this magnetic particle testing technique, dry particles are dusted
onto the surface of the test object as the item is magnetized.
Dry particle inspection is well suited for the inspections conducted
on rough surfaces. When an electromagnetic yoke is used, the AC
or half wave DC current creates a pulsating magnetic field that
provides mobility to the powder. The primary applications for
dry powders are unground welds and rough as-cast surfaces.
Dry particle inspection is also used to detect shallow subsurface
cracks. Dry particles with half wave DC is the best approach when
inspecting for lack of root penetration in welds of thin materials.
Half wave DC with prods and dry particles is commonly used when
inspecting large castings for hot tears and cracks.
performing an inspection using dry particles
Prepare the part surface - the surface should be relatively
clean but this is not as critical as it is with liquid penetrant
inspection. The surface must be free of grease, oil or other moisture
that could keep particles from moving freely. A thin layer of
paint, rust or scale will reduce test sensitivity but can sometimes
be left in place with adequate results. Specifications often allow
up to 0.003 inch (0.076 mm) of a nonconductive coating (such as
paint) and 0.001 inch max (0.025 mm) of a ferromagnetic coating
(such as nickel) to be left on the surface. Any loose dirt, paint,
rust or scale must be removed.
Apply the magnetizing force - Use permanent magnets, an
electromagnetic yoke, prods, a coil or other means to establish
the necessary magnetic flux.
Dust on the dry magnetic particles - Dust on a light layer
of magnetic particles.
Gently blow off the excess powder - With the magnetizing force
still applied, remove the excess powder from the surface with
a few gentle puffs of dry air. The force of the air needs to be
strong enough to remove the excess particles but not strong enough
to dislodge particles held by a magnetic flux leakage field.
Terminate the magnetizing force - If the magnetic flux is
being generated with an electromagnet or an electromagnetic field,
the magnetizing force should be terminated. If permanent magnets
are being used, they can be left in place.
Inspect for indications - Look for areas where the magnetic
particles are clustered.