property of moving charge that is closely related to the electrical phenomenon.

How is the magnetic particle test performed?

Magnetic particle inspection is a non-destructive test used for surface and slightly subsurface quality control of ferromagnetic materials.

Next we explain the steps to follow for a test of this type:

Prepare the surface

The surface to be inspected must be cleaned. So that it is totally free of grease, paint, scale or other materials.

check system

Depending on the type of particle to be used and the magnetization equipment, different points are verified; in general, light intensity, particle concentration, field intensity, etc. are verified.

Configure magnetization

To inspect the part, it must be determined how it is going to be magnetized:

Here are three factors to consider:

  1. Type of current and magnetization technique. As there are different types of electrical current, there are also different techniques to apply the current to a part. It is convenient to review different techniques before applying them, to know which one is the most convenient for the piece we are going to work on.
  2. As with the magnetization method. It is convenient to determine which is the most suitable amperage for the piece that we are going to inspect. The amperage will depend on the piece, the type of metal, and the defect that we want to correct.
  3. Magnetic field intensity. This can be checked with a magnetic flux indicator strip, Hall effect meter, or QQI.

Particle Application

They can be applied in a uniform cloud in the case of dry particles, or with a water or oil-based bath in the case of wet particles.

Application of electrical current

While the particles are being applied, current is applied for inspection, or in some cases, inspection with residual magnetism can be performed.

part inspection

The piece must be evaluated under the appropriate lighting conditions, either with UV light or white light, as the case may be. Indications will be observed, measurements taken, and findings documented.

additional magnetization

This step will depend on the piece and the process that has been followed according to its nature. If additional magnetization is required, steps 3-6 above should be repeated.

demagnetize

After the final inspection is complete, the part will be demagnetized using a demagnetizing device such as: coil, yoke, etc. A residual magnetic field meter (gaussimeter) should be used in order to verify that the residual magnetism is at an acceptable level.

Final cleaning

In addition to demagnetization, it is recommended to give the piece a final cleaning to ensure that any residue of magnetic particles is removed.