Demagnetization

Demagnetization is possible in different ways. One of the most common is to subject the magnetized part to a magnetizing force that continually reverses its direction while it is gradually decreasing in strength. As the decreasing magnetizing force is applied, first in one direction and then in the opposite direction, the residual magnetization of the part is decreased. Generally, a high-intensity demagnetizer is used. The demagnetization is most common but does not demagnetize as deep or complete as a DC step down unit. This decreasing magnetization is accomplished by smaller and smaller hysteresis loops created by the application of decreasing current. A smaller and narrower loop shows lower residual magnetism. All steels have a certain amount of coercive force, making it extremely difficult if not impossible to demagnetize them completely. The only way to completely demagnetize some materials is to heat them to their Curie point or above. Under normal conditions, a part is considered to be satisfactorily demagnetized if, when checked with a field indicator, the magnetic field is below minimum limits. The Code requires demagnetization when the residual field in the part:

• Could interfere with subsequent processing or usage such as machining operations where chips will adhere to the surface of the part of the tip of a tool may become magnetized from contact with the magnetized part. Such chips involve in smooth cutting by the tool adversely affecting both finish and tool life. Other reasons to demagnetize would be in cases where residual magnetism:

• May interfere with electric arc welding operations. Residual magnetic fields may deflect the arc away from the point at which it should be applied.

• May interfere with the functioning of the part itself, after it is placed into service. Magnetized tools, such as milling cutters, hobs, etc., may hold chips and cause rough surfaces, and may even be broken by adherent chips at the cutting edge.

• Moving parts, especially in the oil, may hold particles; for instance, on balls or races of ball bearings, or gear teeth causing wear.

•capable of holding particles that interfere with later applied coatings such as plating or paint. Demagnetization may not be required where:

• Part material is low carbon steel and has low retentivity.

• The material consists of structural parts such as weldments, large castings, boilers, etc., where the presence of a residual field would have little or no effect on the proper performance of the part.

• The the part is to be subsequently processed or heat-treated and in the process will become heated above its Curie point or about 770 °C (1390°F) for steel.

• Apart is to be subsequently re-magnetized in another direction to the same or higher level at which it was originally magnetized as, for example, between the steps of circular and longitudinal magnetizing, for MT purposes.