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? I know that heat would do it, but I
don't want to mess up my hardening. I know mechanical impact can redue
it, but I don't know if that's one of those "theoretical but not
practical" things, or if it'd work. So, simple question - can I just
smack the (un-machined) end of this sucker with a hammer a bunch of
times and get it down to tolerable, or do I need to find someone with
degaussing apparatus of some sort, or am I SOL? It's annoying, to say
the least, to have every chip re-attach itself to the work, invariably
on the layout lines that I'm trying to see.

Thanks,
Dave Hinz

How big a part?

An AC coil is the answer in any case. Preferably one that you can hold
the part in and then draw it away from while the coil is still active.

For small items, the frame les the rotor of a cheap fan or blower motor
will work fine, just place the part oin the feild between the poles and
withdraw it slowly away.
A bunch of wraps of wire and an AC welder would serve for larger items.

Guys to talk to. Engine rebuild shops that magnaflux cranks or heads.
The unit they use (Parker Probe, pretty commonly
http://www.parkreshcorp.com/) can demag as well as magnetise, and is
quite portable.
Non Destructive Testing or Inspection NDT/NDI shops. A fixed Mag
particle bench can run several hundred amps through it's coils. (Leave
your credit cards in the changeroom, or else. Watches too!) Typical coil
on a fixed bench unit should allow you to run a decent sized lunch box
through it.

Watchmakers, clockmakers. Magnetism is a very bad thing, so they demag
parts and tools as required. A little AC coil again, sometimes in the
form of a coil, sometimes the coil is buried in a nice flat plate that
looks like a small benchtop weigh scale. In either case the part is
placed in the field and draw away slowly to minimise the chance of
imparting a bit of magnetism into the part as the field collapses.

Cheers
Trevor Jones