Magnetic particle inspection is quick, inexpensive and sensitive to defects, particularly shallow (0.003 in.) surface cracks and other lineal indications.
It detects small cracks on or near the surface of ferrous alloys that can be magnetized (basically any ferrous alloy except austenitic material). A high-amperage, low-voltage current is passed through the casting, which establishes a magnetic field.
Cracks and defects have magnetic properties different than those of the surrounding material, so their presence will interrupt the magnetic field, causing distortion. Small magnetic particles show the path of the flux line that spreads out in order to detour around the distortion, thereby indicating the shape and position of the crack or void.