AC vs DC for demagnetising 3

[sayee1]

During welding of cs pipelines (API 5L X 52-65 grades) especially the residual magnetism is the pipes (due to say induction heating during the coating) could cause problems during welding. Now traditionally, the equipment for demagnetising are AC equipment. I was told however that the welding rectifiers can be used for demagnetising, does anyone have experience on the same? If yes, please sepecify benefits and disadvantages in using DC over AC for demagnetising.

Thanks and regards
Sayee Prasad R CEng MWeldI MIOMMM


If it moves, train it...if it doesn't move, calibrate it...if it isn't written down, it never happened!

 

[metengr]

Yes, we have used DC from portable welding rectifiers to demagnetize high energy steam lines at Power Plants. We had taken welding leads and coiled them on the pipe in various directions to locally reduce the magnetic field at the weld joint. The benefit was using portable welding equipment without having the need to bring in a larger de-mag unit (high amperage - the type of unit to perform coil or head shots for magnetic particle testing). The disadvantage of using welding rectifiers with DC was time - it may take several iterations to knock the magnetic field down to a workable range for welding.

There are devices on the market that can de-magnetize pipe lines with up to 1000 gauss. They consist of a separate power supply and leads that use AC. We have not used these devices.

 

[Goahead]

You can use DC by changing polarity, and by diminishing the current at each inversion until it is reduced almost to zero. The demagnetizing field however should be opposed to the residual magnetic field, and that may be tricky to obtain.

http://www.welding-advisers.com/

 

[unclesyd]

Make sure you have someone versed in the art if using DC to demag anything or you will have half the world magnetized. I've had to undo many botched jobs, both on fabricated components, turbine rotors, and casings when DC demagnetization attempt went awry.

 

[sayee1]

Thanks guys for reconfirming my understanding on the usage of DC. A star for each of you.

Thanks and regards
Sayee Prasad R CEng MWeldI MIOMMM


If it moves, train it...if it doesn't move, calibrate it...if it isn't written down, it never happened!