Magnetic particle inspection is used to detect surface defects, emerging defects or defects on the inner walls of equipment.
The sensitivity of defect detection depends on the type, direction and intensity of magnetisation on the part, which of course must be ferromagnetic itself, and therefore made of steel. This applies to all grades of steel except for stainless austenitic steels, which are non-magnetic.
The type and general orientation of the magnetic field will depend on those of the electrical current used to produce it either by direct magnetisation using an electromagnet, or by passing the current itself through the part to be inspected (delivering a circumferential field). A continuous current will produce a continuous magnetic field, favourable for detecting defects under the surface. In practice, we seek to use affordable generators, and this leads to the use of alternating currents with half or full wave rectification. The use of an alternating current, and therefore an alternating magnetic field, is common. In this case, a skin effect is created, reinforcing the magnetic field on the surface of the part and increasing, all other things being equal, the power of detection for very fine cracks open to the surface.
Magnetic particle inspection is faster than dye penetrant inspection.