Barry,

Three types of iron-based materials are used in magnetics. The
"Reference Data for Radio Enfineers" has a rather complete table
of metallic core materials.

The "soft iron" variety is almost pure iron, with up to 10% silicon.
The cobalt iron alloys are about 50% cobalt, and have a high saturation
curve. The vast majority of choices, which includes the several of the 
"permalloy" alloys, run from 50 to 80% nickel, and provide the highest 
permeability but lower saturation.

But for many applications low-silicon (4%) soft iron is the best bet. Its 
advantage is that it can be stamped or cold forged, as are the poles 
in speaker magnets, where you can see the stamping streaks in the edges
of the soft-iron donuts. I am using soft iron washers as poles for the 
magnets for the new NSF instrument. (I have posted the magnet design).
In general, I try to design around bought hardware, since few people 
have a lathe or mill.  McMaster has a large selection of large OD/ID 
by 1/4" thick washers.  The cyclotron I worked on had a 13 ton magnet 
with 30" poles and was made with 1" slabs of low silicon soft iron, 
which will not saturate until about 20 kgauss (we ran at about 15 kgauss).
 
But if you dismantle a transformer, the laminates are most likely a
grain-oriented silicon alloy, which  is tempered and stamped; bend
it once and it becomes very soft at the bend. The transformers I modify
to be the poles of the VRDT displacement transducer will not re-assemble
if any of the laminates have been bent because they cannot be straightened.

Regards,
Sean-Thomas
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