Flaws can also develop during the hardening process of material if this is carried out incorrectly. In order to pre-empt costly and hazardous incidents, a magnetic particle test is recommended.

Also known as magnetic particle inspection or MPI, this is a non-destructive type testing (NDT) implemented to discover cracks and other discontinuities on or near the surface of ferromagnetic and electrically conductive materials, and is one of the most used techniques to this end.

The magnetic particle testing procedure is a relatively straightforward method, although it requires both attention and experience so as to avoid false negative results.

The first step of the process consists of evaluating the test subject, which is then prepared for inspection, opting for a cleaning process that does not adversely affect its characteristics.

The test is in fact classified as an NDT since it does not cause unacceptable deformation of test pieces. After the cleaning, the appropriate test method, either white contrast or fluorescent, is selected .

Next, the process involves using AC or DC yokes to magnetise the part under test, before application of the magnetic particles themselves. The specimen can be magnetised either locally or as a whole, with the magnetic poles placed between 7cm and 25cm from each other.

Key to a correct evaluation is the activation of the magnetic charge so that they are immediately free to be attracted to any leakage fields on the test piece.

Additionally, it is imperative to make sure that 100% of the component’s exterior is examined so as not to miss any flaws.

After the element is demagnetised, it is time to visually inspect it for any irregularities.

If there is an exterior or near-surface fracture, the magnetic field will be altered by a local flux leakage at the site of the damage, creating a visible indication of discontinuities on the component’s surface.

The nature and size of the discontinuity can be roughly estimated by the amount, density and shape formed by the clustered magnetic particles.

An inspection by means of magnetic particle testing is suitable for most equipment and pieces composed of ferromagnetic material. Ideal conditions for testing are as-welded, as-cast, as-rolled or as-forged and smooth blasted surfaces.

Industry applications range widely and comprise all those businesses involved in lifting, moving or otherwise handling substantial weights as part of their operations: mining, equipment manufacturing, metal processing and materials handling to name a few.

While law often mandates magnetic particle testing frequency, it is recommended to periodically carry out this inspection so as to avoid unpleasant surprises. Additionally, MPI is a valuable instrument to reveal manufacturing or processing defects, for instance after heat treatment.

Reliance Hexham offers expert magnetic particle testing services thanks to the extensive experience of our skilled team of qualified technicians. Our laboratory is accredited to ISO 17025 – General Requirements for the Competence of Testing.