Low Magnetic Permeability Stainless Springs

[dgallup]

I'm looking to replace 302 stainless springs with something with lower permeability. I think the 302 relative permeability is between 3 and 12 in the springs I have. I was planning on using 316 as I know it's permeability is less than 1.05 at 50% cold work even though the tensile strength of 316 is a bit low. (I've got tight space restrictions). Then someone referred me to:

http://www.diamondwire.com/material-properties.aspx

which says 316 is "Magnetic in spring temper". The same link says 17-7 PH is "Slightly magnetic in spring temper". Everything I see on 17-7 PH says it is very magnetic, relative permeability ~40 @ 200 Oe in anything other than condition A. I can't find anything that says how much cold work is in 316 "spring temper" and what the relative permeability is. I would love 17-7 PH for it's high strength but will it be magnetic in a spring?

What is the best low permeability spring material with reasonable availability and price? I don't think I can afford elgiloy.

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[EdStainless]

You want springs in one of the Nitronic grades, 21-6-9 or 23-13-5.
We cold work them to about 25%, 150ksi UTS and they are less than 1.005.
Worked to about 60% you should have 200ksi and perm no more than 1.02.

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[dgallup]

Thanks Ed. 1.02 perm would be great. How do you measure permeability of a spring? Unfortunately, 200 ksi UTS is too low for my application, the 316 is ~240 ksi, 302 is ~300 ksi (wire sizes are under 0.5 mm). I can barely fit a 316 spring in the space constraints.

Anyone know what the perm of a 316 spring is?

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[TVP]

dgallup,

Ed was just providing you with some general information. You should discuss this directly with Sandvik, Central Wire, etc. in order to get definitive values. Both Sandvik and Central Wire process spring temper 904L, which will have very little magnetism. Central Wire offers Tech Tronic 50 (UNS S20910), which is essentially the same as Nitronic 23-13-5.

http://www.smt.sandvik.com/en/materials-center/material-datasheets/wire/spring-wire/sandvik-2rk66/

http://www.smt.sandvik.com/en/materials-center/material-datasheets/strip-steel/sandvik-13rm19/

http://www.centralwire.com/_assets/bro_spring.pdf

 

[dgallup]

I appreciate the help. Just need a higher strength material than the Nitronics 40 or 50.

First I've heard of 904L, I'll look into it. Hope the availability is decent, so many of these alloys are 2000 lbs and 4 to 6 months.

By the way, Diamond Wire is correcting their web site link above about 17-7 PH, it will have a permeability greater than 40 at 200 Oe.

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[EdStainless]

I just used data at hand. 209 is regularly drawn to full spring temper but I don't know the specific properties.
One reason that 304 and 316 reach the strength that they do is because of the martensite transformation, which also makes them magnetic.
You might look for C276 spring wire also.

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