magnetic particle inspection, the magnetic particles can either
be applied to the component while the magnetizing force is applied,
or after it has been stopped. Continuous magnetization describes
the technique where the magnetizing force is applied and maintained
while the magnetic particles are dusted or flowed onto the surface
of the component. In a wet horizontal testing unit, the application
of the particles is stopped just before the magnetizing force
is applied; but, since particles are still flowing over and covering
the surface, this is considered continuous magnetization. Residual
magnetization, on the other hand, describes the technique where
the magnetizing force is applied to magnetize the component and
then stopped before applying the magnetic particles. Only the
residual field of the magnetized component is used to attract
magnetic particles and produce an indication.
The continuous technique is generally chosen when maximum sensitivity
is required because it has two distinct advantages over the residual
technique. First, the magnetic flux will be highest when current
is flowing and, therefore, leakage fields will also be strongest.
Field strength in a component depends primarily on two variables: the
applied magnetic field strength and the permeability of the test
object. Viewing the upper right portion of the hysteresis loop
below, it is evident that the magnetic flux will be the strongest
when the magnetizing force is applied. If the magnetizing force
is strong enough, the flux density will reach the point of saturation.
When the magnetizing force is removed, the flux density will drop
to the retentivity point. The two gray traces show the paths the
flux density would follow if the magnetizing force was applied
and removed at levels below that required to reach saturation.
It can be seen that the flux density is always highest while the
magnetizing current is applied. This is independent of the permeability
of a material.
However, the permeability of the material is very important.
High permeability materials do not retain a strong magnetic field
so flux leakage fields will be extremely weak or nonexistent when
the magnetizing force is removed. Therefore, materials with high
magnetic permeability are not suited for inspection using the
residual technique. When the residual technique is used to inspect
materials with low permeability, care should be taken to ensure
that the residual field is of the necessary strength to produce
an indication. Defects should be relatively large and surface
breaking to have a high probability of detection using the residual
The second advantage of the continuous technique is that when
current is used to generate the magnetizing force, it can provide
added particle mobility. Alternating or pulsed direct current
will cause the particles to vibrate and move slightly on the surface
of the part. This movement allows the particles to travel to leakage
sites. More particles mean brighter indications compared to those
formed using the residual technique.
One disadvantage of the continuous method is that heating of
the component occurs when using direct magnetization. For example,
when prods are used, they may create areas of localized heating
when the continuous technique is used. This may be acceptable
on components that will be further processed (removing this condition),
but machined or in-service components may be adversely affected
by this condition.
While generally not recommended, the residual technique does
have its uses. It is commonly used in automated inspection systems
to inspect materials with high retentivity. To speed throughput,
automated systems often magnetize the parts and then submerge
them in an agitated magnetic particle bath or pass them through
a spray station. Closely controlled automated systems provided
good results using the residual magnetism technique.