9: Non-destructive Examination
API ICP Self Study Materials
API ICP Self Study Materials
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9.4 MAGNETIC PARTICLE EXAMINATION (MT)<br />
9.4.1 General<br />
Magnetic particle examination is effective in locating surface or near surface<br />
discontinuities of ferromagnetic materials. It is most commonly used to<br />
evaluate weld joint surfaces, intermediate checks of weld layers and backgouged<br />
surfaces of the completed welds. Typical types of discontinuities that<br />
can be detected include cracks, laminations, laps, and seams.<br />
In this process, the weld (and heat-affected zone) is locally magnetized,<br />
creating a magnetic field in the material. Ferromagnetic particles are then<br />
applied to the magnetized surface and are attracted to any breaks in the<br />
magnetic field caused by discontinuities as shown in Figures 21 and 22.<br />
Figure 21 shows the disruption to the magnetic field caused by a defect open<br />
to the surface. Ferromagnetic particles will be drawn to the break in the flux<br />
field. The pattern of the particles will be very sharp and distinct. Figure 22<br />
illustrates how a sub-surface defect would also disrupt the magnetic lines of<br />
flux. The observed indication would not be as clearly defined, as would a<br />
defect open to the surface.
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