wrote:
On Sat, 05 Feb 2011 19:13:04 -0800, Winston
wrote:

Snip
Er. Make that three concentrations vs two concentrations.


--Winston


The permeability figures quoted in your reference are for
quenched 0.9% carbon steel. This is not a mild steel but a high
carbon steel - file hard when quenched.

Mild steel is typically 0.1% carbon and magnetically pretty
similar to soft iron.

Jim

Ah! Good catch!

Still, at least a 2.5:1 advantage to soft iron WRT mild steel:

http://www.microwaves101.com/encyclo...cmaterials.cfm

2.5 times better performance for only 20% more money is probably
worth it. See for example McMaster 89175K27 at $169.71 each vs
8910K487 at $142.31 each. I agree that paying ca. $680 for core
material for each prototype would be beyond the budget for most
hobbyists, though!

--Winston-- Like me, for example!


The 2.5:1 permeability difference is only significant at
extremely small air gap and medium flux densities.

With 5"iron length and .005" residual air gap the working
permeability of the iron circuit drops to 877 for silicon iron
and 667 for mild steel.

The above assumes that the iron is working somewhere near its
maximum permeability flux density. Typical electromagnets work at
higher flux densities where the permeability is starting to drop.

Kay and Laby "Physical and Chemical Constants" shows how this
varies for both 3% oriented silicon steel and mild steel.

Excitation Oersteads 10 50 500

Mild steel flux density 14,000 17,000 21,000

Silicon steel flux density 17,800 19,000 20,300

Do you happen to know where 'malleable iron' fits in
this chart?

Good information. Thanks Jim!

--Winston


Malleable iron is a heat treated variant of cast iron and since
this is similar to an annealing processs I would expect similar
or somewhat better permeablity.

Oersteds 10 50 500

Cast iron `5,000 8,500 14,000

Annealed 1% Carbon Steel 6,500 16,200 20,200

Swedish Soft Iron 14,800 17,000 21,000


Interesting! I hadn't heard of Swedish Soft Iron before.

Thanks!

--Winston