ABSTRACT
There are no universal rules regarding magnetization levels for MFL inspection. The criterion for good inspection is that the value of the leakage field from the smallest required flaw is relatively large in comparison to the noise that the magnetization process causes from surface and subsurface irregularities in the part, or other nonrclevant indications. The application of this criterion to the magnetic particle inspection of relatively smooth, shiny steel surfaces for tight cracks may require a different type and level of field intensity from that required for the detection of small pits on the inside surfaces of tubes, when the sensor is constrained to scan only the outer surface. While a low active AC field may be specified for the former, so as to hold the particles to relevant cracks while moving them away from nonrclevant surface indentations and extraneous flux leakage caused by local changes in material permeability, a high DC field is required for the latter, the value of which depends upon part wall thickness and surface MFL from the same causes. In effect, each case should be treated on its own merits, and the inspector should beware of applying methods and formulae from one particular application to another.
