Magnetic Particle Testing (MPT/MT)
Magnetic particle testing or MPT is a nondestructive testing method for locating surface and near surface discontinuities in ferromagnetic materials. It depends for its operation on the fact that when the material or part under test is magnetized, discontinuities that lie in a cause leakage field to the direction of the magnetic field will cause a leakage field to be formed at and above the surface of the part. The presence of this leakage field, and therefore the presence of the discontinuity, is detected by the use of finely divided ferromagnetic particle applied over the surface, some of the particle being gathered and held by the leakage field. This magnetically held collection of particle forms an outline of the discontinuity and generally indicates its location, size, shape and extent. Magnetic particles are applied over a surface as dry particles, or as wet particle in a liquid.
Ferromagnetic materials include most of the iron, nickel and cobalt alloys. These materials lose their ferromagnetic properties above a characteristic temperature called the Curie point which is approximately 760◦ C for most of the ferromagnetic material.
Applications
The principal industrial uses of magnetic article testing are final inspection, receiving inspection, in process inspection and quality control, maintenance and overhaul in the transportation industries, plant and machinery maintenance and inspection of large components.
Limitations
Thin coatings of paint and other non-magnetic coverings, such as plating; adversely affect sensitivity of magnetic particle inspection. Other limitations are:
- Magnetic particle inspection methods will work only on ferromagnetic materials.
- For best results, the magnetic field must be in a direction that will intercept the principle plane of the discontinuity. Sometimes this requires two or more sequential inspections. With different magnetizations.
- Demagnetization following magnetic particle testing is often necessary.
- Post cleaning to remove remnants of the magnetic particle clinging to the surface may be required after testing and demagnetization.
- Exceedingly large currents sometimes are required for very large parts.
- Care is necessary to avoid local heating and burning of finished parts or surface at the points of electric contact.
- Although magnetic particle indications are easily seen, experience and skill in interpreting their significance are needed.