Magnetic Particle Inspection (MPI) is an effective process for discovering flaws in investment castings. This method is best for detecting cracks and flaws in the surface and just below. It can be used on all alloys that can be strongly magnetized including irons and ferritic steels. Austenitic steels are typically not good candidates. After a casting is magnetized, very fine iron particles are applied to the surface. If there is a flaw, the magnetic field will be disrupted, and particles will concentrate in those areas, even if the flaw is very small. A hairline crack below the surface or not visible to the eye will appear as a line of iron powder particles on the surface.