T.O. 33B-1-13-60discontinuities. This produces an increase in sensitivity over the mere flowing of the bath over the surface of the partas it is being magnetized by the continuous method. It should be noted, however, that the location of the discontinuityon the part as it is immersed affects particle buildup. Build-up will be greatest on horizontal upper surfaces, and lesson vertical surfaces or horizontal lower surfaces. Also, rapid withdrawal from the bath or spray can wash offindications held by extremely weak leakage fields, and care must be exercised in this part of the process. The residualmethod, either wet or dry, has many attractive features and finds many applications, even though the continuousmethod has the inherent advantage of greater sensitivity.3.5.4.2 ContinuousMethod.The reason for greater sensitivity for the continuous method is straightforward. When the magnetizing force is appliedto a ferromagnetic part, the field rises to a maximum. Its value or intensity is derived from the strength of themagnetizing force and the material permeability of the part. When the magnetizing force is removed, the residualmagnetism in the part is always less than the field present while the magnetizing force was acting. The amount ofdifference 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 beendeveloped for the continuous method, which make it faster than the residual. The indication is produced at the time ofthe two magnetization shots and the sixty second migration of the magnetic particles as the residual vehicle drains fromthe part. The residual method requires two steps, magnetization and application of particles, plus the added time forindications to build up if the immersion method is used. The continuous method is preferred unless specialcircumstances make the residual method more desirable.3.5.4.2.1The continuous method is the only effective one to use on low carbon steels or iron having little retentivity. It isfrequently used with AC on such materials because the alternating current field produces excellent mobility of theparticles. With the wet method the usual practice is to flood the surface of the part with the bath, then simultaneouslyterminate bath application and apply the magnetizing current momentarily. Thus the magnetizing force acts on theparticles in the film of the bath as they are draining over the surface. Strength of the particle bath has beenstandardized to supply a sufficient number of particles in the film to produce good indications with this technique. Itshould be noted that the continuous method requires more attention and alertness on the part of the inspector than doesthe residual method. Careless handling of the bath/current application sequence can seriously interfere with reliableresults.3.5.4.2.2Probably the highest possible sensitivity obtainable for very fine defects is achieved by immersing the part in the wetbath, magnetizing the part for a short time while immersed, and continuing to magnetize while the part is removedfrom the bath and while the bath drains from the surface.3.5.5 DryPowderMagneticParticles.CAUTIONDry powder method SHALL NOT be used on aerospace vehicles or aerospace partswithout specific approval of the appropriate engineering authority for the individualinspection 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 whollybelow 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 coatingmethod are similar in all colors making the particles equally efficient. The choice of powder is then determined
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