Equipment, Materials, and Magnetization/Particle Application Methods
Equipment families
Match equipment to geometry. Portable AC electromagnetic yokes must pass a lifting-power check - commonly 10 lb (4.5 kg) at the maximum rated pole spacing - and the poles should straddle the area of interest. Stationary benches provide head shots (circular magnetization), coils and central conductors (longitudinal magnetization), and prods (local circular field). Wet fluorescent benches give the highest sensitivity for production; dry powder suits portable work, rough surfaces and higher temperatures.
Calculating magnetizing current
For circular magnetization by current through the part or a central conductor, use I = 30 000/(L/D) amperes, where L/D is the length-to-diameter ratio of the part or the section being examined. For longitudinal magnetization in a coil, use NI = 45 000/(L/D) ampere-turns, applied within about ±10 %; below L/D = 4, use a cable wrap or central conductor rather than a coil. Prod current is set by spacing (commonly 90–110 A per inch of spacing, prods at least 3 in. apart) and successive prod placements must overlap.
Worked example: a bar 10 in. long and 1 in. in diameter has L/D = 10. A coil needs 45 000/10 = 4 500 ampere-turns, so a 10-turn coil carries 450 A; the acceptable band is roughly 405–495 A.
Traps
Adjust for hollow parts by using the effective (wall) diameter when a central conductor is used. Remember continuous technique is required when residual magnetism is too weak to hold particles. Particles come wet or dry, fluorescent or nonfluorescent, and color-contrast, sized for different defect widths. Black-light intensity at the surface and bath concentration by settling test are classic process-control items.