Sunday, April 25, 2010

Demagnetization

There are several steps in a magnetic particle inspection.
  1. Clean the surface of the ferromagnetic material
  2. Magnetize the material
  3. Spray or pour the magnetic particles
  4.  Visual inspection
  5.  Demagnetization
  6.  Cleaning
Demagnetization is done to disorganize the micro atomic arrangement which produce a magnetic field. After demagnetization the material will act as a normal metal.
The magnetized components can cause problems such as,


  • 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.
  • Attract unnecessary machine-parts such as bolts/nuts, which lead to further damage while the machine is in use.
There are several laboratory techniques that are available for separating various components of demagnetization. Paleomagnetists rely on the relationship of relaxation time, coercively, and temperature in order to remove (demagnetize) low stability remanence components. The fundamental principle that underlies demagnetization techniques is that the lower the relaxation time τ, the more likely the grain will carry a secondary magnetization. The basis for alternating field (AF) demagnetization is that components with short relaxation times also have low coercivities. The basis for thermal demagnetization is that these grains also have low blocking temperatures.
In AF demagnetization, an oscillating field is applied to a paleomagnetic specimen in a null magnetic field environment.
 All the grain moments with coercivities below the peak AF will track the field. These entrained moments will become stuck as the peak field gradually decays below the coercivities of individual grains. Assuming that there is a range of coercivities in the specimen, the low stability grains will be stuck half along one direction of the AF and half along the other direction; the net contribution to the remanence will be zero. In practice, we demagnetize specimens sequentially along three orthogonal axes, or while “tumbling” the specimen around three axes during demagnetization.

Thermal demagnetization exploits the relationship of relaxation time and temperature. There will be a temperature below the Curie temperature at which the relaxation time is a few hundred seconds. When heated to this temperature, grains with relaxation times this short will be in equilibrium with the field. This is the unblocking temperature. If the external field is zero, then there will be no net magnetization. Lowering the temperature back to room temperature will result in the relaxation times growing exponentially until these moments are once again fixed. In this way, the contribution of lower stability grains to the NRM can be randomized. The curie temperature for a low carbon steel is 770C. When steel is heated above its curie temperature, it will become austenitic and loses its magnetic
The NRM is the sum of two components carried by populations with different coercivities. The distributions of coercivities are shown in the histograms to the left in Figure below.
Figure:  Principle of progressive demagnetization. Specimens with two components of magnetization (shown by heavy arrows on the right hand side), with discrete coercivities (plotted as histograms to the left). The original “NRM” is the sum of the two magnetic components and is shown as the + in the diagrams to the right. Successive demagnetization steps (numbered) remove the component with coercivities lower than the peak field, and the NRM vector changes as a result. a) The two distributions of coercivity are completely separate. b) The two distributions partially overlap resulting in simultaneous removal of both components. c) The two distributions completely overlap. d) One distribution envelopes the other. [Figure redrawn from Tauxe, 1998.]
Two components of magnetization are shown as heavy lines in the plots to the right. In these examples, the two components are orthogonal. The sum of the two components at the start (the NRM or demagnetization step ‘0’) is shown as a + on the vector plots to the right. After the first AF demagnetization step, the contribution of the lowest coercivity grains has been erased and the remanence vector moves to the position of the first dot away from the +. Increasing the AF in successive treatment steps (some are numbered in the diagram) gradually eats away at the remanence vectors (shown as dashed arrows and dots in the plots to the right) which eventually approach the origin.