Drawing of a Tool Steel Ring Specimen (Ketos Ring) with Artificial Sub-Surface Defects
For the inspection of finished parts, such as the machined and ground shafts and gears, direct current is
frequently used. Although AC is excellent for the location of fine cracks that actually break the surface, DC is
better for locating the very fine non-metallic stringers that can lie just under the surface.
These comparisons point up the importance of choosing the right current type to give the best indications
possible, and show how the choice will vary, depending upon the nature and location of the defects sought.
Half-wave current provides the greatest sensitivity for detecting discontinuities that lie below the surface,
particularly when using dry powder and the continuous technique. The pulsation of the half-wave current
vibrates the magnetic particles, thereby aiding their migration across a surface to form indications at
discontinuities. This particle mobility, which is very pronounced when dry magnetic powder is used,
contrasts with the relative immobility of the powder when pure direct current is used. There is some skin
effect when half-wave current is used, due to the pulsating magnetic fields produced by this current.
However, the effect on field penetration is small at the usual frequencies of 50 and 60 Hertz.
The particles used in magnetic particle testing are made of ferromagnetic materials, usually combinations of
iron and iron oxides, having a high permeability and low retentivity. Particles having high permeability are
easily attracted to and magnetized by the low-level leakage fields at discontinuities. Low retentivity is
required to prevent the particles from being permanently magnetized. Strongly retentive particles will cling
together and to any magnetic surface, resulting in reduced particle mobility and increased background
Magnetic particles may be applied as a dry powder or wet suspension, in the latter case using either water or
a high f lash point petroleum distillate as a suspension vehicle. Dry powders are available in various colors so
the user can select the color that contrasts best with the color of the surfaces upon which they are used.
Colors for use with ordinary visible light are red, gray, black or yellow. Red and black colored particles are
available for use in liquid suspensions and visible light. Fluorescent yellow-green particles are used only in