Degaussing would be the best bet to *remove* the magnetism.
There are other methods to work around the problem:
1. Use a handheld gaussmeter to find null points along the joint (these are areas where the magnetic field is going from positive to negative and cross through zero) and weld in those areas first. Welding in these areas will shift the magnetic field, so don't mark a bunch and then start welding - do them one at a time.
2. Improve grounding - try getting a machine ground directly behind the area to be welded.
3. Use a "shunt" to give the magnetic field somewhere else to be other than the beveled area. This can be accomplished by tacking a "backing bar" on the side opposite where you want to weld.
4. Wrap the pipe with the welding lead ....this is trial and error, but can "deflect" the field away from the joint during welding.
Never an easy fix. I've worked with sections that were so highly magnetized that the fitters "hung their angle grinder" byt simply sticking it to the pipe!
Depending on how strong the field is, it may take several welding machines to counteract the field. Start with one machine in the off position and wrap leads about 8-10 turns around the pipe next to the weld and ground the stinger end back to the machine. Make sure the machine is turned down low, then turn it on. Slowly turn up the amperage until the field starts to weaken. If it gets stronger, swap the end of the leads from positive to negative. It may take a few hundred amps to do this, so be careful not to overheat the machine.
I haven't tried this, but AC is also supposed to remove magnetism in metal. You might try using something like a Lincoln AC225, wrap the leads as described above and slowly increase the current. Start with the coils at the weld joint and drag the coils back away from the weld joint a few feet. You might need to build a wood _not metal) frame to wrap the leads around to make dragging the coils easier. You should be able to drag the field away from the weld and hold it there.
Once you can get some strongbacks welded across the joint, the field strength will get weaker. If a backing ring is permitted, that will help also. As the weld groove is filled the field will continue to weaken.
Another thing you can do if you have the room, is to take a pocket magnet to find magnetic north and orientate the pipe ends north and south. Give the 5 wraps on the ground lead on the north pipe. If you have enough lead, place the ground clamp on the opposite end of the joint being welded with the 5 wraps. Give the Electrode lead 4-5 wraps in the same direction you wrap the ground lead. Also running DC straight on the bead will sometimes help.
BABRT's
When using an ac machine wrapping the leads may not be required. Use two work clamps and enough cable to make two connections. Magnetic clamps may not work well when ac is passed through them. Set the machine for current to maintain 100% duty cycle. Dead short the leads connected to the pipe. The voltage will be about 1 volt and if you set 200 amps you have a 200 watt load. The machine will have about 20-volts at 120 amps or 2400 watts with a 20% duty cycle. Just make sure the cable is sized so it doesn't get hot. Run the ac current through the pipe while welding with a second machine's dc weld output.
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