The residual method is a method of testing in which magnetic particles are applied to test objects after the test objects have been magnetized. The residual method is used only when test objects are magnetized with direct current. This method of testing is used when test objects have sufficient retentivity to form adequate magnetic particle indications at discontinuities. This method is used with both longitudinal and circular magnetization techniques, direct contact
and central conductor methods.

Residual testing requires the test objects to be retentive enough to hold magnetic particle indications at discontinuities. Usually, the use of the residual method is limited to the search for discontinuities that are open to the surface, such as cracks. Detection of subsurface discontinuities requires the stronger leakage fields at discontinuities that exist while the test object is being magnetized, as when the continuous method is used. Residual testing permits the magnetizing of test objects at one time and the application of magnetic particle media at a subsequent time.

When the central conductor method is used, testing of holes or bores is facilitated, since testing takes place after removal of the central conductor.
Care must be taken in the handling of test objects that have been magnetized, particularly test objects having smooth or machined surfaces, to avoid their being rubbed together or against other ferromagnetic test objects. Such rubbing may produce localized magnetized areas on surfaces that will attract and hold magnetic particles.

Magnetic particle indications produced on these areas are nonrelevant and are called magnetic writing. A technician maynotice 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 test object. 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.

Amperages used with the residual method need be only great enough to magnetize the test object sufficiently to show the type of discontinuity being sought. Some gross discontinuities may require only weakly magnetized test objects, and others, being more difficult to find, may require the maximum residual field obtainable. The residual magnetic field retained in a test object is always less than the magnetic field strength that produced it. A maximum residual field strength results when the magnetization level within the test
object 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 method.

Technician experience is very helpful for determining the amperage required to test an object using the residual method. A technician should first determine whether or not an object can be tested using the residual method. Any object to be tested using this method requires that the object be retentive enough so that magnetic particle indications will be formed at discontinuities. A rough
determination of a test object retentivity can be made by magnetizing the test object in a coil having field strength of about 10 000 direct current ampere turns. If after magnetizing the test object can lift and hold an ordinary paper clip, the test object is retentive enough for residual testing. If the test object will not hold a paper clip, the use of the residual method can be ruled out.

Magnetic particle testing media may be applied by stream, spray or immersion of the test object in a tank. Extra care is required when using the immersion method, particularly with test objects that have smooth surfaces, to avoid removing any indications by the rapid removal of a test object from the bath. To ensure uniform magnetic bath concentration, the bath must be continuously agitated. The bath concentration must be maintained within specified limits. Too weak a particle concentration will produce weak indications, and in
borderline cases may cause fine discontinuities to go undetected.

Too heavy a concentration produces heavy background accumulations that reduce color contrast. Most magnetic particle indications produced using the residual method appear quickly on a test object. Longer times are required
when discontinuities are extremely fine. Formation of the indications can sometimes be speeded up by holding the test object in a position that will allow residual bath drainage to flow across the suspected areas.

After a magnetic particle test, the test object could still be retentive enough to be tested residually, depending on the nature of the discontinuities expected to be found. In this case, the technician must resort to testing of the object(s) using the continuous method, testing for indications at discontinuities, then removing these indications and reapplying the media to see if residual indications are produced. The amperage used to form the indications found
during the continuous method will give a technician some indication of amperage to use for residual testing.
The application of magnetic particle media with residual testing is simply a matter of covering the area to be tested. Care should be taken in the case of wet media to ensure that test objects are adequately flooded using low velocity streams or sprays. The test objects should be positioned to take advantage of any particle flow resulting from drainage on a test object’s surface. Some test objects may need a longer drain time than others because indications may be
slower in forming on smooth surfaces. In some cases, on bearing rollers for example, formation of fine indications may be enhanced by immersing the magnetized test object in liquid media for a considerable time. This gives the leakage fields time to attract and hold the maximum number of particles resulting in an increase in sensitivity.

Dry powders are used to detect gross indications. Care must be taken when applying dry magnetic powders to test objects that have been magnetized to avoid getting too much powder on a test surface. A combination of a light blowing and tossing action, either from a handheld container or a pressurized powder blower, is needed. 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 method has several disadvantages. It is more difficult to apply to interior surfaces of a  test object than is wet media, it is more difficult to completely cover a test object in a short time and removal of powder from a test object can be a problem, particularly when the test object has not been demagnetized. A cylindrical test object would be held in a near
vertical position, allowing the drainage flow across circumferential (transverse) cracks.
Although the residual method is not as widely used today as the continuous method, it does have some advantages that make it attractive in some circumstances. The residual method is capable of close control, and of giving uniform results to a greater degree than the continuous method. The fact that it is applicable only to test objects having relatively high retentivity is why the method is not used more extensively.

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