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Magnetic permeability of 430 SST
2

Magnetic permeability of 430 SST

Magnetic permeability of 430 SST

(OP)
Does anybody have any personal experience or source of information on heat treatment of 430 to maximize permeability?  I will be making a vacuum chamber from 6" diameter 430.  The material will be heavily machined and welded and I plan to anneal it to remove any magnetic hot spots.  Any information on grain growth and it's effect on permeability would also be useful.

RE: Magnetic permeability of 430 SST

Here is a technical paper by Carpenter on the development of enhanced magnetic properties for their ferritic stainless steels. It does provide background information on the effects of alloying and grain size regarding various magnetic properties;

http://crswnew.cartech.com/wnew/TechArticles/TA00012.html

RE: Magnetic permeability of 430 SST

(OP)
Thanks for the response, it is appreciated.

RE: Magnetic permeability of 430 SST

Here is our spec, sorry the formatting does not copy:

3.3.2 Annealing
Temperature, time, atmosphere and rate of cooling shall be as
specified in Sections 3.3.2.1 through 3.3.2.3.
3.3.2.1 Annealing Temperature and Time
The parts shall be heated to the temperature range in
the chart below and held at the selected temperature
for 2 to 4 hours.
Silicon content Annealing Temperature
Less than 1% 788-843°C (1450-1550°F)
1% or more 843-1000°C (1550-1830°F)
Note: Furnace control thermocouple(s) may be used
for the temperature and time measurements provided
sufficient allowance is made to ensure all the parts in
the load reach the selected temperature for the
specified time.
3.3.2.2 Atmosphere
A dry hydrogen or vacuum atmosphere shall be used
during the heating and soaking periods of the cycle.
If a vacuum atmosphere is used, of 1X10-3 millimeters
(1 micron) of mercury or less shall be maintained
throughout the soaking period.
3.3.2.3 Cooling Rate
Furnace cool to 425°C (800°F) at a rate not
exceeding 56°C (100°F) per hour. Below 425°C
(800°F) cooling rate is optional. Nitrogen with a dew
point of -50°C (-60°F) or lower can be used to
assist cooling. Below 180°C (356°F) air cooling is
permissible.

I recommend the dry hydrogen atmosphere if you want a bright, scale free finish.  If you want maximum magnetic properties be sure to get "Solenoid Quality" material, not generic 430.

RE: Magnetic permeability of 430 SST

(OP)
Thanks for the response dgallup.  

Is this an internal spec you use or is it something you got from a standard?  I have seen similar numbers but slightly lower.  Ranging from 705 to 790 C.

430F would be a good choice except for the sulfur content.  It's too high to be used in an ultra high vacuum application.  

I'm sort of forced into a hard spot on this.  Higher temps make bigger crystals increasing magnetic properties but reducing hardness, essential for making a vacuum seal.  

The paper referenced by metengr also shows carbon content on magnetic properties, less carbon being better. This might argue doing a hydrogen anneal rather than a vacuum anneal.  

RE: Magnetic permeability of 430 SST

That is our internal spec.  

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