Situations Not Requiring Demagnetization

T.O. 33B-1-1 3-78 e. May prevent proper cleaning of the part after inspection by holding particles magnetically to the surface of a part. f. May interfere with subsequent magnetization requirements. g. May hold particles that interfere with later applications of coatings such as plating or paint. 3.6.4.2 Situations Not Requiring Demagnetization. Demagnetization is not usually required when: a. The parts are not aircraft parts and have low retentivity. In this case, the residual field is low or disappears after the magnetizing force is no longer acting. An example is low-carbon plate such as that used for low strength weldments, tanks, etc. b. The material in question consists of non-aircraft structural parts such as weldments, large castings, boilers, etc., where the presence of a residual field would have no effect on other components or the proper service performance of the part. c. If the part is to be subsequently processed or heat-treated and in the process will become heated above the Curie point, or about 770°C (about 1418°F). Above this temperature steels become nonmagnetic, and on cooling are completely demagnetized when they pass through the reverse transformation. d. The part will become magnetized anyway during a subsequent process, for example, when held in a magnetic chuck. e. A part is to be subsequently magnetized in another direction to the same or higher level at which it was originally magnetized, for example, between circular and longitudinal magnetization for magnetic particle inspection. f. The magnetic field contained in a non-aircraft finished part is such that there are no external leakage fields measurable by ordinary means, i.e., the field produced during magnetic particle inspection with circular magnetization. 3.6.4.2.1 The requirement cited in paragraph 3.6.4.2e is sometimes a cause of confusion. A residual magnetic field in a ferromagnetic material exists because there is a preferred orientation of the magnetic domains caused by a previously applied magnetic field. A residual magnetic field perpendicular to a previously established residual field can only be produced by application of a magnetic field in the perpendicular direction strong enough to rotate the domain 90 degrees. Because the preferred orientation of the domains has been rotated 90 degrees, the previous residual field no longer exists. For this reason, longitudinal magnetization, strong enough to produce indications of discontinuities in a part that previously had a residual circular magnetic field, reduces the circular residual field to zero. If the magnetizing force is not of sufficient strength to establish the longitudinal field, the strength SHALL be increased, or other steps taken to insure that a residual longitudinal field actually has been established. For example, a large part having a large L/D ratio may require multiple longitudinal shots along its length to eliminate the circular field. Rotation of the preferred orientation of the magnetic domains also occurs when a circular residual field is produced in a part with an existing residual longitudinal field. 3.6.4.2.2 If the two fields, longitudinal and circular, are applied simultaneously, an applied field results that is a vector combination of the two in both strength and direction. If the magnitude of this resultant applied field is large enough, then a residual field will be produced in this same direction. If, however, the fields are induced sequentially the last field applied, if strong enough to produce a residual field, will eliminate the residual field from the previous magnetization. A convenient method of assuring reduction of a residual magnetic field in one direction and establishing a field in a perpendicular direction is to slightly increase the magnetizing force of the second shot.