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Demagnetization of Long Hollow Shaft in MPI

Posted by: NDTman , E-mail: Address, on March 16, 2008 at 10:45 :

Dear Friends,

I have tried demagnetizing a long hollow shaft (approximately 0.500 inch thick, 9 feet long and having a O/D of 8 inches) on a MPI Bench Unit but find it difficult to achieve less than 3 gauss on all areas of the shaft as per the ASTM E1444-05 requirement.

This shaft has been previously magnetized longitudinally for inspection using 4ooo Amps FWDC.

I have used the machine's Coil which was set to 4500 Amps on AC Demag Mode to demagnetize the shaft but it is not effective.

Is it the right thing to do if I insert a flexible cable (2 inch O/D) through the internal diameter of the shaft and pass AC and DC current through the flexible cable to demagnetize the shaft? If I could use this method, what current should I use for AC and DC to demag?

Please advise the appropriate method and current to use for this demagnetization to be effective. Thank you in advance.

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