Continuous Technique

T.O. 33B-1-1 3-28 suspension must be continuously agitated. The 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. Equally undesirable, too heavy a concentration produces heavy background accumulations that reduce contrast. 3.3.8.1.7 Most magnetic particle indications produced using the residual technique appear quickly on a part. Longer times are required when discontinuities are extremely fine. Formation of the indications can sometimes be speeded up by holding the part in a position that will allow residual suspension drainage to flow across the suspected areas. In the case of a cylindrical part, it would be held in a near vertical position allowing the drainage flow across circumferential (transverse) cracks. 3.3.8.1.8 Although the residual technique is not as widely used today as the continuous technique, it does have some advantages that make it attractive in some circumstances. The residual approach is capable of close control, and of giving uniform results to a greater degree than the continuous technique. 3.3.8.2 Continuous Technique. The continuous technique is used primarily with liquid suspensions although occasionally dry powder is more appropriate. This technique requires that the magnetizing force be present while the liquid suspension is present on the part in sufficient quantity for the particles to be highly mobile. When the current is on, the maximum flux density will be created in the part and the maximum leakage flux will be present at a discontinuity to attract the magnetic particles to form an indication. To leave the current on for long periods of time is not practical or necessary in most instances. However, when using dry particles and either AC or HWDC as the magnetizing current, the current is sometimes kept on for minutes at a time. The heavy current required for proper magnetization can cause overheating of parts and contact burning or damage to the equipment if allowed to flow for any appreciable time. In practice, the magnetizing current is normally on for only a fraction of a second at a time since the real requirement is that a sufficient number of magnetic particles have been applied and are in the magnetized zone. These particles must be free to move while the magnetizing current flows. The bath ingredients are so selected and formulated that the particles can and do move through the film of liquid on the surface of the part and form strong, readable indications. This is one reason why the viscosity of the bath and bath concentration are so important, since anything that reduces the number of available particles or slows their movement will impede the build-up of indications. 3.3.8.2.1 The reason for greater sensitivity for the continuous method is simple and basic. When the magnetizing force is applied to a ferromagnetic part, the flux density rises. 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 difference depends on the retentivity of the material. Consequently, the continuous technique, for a given value of magnetizing current, will always be more sensitive than the residual technique. The continuous technique can be faster than the residual since the indication starts to form at the time of magnetization, whereas the residual method requires magnetization and then application of particles, plus the added time for indications to build up if immersion is used. Parts made of low retentivity materials, such as low carbon steel must be inspected using the continuous technique since residual leakage fields at discontinuities in these materials are too weak to produce good magnetic particle indications. It is frequently used with AC on such materials because the alternating current field produces excellent mobility of the particles. 3.3.8.2.2 Liquid suspensions are usually used with the continuous technique. The exception is when small, subsurface defects must be found. In this case, under some conditions, a dry particle continuous technique can produce slightly greater sensitivity. Timing of the liquid suspension application and the magnetizing current is critical to form good indications. The area of the part to be inspected must be completely flooded with suspension and then the current SHALL be applied at least twice in rapid succession. Turning off or diverting the suspension flow before the final applications of current ensures that the force of the flow will not interfere with the formation of indications. Extra care