Magnetic Particle Inspection in Welding

Principle – Magnetic Particle Inspection

Magnetic particle inspection is a method of detecting invisible cracks and other defects in ferromagnetic materials such as iron and steel. It is not applicable to nonmagnetic materials. The inspection process consists of magnetizing the part and then applying ferromagnetic particles to the surface area are to be inspected. If a defect is present, the magnetic lines of force will be disturbed and opposite poles will exist on either side of the defect.

The magnetized particles form a pattern, as shown in image, in the magnetic field between opposite poles. This pattern known as “indication” assumes the approximate shape of the surface projection of the defect.

In this experiment, commercially available magnetic powder manufactured for NDT inspection will be used. A strong U shape magnet will be used to provide the necessary magnetic field at the inspected area.

This test is used to detect cracks, porosity and inclusions in the welding. It is mainly used for testing ferromagnetic materials (those that can be magnetized). Magnetic particle inspection can detect surface and near surface defects.

Steps used in the testing

The following steps are applied during the inspection;

• The surface of the specimen is roughly cleaned wiping with a piece of textile.
• The fluorescent magnetic spray is applied on the surface being inspected.
• Magnetic field is applied with a strong magnet to the location of interest. The iron powder is attracted to the crack and the iron powder will be gathered near the cracks.
• The spots where the fluorescent magnetic particles accumulated is inspected under UV light.

Advantages

1. Large surface areas of complex parts can be inspected rapidly.
2. The test can detect surface and subsurface flaws.
3. Surface preparation is less critical than it is in penetrant inspection.
4. Magnetic particle indications are produced directly on the surface of the part and form an image of the discontinuity.
5. Equipment costs are relatively low.

Limitations – Magnetic Particle Inspection

• Only ferromagnetic materials can be inspected.
• A proper alignment of magnetic field and defect is critical.
• Large currents are needed for very large parts.
• It requires relatively smooth surface.
• Paint or other nonmagnetic coverings adversely affect sensitivity.
• Demagnetization and post cleaning are usually necessary.