Current Level and Particle Application
Two methods of processing are used in magnetic particle testing. The method to use in a given case depends on the magnetic retentivity of the test object being tested and the desired sensitivity of the testing to be made. Highly retentive test objects may be tested using the residual method. The continuous method must be used on test objects having low retentivity. For a given magnetizing current or applied magnetizing field, the continuous method offers the greatest sensitivity for revealing discontinuities.
Wet method media may be applied to retentive test objects, which have been magnetized using the induced current method, while the test object is being magnetized or at any subsequent time. If applied to low retentivity test objects while being magnetized, careful control of the bath application is required to prevent washing away of indications after the magnetizing current has ceased.
On some test objects having very fine transverse discontinuities, it may be necessary to apply the liquid media (bath) while the test object is in a vertical or near vertical position. This allows the liquid to flow downward by gravity and across the discontinuities assisting in the formation of those indications that may be slow to develop.
Test objects having fine grinding cracks may require this technique. In extreme cases, as much as 5 s may be needed to fully develop slow forming indications. For test objects having extensive grinding cracks, fatigue or heat treat cracks, 0.5 to 1 sis usually enough time to form visible indications, particularly when the media is fluorescent.
Test objects made of low retentivity steels must have wet method media applied while they are being magnetized. Test objects that have smooth machined surfaces will need extra care when handled and tested to prevent washing away of indications by liquid retained on the surface. On test objects having rougher surfaces, such as those produced by sand blasting, indications are usually held by mechanical bond so that washing away of indications is less of a problem. However, the rough surface finish tends to accumulate more particles, so bath concentrations must be carefully maintained.