Dave Hinz wrote:

Building a project out of a piece of 4140 prehard. The local custom
metalcutting shop provided the steel. Problem is, it's magnetic. Not
just a little magnetic, it's "you could use this thing to pick up nails,
small children, and battleships" magnetic. I asked them if they have a
demag ring, they don't (and were surprised but will check their stock...
ok, great, but doesn't help me much at the moment).

So how do I demag this sucker?

How big is the part? For very small parts, I use a Weller soldering
gun, the
kind with the two posts sticking out the front, and the copper "wishbone".
If necessary, I can make a ring of #12 wire that has a bigger opening than
the standard element.

For larger pieces, do you have an AC TIG welder? If not, then try an AC
buzz-box. Make a loop of several turns of ground cable big enough to
surround the part, and short the electrode holder to the ground clamp.
If TIG, set it for something like 50 A, if buzz-box, set it for the lowest
current possible. Place the looped cable around the part, and turn on the
welder, or step on the TIG pedal. Remove the cable while the welder
is still on, then turn it off when the cable is away from the part. You
may have to repeat this process several times in different starting
orientations to completely demagnetize the part. You may have to turn
up the current if the amp-turns are not enough to do the job.

What you are doing here is passing an AC electromagnet over the part,
and gradually drawing it away. The 60 times per second alternating
field of decreasing intensity gradually reduces the remanent field to
zero. (When I say gradually, it only takes a couple of seconds.)

Jon