After conducting a magnetic particle inspection, it is usually
necessary to demagnetize the component. Remanent magnetic fields
- affect machining by causing cuttings to cling to a component.
- interfere with electronic equipment such as a compass.
- create a condition known as "arc blow" in the welding
process. Arc blow may cause the weld arc to wonder or filler
metal to be repelled from the weld.
- cause abrasive particles to cling to bearing or faying surfaces
and increase wear.
Removal of a field may be accomplished in several ways. This
random orientation of the magnetic domains can be achieved most
effectively by heating the material above its curie temperature.
The curie temperature for a low carbon steel is 770oC
or 1390oF. When steel is heated above its curie temperature,
it will become austenitic and loses its magnetic properties. When
it is cooled back down, it will go through a reverse transformation
and will contain no residual magnetic field. The material should
also be placed with it long axis in an east-west orientation to
avoid any influence of the Earth's magnetic field.
It is often inconvenient to heat a material above its curie temperature
to demagnetize it, so another method that returns the material
to a nearly unmagnetized state is commonly used. Subjecting the
component to a reversing and decreasing magnetic field will return
the dipoles to a nearly random orientation throughout the material.
This can be accomplished by pulling a component out and away from
a coil with AC passing through it. The same can also be accomplished
using an electromagnetic yoke with AC selected. Also, many stationary
magnetic particle inspection units come with a demagnetization
feature that slowly reduces the AC in a coil in which the component
A field meter is often used to verify that the residual flux
has been removed from a component. Industry standards usually
require that the magnetic flux be reduced to less than 3 gauss
after completing a magnetic particle inspection.