Magnetic particle inspection is a nondestructive method for detecting cracks, seams, inclusions, segregation, porosity, and similar discontinuities in magnetic materials.  It is not applicable to non-magnetic materials.  This method will detect surface discontinuities which are too fine to be seen by the naked eye, defects which lie slightly below the surface and - with special equipment - even deep-seated discontinuities. 
 Not all discontinuities in metal are detrimental to its efficient service.  The inspector must be able to interpret magnetic particle indications and decide which discontinuities are to be regarded as defects.

Since one can expect a wide variation in evaluation of the results, the following points should be agreed upon when inspection is being considered:
1.     What techniques to use (specified in detail);
2.     What types of discontinuities shall be rejected;
3.     What types of discontinuities may be accepted;
4.     The definition of reworking and subsequent retesting which may be permissible. In magnetic particle inspection a magnetic field is induced in the piece to be inspected and the piece is covered by finely divided magnetic particles.  The magnetic field escapes from the work piece at discontinuities, and the magnetic particles align themselves with magnetic poles produced at the edges of the discontinuity to form a pattern which outlines the shape of the discontinuity.  Information on magnetic particle inspection is provided by the following specifications:
             ASTM E 1444 Practice for Magnetic Particle Inspection
            ASTM E 709 Guide for Magnetic particle Inspection
            ASTM E 125 Reference Photographs for Magnetic particle
            Indications on Ferrous castings (revised 1993)

  Source:  Investment casting Institute Handbook 1997 Edition