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Quality and Inspection Process Knowledge
Quality and Inspection Services
Magnetic particle inspection is a method for detecting cracks,
laps, seams, voids, pits, subsurface holes, and other surface, or slightly
subsurface, discontinuities in ferro-magnetic materials. Magnetic particle
inspection can be used only on ferro-magnetic materials (iron and steel). It can
be performed on raw material, billets, finished and semi finished materials,
welds, and in-service assembled or disassembled parts. Magnetic particles are
applied over a surface either dry, as a powder, or wet, as particles in a liquid
carrier such as oil or water. Common uses for magnetic particle inspection are;
final inspection, receiving inspection, in-process inspection; and quality
control, maintenance, and overhaul.
PRINCIPLES OF OPERATION: Magnetic particle inspection uses the
tendency of magnetic lines of force, or flux, of an applied field to pass
through the metal rather than through the air. A defect at or near the metal’s
surface distorts the distribution of the magnetic flux and some of the flux is
forced to pass out through the surface. (See figure 1.) The field strength is
increased in the area of the defect and opposite magnetic poles form on either
side of the defect. Fine magnetic particles applied to the part are attracted to
these regions and form a pattern around the defect. The pattern of particles
provides a visual indication of a defect. (See figure 2.)
Figure 1
Figure 2
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