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Newsgroups: rec.crafts.metalworking
Subject: Re: non-magnetic stainless
From: (John Whitmore)
Date: 18 Sep 1995 21:16:20 GMT

In article <> (Alan Frisbie) writes:
>In article <4358ch$>, 
> (John VerBurg) writes:
>> Hi. I am in need of some stainless steel that must be completely 
>> non-magnetic. I understand that the 300 series stainless is suppose to be 
>> non-magnetic, but I have heard that some are "more" non-magnetic than 
>> others.

>I just tested several of my wife's Farberware stainless steel
>saucepans with a magnet.   ...
>I held the pan sideways for this test.   When the magnet is 
>placed against the sides (nearest the opening of the pan),
>it attracts strongly.   However, as I slide it towards the
>bottom of the pan, it attracts less and less, eventually
>falling off.

	300-series stainless is nominally nonmagnetic, because
any magnetic field applied to it induces only a small temporary
magnetization.  But, STRAINS in 300-series stainless cause
flux pinning, and the result is that worked stainless can
take a permanent magnetic moment.  Strain also changes the
material density enough to create regions of high permeability
in an otherwise nonmagnetic material.  Magnetizability
of ferrous alloys depends on a delicate balance of energy
minimization (which acts against magnetization) and
electron exchange (which acts to cause magnetization), and
the balance can be tipped by VERY small changes, like the
compression of solid steel.  Remember that a hammer blow
can cause spontaneous magnetization of steel objects by
sending a pressure wave through the material that demagnetizes
the bulk, so that the material spontaneously takes on
aligned magnetization with the Earth's field when the
wave has passed.

	There are magnetic 'tags' applied to salmon fingerlings
at local hatcheries, which consist of crimped SS wire.  The
magnetization stays around for the life of the fish (decades).

	The magnetization of a sample is often history-dependent,
so all materials properties data is for annealed metal.  The
pan might have been annealed where it contacts the
the burner, and sufficiently strained to hold a fair magnetization
where it was deep-drawn.

	For sensitive use inside a superconducting magnet, we've
used ceramics (alumina), molybdenum, and copper.

	John Whitmore

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