T.O. 33B-1-1
3-60
discontinuities. This produces an increase in sensitivity over the mere flowing of the bath over the surface of the part
as it is being magnetized by the continuous method. It should be noted, however, that the location of the discontinuity
on the part as it is immersed affects particle buildup. Build-up will be greatest on horizontal upper surfaces, and less
on vertical surfaces or horizontal lower surfaces. Also, rapid withdrawal from the bath or spray can wash off
indications held by extremely weak leakage fields, and care must be exercised in this part of the process. The residual
method, either wet or dry, has many attractive features and finds many applications, even though the continuous
method has the inherent advantage of greater sensitivity.
3.5.4.2
Continuous Method.
The reason for greater sensitivity for the continuous method is straightforward. When the magnetizing force is applied
to a ferromagnetic part, the field rises to a maximum. Its value or intensity is derived from the strength of the
magnetizing force and the material permeability of the part. When the magnetizing force is removed, the residual
magnetism in the part is always less than the field present while the magnetizing force was acting. The amount of
difference depends on the retentivity of the material. The continuous method, for a given value of magnetizing current,
is always more sensitive than the residual as determined by the strength of field in the part. Techniques have been
developed for the continuous method, which make it faster than the residual. The indication is produced at the time of
the two magnetization shots and the sixty second migration of the magnetic particles as the residual vehicle drains from
the part. The residual method requires two steps, magnetization and application of particles, plus the added time for
indications to build up if the immersion method is used. The continuous method is preferred unless special
circumstances make the residual method more desirable.
3.5.4.2.1
The continuous method is the only effective one to use on low carbon steels or iron having little retentivity. It is
frequently used with AC on such materials because the alternating current field produces excellent mobility of the
particles. With the wet method the usual practice is to flood the surface of the part with the bath, then simultaneously
terminate bath application and apply the magnetizing current momentarily. Thus the magnetizing force acts on the
particles in the film of the bath as they are draining over the surface. Strength of the particle bath has been
standardized to supply a sufficient number of particles in the film to produce good indications with this technique. It
should be noted that the continuous method requires more attention and alertness on the part of the inspector than does
the residual method. Careless handling of the bath/current application sequence can seriously interfere with reliable
results.
3.5.4.2.2
Probably the highest possible sensitivity obtainable for very fine defects is achieved by immersing the part in the wet
bath, magnetizing the part for a short time while immersed, and continuing to magnetize while the part is removed
from the bath and while the bath drains from the surface.
3.5.5
Dry Powder Magnetic Particles.
CAUTION
Dry powder method SHALL NOT be used on aerospace vehicles or aerospace parts
without specific approval of the appropriate engineering authority for the individual
inspection requirements.
3.5.5.1
General.
The dry powder method is used for the inspection of welds and castings where the detection of defects lying wholly
below the surface is considered important. The particles used in the dry method are provided in the form of a powder.
They are available in red, black, yellow, and gray colors. The magnetic properties, particle size and shape, and coating
method are similar in all colors making the particles equally efficient. The choice of powder is then determined
