Magnetic Particle Examination – Magnetic Particles
The magnetic particles that are applied to the surface of a workpiece when it is magnetized are of two types, dry and wet; and they are classified according to how they are carried to the workpiece. Dry particles are carried to the surface of the workpiece by air, whereas wet particles are carried by a liquid. Both dry and wet particles must have a high magnetic permeability so that they can be attracted by relatively low levels of magnetic leakage at flaws to assure a high sensitivity for detection. Furthermore, the particles must have suitable shape and size for adequate mobility on the surface of the workpiece to move to the locations of magnetic leakage at flaws.
Dry particles should be applied to the surface of the workpiece with low velocity air as a uniform cloud with a minimum of turbulence. This will allow the particles to be attracted by the magnetic leakage at flaws while they are suspended in air and their movement is not unduly influenced by air currents and gravity. Dry particles are especially useful for rough surfaces that inhibit the flow of a liquid, and they are somewhat more sensitive than wet particles for detecting subsurface flaws. Another advantage of dry particles is that they can be applied to surfaces that are too warm for liquids, and therefore can permit some pressure vessels to be examined during operation. The dry particles are usually colored yellow, red, or black to contrast with the workpiece surface. Dry particles are also available with fluorescent coatings that give them a very high visibility in ultraviolet (black) light.
Wet particles are suspended in light oil or water, and the slurries are generally applied to the surface of the workpiece by spraying. The concentration of suspended particles in the liquid must be high enough to give observable indications of fine flaws, but too high a concentration can result in a confusing background that obscures the indications of flaws. Frequent agitation is necessary to maintain a uniform concentration of particles in the slurry. The best way to determine proper concentration of particles in the slurry is to perform a test with a specimen that contains known discontinuities.
Wet particles are more sensitive than dry particles for detecting fine surface cracks. They generally have a fluorescent coating that makes them highly visible in ultraviolet (black) light, and they will generally adhere to the surface of the workpiece after the liquid has evaporated.
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