Magnetic Particle Testing

Magnetic Particle Testing (MT)

Magnetic Particle Testing (MT) is a widely established non-destructive method for the detection of surface breaking flaws as well as sub-surface flaws with depths of 2-3mm. The technique is refined by inducing a magnetic field in ferromagnetic materials. The surface is coated with a white contrast paint and over this paint iron particles suspended in liquid are applied. Surface and near surface flaws produce magnetic poles and iron particles are pulled in and concentrated, producing a visible indication of the defects on the surface of the material. The length of an indication can be accurately determined; however the depth cannot be determined with this technique.

Magnetic Particle Testing is a moderately easy test method that can be applied to ferromagnetic materials including finished articles, billets, hot rolled bars, castings and forgings. Magnetic particle testing requires magnetising the piece to be examined, applying the inspection medium or particles, and interpreting the patterns formed by the particles when they are attracted to the magnetic leakage field created by discontinuities in the part.

When inspecting a test part with the magnetic particle method it is essential for the particles to have an unimpeded path for migration to both strong and weak leakage fields alike. The part’s surface should be clean and dry before inspection. Magnetic Particle Testing can be performed using either dry particles, or particles suspended in a liquid.

Fluorescent Magnetic Particle Testing is more sensitive than the visible method. It is only available as a wet method. Contrast paint is not normally used. It requires the use of a UV-A lamp and a darkened area for viewing and interpreting indications. There are a variety of methods used to provide the magnetic field: yoke method (indirect), prod/clamp method (direct) and bench method – head shot (direct), coil (indirect). Typically, Magnetic Particle Testing will require access to a power source to create the magnetic field and for the UV-A lamp necessary for the fluorescent medium.