MAGNETIC PARTICLE TEST CONTINUOUS METHOD
The advantage of greater sensitivity for the continuous method is simple and basic. When the magnetizing force is applied to a ferromagnetic test object, the field within the test object rises to a maximum. This value or intensity is derived from the strength of the magnetizing force and the material permeability of the test object.
When the magnetizing force is removed, the residual magnetism in the test object 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 test object.
Techniques have been developed for the continuous method that make it faster than the residual method. The indication is produced at the time of magnetization, whereas 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.
The continuous method can be used with the direct contact method, the central conductor method or with a coil for longitudinal magnetization. Test objects made of low retentivity materials, such as low carbon steel, must be tested using the continuous method because residual leakage fields at discontinuities in these materials are too weak to produce good magnetic particle indications.
Timing of the wet media bath application and the magnetizing current is very important when using the continuous method. A test object should be flooded with wet media before the current is applied. At the moment of, or just before, application of current, the media stream is turned off or diverted from the test object before the magnetic pulse ends. This allows the test object to be covered with a mobile layer of fluid when current is applied. Indications can then
form at discontinuities. Turning off or diverting the bath stream before the magnetizing field is terminated ensures that the force of the bath stream will not interfere with the formation of indications.
Maintaining the magnetizing field for a short time after the bath stream ceases allows time for magnetic particle indications to form, taking advantage of additional magnetic particles being made available by surface drainage. On weakly retentive test objects, extra care is needed after magnetizing to minimize any washing away of indications by the retained bath on a test object’s surface. It should be noted, however, that the continuous method requires more attention and alertness on the part of the technician than does
the residual method. Careless handling of the bath current sequence
can interfere seriously with reliable results. Normally the duration of
the coil magnetizing shot will vary from about 0.5 to 1 or 2 s, depending on the difficulty involved in showing the discontinuities being sought. In some instances, when large forgings or steel castings are to be tested using the continuous method with bath application by hand hose, the magnetizing current may be”left on from 5 to 10 s, during which time the test object may be repeatedly swept with the bath spray.
The magnetizing field is maintained for 1 to 2 s after the final spray has ceased or been diverted. This latter technique also applies to circular magnetization when large test objects are processed using contact or central conductors.