Industrial Magnetic Particle Testing (MT)

Magnetic particle testing (MT) locates surface and shallow sub-surface discontinuities in ferromagnetic materials. When the part is magnetised, a crack interrupts the magnetic flux and creates a leakage field at the surface; the ferromagnetic particles applied over the area gather in that field and form an indication that reproduces the shape, length and orientation of the defect.

The method works only on materials that can be magnetised: carbon steels, ferritic steels, low-alloy steels and cast irons. It does not apply to austenitic stainless steels, aluminium, copper or their alloys. Magnetisation is produced with an electromagnetic yoke, prods or a coil, and the field has to run across the discontinuity being sought, which is why every area is covered in two directions 90° apart so that cracks parallel to the field are not missed.

The particles are applied as dry powder or in a wet suspension, with visible pigment for evaluation under white light or fluorescent pigment for evaluation under ultraviolet light in a darkened area. On tight cracks with very close faces, such as fatigue cracks, the method is more sensitive than penetrant testing, because it does not depend on a liquid reaching the inside of the defect. Once the test is finished the part is cleaned and, where residual magnetism would affect service, machining or later welding, it is demagnetised.

At Compañía Técnica the test is carried out and interpreted by technicians qualified to IRAM-ISO 9712, following ASNT recommended practices and the acceptance criteria of the ASME code or of the client specification. It is the usual method on welded joints, shafts and drive axles, forged parts, hooks and lifting components, both in fabrication and on equipment in service. The report records the magnetisation method, the type of particles, the lighting used, and every indication with its location and extent on a sketch of the part.