Magnetic Particle Test
Magnetic particle testing (MT) is a nondestructive testing (NDT) method for detecting discontinuities that are primarily linear and located at or near the surface of ferromagnetic components and structures. MT is governed by the laws of magnetism and is therefore restricted to the inspection of materials that can support magnetic flux line. Metals can be classified as ferromagnetic, paramagnetic, or diamagnetic.
Ferromagnetic metals are those strongly attracted to a magnet and can become easily magnetized. Examples include iron, nickel, and cobalt.
Paramagnetic metals such as austenitic stainless steel are very weakly attracted by magnetic forces of attraction and cannot be magnetized.
Diamagnetic metals are very slightly repelled by a magnet and cannot be magnetized. Examples include bismuth, gold, and antimony.
Only those metals classified as ferromagnetic can be effectively inspected by MT. In order to understand MT, one should have a basic understanding of magnetism and electromagnetism.
Prods, yokes, coils suitable for inducing magnetism into the weld. Power source (electrical). Magnetic powders. Some applications require special facilities and ultraviolet lights.
Most weld discontinuities open to the surface. Some large voids slightly subsurface. Most suitable for cracks.
Relatively economical and expedient. Inspection equipment is considered portable. Unlike dye penetrants, magnetic particle can detect some discontinuities slightly below the surface.
Applicable only to ferromagnetic materials. Parts must be cleaned before and after inspection. Thick coatings may mask rejectable discontinuities. Some applications require parts to be demagnetized after inspection. Magnetic particle inspection requires use of electrical energy for most applications.
Source: American Welding Society Inc., The McGraw-Hill Companies Inc.