Degaussing - theory
Magnetic Work Pieces
Causes of Magnetisation
The reasons for magnetization of work pieces are various. Fairly often, they cannot easily be ascertained in practice. The main causes are artificial magnetic fields acting in direct vicinity of the work pieces. These magnetic fields can be of intended or unintended origin, as for example: magnetic transport, induction hardening, magnet gripper, magnet chuck devices and others. Mechanical vibrations and cold forming under the influence of those magnetic fields reinforce or enhance the process of magnetization. The effects can be the following:
- metal scabs and swarf stick to the work piece
- sintered tools wear off faster
- down times for robots / automatic feeding systems due to parts sticking together
- magnetic field sensors are falsely activated
- measurement errors at highly sensitive measuring instruments
- faulty welding seams
- electron beam welding becomes impossible
- irregular thickness of layers at hard chromium plating
- titanium nitride coatings:
irregular repartition of the gaseous coating material - edges breaking off at edwc (electric discharge wire cutting)
Principle
Inside a ferromagnetic crystal, a larger amount of atoms always has the same orientation. This area can be considered from the outside as a domain (Weiß domain); its volume ranges from 0.001 to
0.1 mm³. If these areas also have the same orientation, a work piece is magnetic.
Degaussing is achieved by reversing the homogenous orientation of the Weiß domains and thus creating an extreme disorder with the effect that the magnetic impact of the different areas is being neutralized.
In practice, mainly the following methods are used:
- The work piece is exposed to a strong alternating magnetic field, which is constantly being reduced to zero.
- The work piece is lead through a strong alternating magnetic field, at a slow and constant speed.
- The work piece is heated to >800°C (exceeding Curie-point), and slowly cooled down in a neutral spot (which is exposed to the magnetic field of the earth only).
Since the degaussing effect of the alternating field is at its optimum only if it has the same orientation as the magnetic field of the work piece, divers methods of creating the degaussing field will be necessary (degaussing coils, degaussing yokes).