Care must be taken in the handling of parts that have been magnetized, particularly parts having smooth or machined
surfaces, to avoid their being rubbed together or against other ferromagnetic parts. Such rubbing may produce
localized magnetized areas on surfaces of parts that will attract and hold magnetic particles. Magnetic particle
indications produced on these areas are non-relevant and are called magnetic writing. An inspector may notice that
magnetic writing indications are not as sharp as those produced at surface cracks and is cautioned against
misconstruing such indications as being caused by subsurface discontinuities. Whether an indication is caused by
magnetic writing or by a subsurface discontinuity can be determined by demagnetizing and reprocessing the part.
Demagnetizing will remove the magnetic writing. If the indication returns after demagnetizing and reprocessing, it is
an indication of a discontinuity at or near the surface.
Currents used with the residual technique need be only great enough to magnetize the part sufficiently to show the type
of discontinuity being sought. Some gross discontinuities may require only weakly magnetized parts, and others, being
more difficult to find, may require the maximum residual field obtainable. The residual magnetic field retained in a
part is always less than the applied magnetic field strength that produced it. A maximum residual field strength results
when the magnetization level within the part reaches magnetic saturation. The use of magnetizing currents greater
than those needed to produce the maximum saturation field strength, are of no value with the residual technique.
Inspector experience with typical parts that have discontinuities is very helpful to determine what current levels should
be used to inspect a part using residual magnetism. In the absence of such experience, an inspector should first
determine whether or not a part could be inspected using the residual approach. Any part to be inspected must be
retentive enough so that magnetic particle indications will be formed at discontinuities in the parts. A rough
determination of a part's retentivity can be made by magnetizing the part in a coil with the maximum DC current
available. If, after magnetization, the part will lift and hold an ordinary steel paper clip, chances are good the part is
retentive enough for residual inspection. If the part will not hold a paper clip, residual techniques may still be possible.
The part could still be retentive enough to be inspected residually, depending upon the nature of the discontinuities
expected to be found. In this case, the inspector must resort to testing of the part, or parts, using the continuous
technique, inspecting for indications at discontinuities, then removing these indications and reapplying the magnetic
particle media to see if residual indications are produced. The current used to form the indications found with the
continuous technique will give an inspector some indication of the current level needed for residual inspection.
The application of magnetic particle media for residual inspection is simply a matter of covering the area to be
inspected. Care should be taken with a liquid suspension to ensure that the parts are adequately covered using low
velocity streams or sprays, and that the parts are positioned to take advantage of any particle flow resulting from
drainage on a part's surface. Some parts may need a longer drain time than others, since on smooth surfaces
indications may be slower in forming. In some cases, on bearing rollers for example, formation of fine indications may
be enhanced by immersing the magnetized part in liquid media for a considerable time. This permits time for the
leakage fields to attract and hold the maximum number of particles resulting in an increase in sensitivity.
Care must be taken when applying dry magnetic powders to parts that have been magnetized to avoid getting too much
powder on a part's surface. A combination of a light blowing and tossing action is needed, either from a hand-held
container or a pressurized powder blower. Additional care is also required when removing any excess powder from a
surface so as not to hinder formation of indications or remove indications already formed. The use of dry powder with
the residual technique has several disadvantages. It is more difficult to apply to interior surfaces of a part than is a
liquid suspension and it is more difficult to completely cover a part in a short time.
Liquid suspensions may be applied by stream, spray, or immersion of the part in a tank containing the media. Extra
care is required when using the immersion technique, particularly with parts that have smooth surfaces, to avoid
removing any indications by the rapid removal of a part from the bath. To ensure uniform concentration the