View Full Version : Demagnetization

05-27-2004, 08:18 AM
I work at a power plant, and sometimes when we replace large sections of pipe, we have a problem with the pipe becoming magnetized. The magnetization causes problems with the welding procedure, so we have tried several unusual methods of demagnetizing the pipe. It was suggested that we hit it with a really big hammer, but I think he just wants to justify buying a "testing hammer." Anyway, I thought I'd ask all you experts out there if you had any idea why the pipe might have become magnetized in the first place, or any ideas on how to demagnetize it. Thanks.


05-27-2004, 10:54 AM
I get salvage steel that is magnetized from the magnetic crane used to lift it. Even old rusted stuff has a residual magnetism in it.
I've never figured out a way to get rid of it in large pieces of metal. Using AC overcomes the problem though.

Rocky D
05-27-2004, 12:58 PM
Demagnitizing is usually not done, to my knowledge, just weld it with AC like rferg says. While hammering will demag something small, I doubt it will do anything at all to your pipe.

However, if you have to code weld it, AC may not be an option, whereas you might have to electrically demag it. I believe there are companies that do this.

05-27-2004, 01:37 PM
Got to thinking about the problem of magnetism. If you can't use ac, wonder if slapping one of those powerful rare earth magnets up against the side so that the poles bucked each other would help. Might use a compass to tell when the two cancelled each other exactly. You would have to move the magnet every so often but bet it wold be a lot cheaper to have the pipes demagnetized.
Another idea might be to use a compass to find the polarity of the magnetism on two pices of pipe and put them togther so that they are opposite.
Just a thought.

05-27-2004, 03:42 PM
1. Heating will often demagnetize metal. Try preheatng the weld area with a torch. I don't know how hot you have to get it to cause the molecules to "forget" to line up.

2. You can demagnetize an object by putting it in a strong alternating magnetic field. The device is basically a length of metal (the right kind of metal), wrapped with wire (in the form of a coil), thatis connected to 120V AC. I doubt mild steel would work. Maybe a Google search for a hand held demagnetizer will turn up something.

05-28-2004, 11:30 AM
From what I understand of magnetic fields, you should be able to demagnetize your metal with a deguassing wand.
Definition of deguassing:
"degaussing: [A] procedure that reduces the magnetic flux to virtual zero by applying a reverse magnetizing field. Synonym demagnetizing."

05-28-2004, 12:53 PM
Trouble is, in order to get rid of the magnetic field with heat, you would have to heat the WHOLE pipe. as long as any of it is magnetized, the lines of force will extend though the whole thing.

Mike W
05-28-2004, 01:26 PM
You can de-magnetize a screwdriver by putting it in a coil of wire that has a few amps of 60 Hz current flowing in it. You slowly withdraw it. ;)

05-28-2004, 07:14 PM
Wrapping work DC welding lead a few turns around magnetized pipe will counter the pipe's magnetism. Wrapping work lead one way will increase magnetism and the other way will decrease pipe's magnetic field. You will have to experiment with number of turns.

06-01-2004, 12:28 AM
I know how you feel, can get frustrating.
I had the same thing happen to me in a power plant, with my fitters help, we managed to solve the problem.
Our job was to tie feedwater piping from the economizer to the boiler feed pump. The line was 12" heavy wall prepped with a "J" groove. The pipe material was A106 Gr.B carbon seamless pipe.
The procedure was tig root with 70s-2 wire and stick 7018 A-1 fill and cap.
My fitter had warned me that I might run in to arc blow, because it had happened on the previous feedwater piping on another boiler in the same plant. Anyway much to my dismay I struck an arc with the tig torch and the arc wondered off. Not only that, a piece of filler wire would stick to the end of the joint if placed on it.
Our QC guy used a meter to measure the strength of the field and it went off the scale.
My fitter told me we would tack flat bar all the way around to bridge the joint so as to neutralise the magnetic field. We left the bottom quarter open so I had a place to start with. I rooted the joint on opposite quarters, knocking the bars off as each section was completed (nice to have an experienced fitter).
It was that way for all tie in joints, and each time we bridged the joint the magnetic field was gone.
Worked for me, give it a shot.