Magnetic particle inspection is an effective way to detect product defects

At the moment, there are many methods for checking the condition of surfaces of industrial products made of ferromagnetic materials. They help determine the quality of products. One of the proven and recognized methods is magnetic particle inspection. This method allows you to identify various kinds of defects - cracks, flaws in welded joints and so on.

Description of the method

Magnetic particle inspection is designed to test different products: from large to small items, including all kinds of their shapes. It is an effective method of verification. In this case, certain features of the material must create a magnetization. It is necessary for the formation of a field for eliminating product flaws. This will promote the attraction of particles of a special powder. For a greater chance of finding flaws, it is necessary that their plane be at right angles to the flux current. This is an important condition. In the case of a reduction in this angle, the ability to detect defects is reduced.

Feature of the method

Magnetic particle inspection method depends on certain properties of the material of products. Their presence is important in this case. These include:

• Magnetic induction.
• Coercive force.
• Residual magnetization.
• The presence of roughness on the surface of the product.
• The greatest magnetic permeability.
• Illumination of the surface of the product.
• The strength of the magnetic field and its location relative to the plane of the defect.

Application area

This method is used in various industries. In general, magnetic particle testing is used in machine, ship and automobile industries. It is also used in the oil and gas industry. Magnetic particle inspection (GOST 21105-87) is characterized by high productivity, sensitivity, which allows to detect the presence of flaws at the time of their appearance. This is its important property.

Magnetic particle inspection: method steps

1. Preparation of the product for inspection. The surface is cleaned from dirt, rust and various oils. If the surface of the product is dark, then the magnetic powder is not visible. In this case, a layer of white paint is applied to the part.
2. Magnetization of the product. This is the main stage of control. It is important to choose the correct magnetization method. This determines the sensitivity and likelihood of finding flaws.
3. Coating the surface of the product with a magnetic powder or suspension. This stage requires considerable care. When the suspension is applied, the product is immersed in a container with this liquid, or, more technically, it is poured from the hose under a weak pressure. The use of dry powder is characterized by its application by air flow.
4. Inspection of the product. Reading the picture and sorting. The inspector should inspect the part visually or with the use of optical instruments after applying and removing the magnetic substance. To sort out, you need to compare with the existing photos of different flaws.
5. Demagnetize and remove the remaining indicated indicator. This stage is final. In this case, there are two demagnetization options:

• The most effective is the heating of the product to the Curie temperature point. In this case, the magnetic properties of the material are lost. However, this method is used very rarely. This is due to the fact that with this heating, there may be a change in the mechanical properties of the material of the product, which in many cases is unacceptable.
• The second method is based on carrying out the demagnetization of a part by means of an alternating magnetic field with an amplitude that should uniformly decrease from a certain maximum point to zero. This method is more popular. The frequency of the applied magnetic field depends on the size and shape of the part being checked.