Magnetic Particle Inspection - Demagnetizing Parts

Demagnetizing Parts

After the part has been magnetized it needs to be demagnetized. This requires special equipment that works the opposite way of the magnetizing equipment. The magnetization is normally done with a high current pulse that reaches a peak current very quickly and instantaneously turns off leaving the part magnetized. To demagnetize a part, the current or magnetic field needed has to be equal to or greater than the current or magnetic field used to magnetize the part. The current or magnetic field is then slowly reduced to zero, leaving the part demagnetized.

• AC demagnetizing
  • Pull through AC demagnetizing coils: seen in the figure to the right are AC powered devices that generate a high magnetic field where the part is slowly pulled through by hand or on a conveyor. The act of pulling the part through and away from the coil's magnetic field slows drops the magnetic field in the part. Note that many AC demagnetizing coils have power cycles of several seconds so the part must be passed through the coil and be several feet (meters) away before the demagnetizing cycle finishes or the part will have residual magnetization.
  • AC Decaying demagnetizing: this is built into most single phase MPI equipment. During the process the part is subjected to an equal or greater AC current, after which the current is reduced over a fixed period of time (typically 18 seconds) until zero output current is reached. As AC is alternating from a positive to a negative polarity this will leave the magnetic domains of the part randomized.
  • AC Demag does have significant limitations on its ability to demag a part depending on the geometry and the alloys used.
• Reversing Full Wave DC demagnetizing: this is a demagnetizing method that must be built into the machine during manufacturing. It is similar to AC decaying except the DC current is stopped at intervals of half a second, during which the current is reduced by a quantity and its direction is reversed. Then current is passed through the part again. The process of stopping, reducing and reversing the current will leave the magnetic domains randomized. This process is continued until zero current is passed through the part. The normal Reversing DC demag cycle on modern equipment should be 18 seconds or longer. This method of demag was developed to overcome the limitations presented by the AC Demag method where part geometry and certain alloys prevented the AC Demag method from working.
• Halfwave DC demagnetizing (HWDC): this process is identical to Full Wave DC demag except the waveform is Halfwave. This method of demagnetization is new to the industry and only available from a single manufacturer. It was developed to be a cost effective method to demagnetize without needing a Full Wave DC bridge design power supply. This method is only found on single phase AC/HWDC power supplies. HWDC Demag is just as effective as Full Wave DC without the extra cost and added complexity. Of course other limitations do apply due to inductive losses when using HWDC waveform on large diameter parts. Also, HWDC effectiveness is limited past 16 inches in diameter using a 12 volt power supply.