25 Magnetic Particle Examination (MT)

Magnetic particle examination (MT) is a test in which colored iron particles are dusted across the welded surface and an electromagnet is placed so that the iron gathers in specific ways to reveal a discontinuity.  The test is moderately expensive and requires training to conduct properly.

MT is used to detect surface or very near subsurface discontinuity indications in ferromagnetic metals. Crack types detected by MT include crater cracks, transverse cracks, longitudinal cracks, and toe cracks.

Procedure

When a magnetic field is established (by circular or longitudinal magnetization) in a piece of ferromagnetic material containing one or more discontinuities, magnetic poles are set up at the discontinuities. Colored magnetic particles are applied to the material (by the dry or wet magnetization method). Impurities or discontinuities in the magnetized material interrupt the lines of magnetic force, showing the size, shape, and location of defects, as the colored particles concentrate around the defects. The patterns are usually characteristic of the type of discontinuity detected.

Maximum sensitivity with MT is obtained from linear discontinuities either oriented perpendicular to or laying at a 45° angle to the lines of magnetic flux. For this reason, each area should be examined twice, with the lines of magnetic flux during the second examination approximately perpendicular to the lines of flux during the first examination.

For efficient coverage of welds when using the prod method, prods must be crisscrossed and spaced appropriately. This is to ensure coverage over the entire weld, so the inspector doesn’t miss any discontinuities. Wet or dry colored iron particles are applied to the surface while the magnetizing current is switched on and the prods are in contact with the surface.

After examination, demagnetization is mandatory for parts in critical service, such as engines and aircraft, that have been strongly magnetized. Filings, grindings, and chips resulting from operational wear are attracted to magnetized parts and interfere with performance.

Key Terms

Circular magnetization is a concentric magnetic field produced by a straight conductor, such as a piece of wire, carrying an electrical current. Circular magnetization is produced by a contact head, central conductor, or prods.

Longitudinal magnetization is a magnetic field produced when the current-carrying conductor is coiled and the magnetic field is parallel to the axis of the coil. The magnetic field strength produced within a coil increases in proportion to the number of loops within the coil. Longitudinal magnetization is achieved by coil or yoke.

The magnetic leakage field is the magnetic field that leaves or enters the surface of a part at a discontinuity or change in section configuration of a magnetic circuit. When a part with discontinuities is magnetized, a magnetic leakage field is produced at the discontinuities. Colored iron particles congregate at leakage fields and indicate the approximate shape of a discontinuity.