Annealing SS430 to maximize magnetic permeability 3

[Bfrost]

Hi,

I recently bought some SS430 tubes from China, expecting a certain magnetic permeability. However, the magnetic permeability seems very low when I tried using it as a core. Further research shows that the cold working involved in making it into a tube probably reduced its magnetic properties, and annealing it should bring it back. I've tried a few options so far:

- heat to 800C for 10 minutes and then air cool down to room
- Heat to 800C for 10 minutes and water quench (suggested by manufacturer)
- Heat to 800C for 10 minutes, lower temperature to 600C in oven over 2 hours, and then air cool

None of them seem to have improved the magnetic properties significantly. Any thoughts / ideas on what I should do to return it to its expected values (what I found in literature)? Thanks.

Kazi

 

[btrueblood]

If you have fairly "dirty" steel, it may not ever get the permeability you want (see ref. below). If you want the highest possible permeability, repeatably, you should look for 430F or 430FR "solenoid quality" steels.

https://www.cartech.com/en/alloy-te...icles/magnetic-properties-of-stainless-steels

 

[MagMike]

Most stainless steel producers do not control the magnetic permeability, even for the 400 series.

Of the three annealing options you listed, the third one was the best. If that didn't bring you the permeability you want, then you might need to explore other alloys or suppliers that are willing to control/guarantee a particular permeability value.

Just out of curiosity, what level of permeability did you need/require?

 

[EdStainless]

What is the C content of this material?
This should be 430S, low C if you want higher perm.
IF this is high C material (martensitic) then even reaching a value of 1000 could be difficult.
If it is the low C (ferritic) alloy then you should be able to get 1500-2000
Is the saturation OK?

Quenching from 800C is correct, you should be able to verify this by micros.

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P.E. Metallurgy, Plymouth Tube

 

[Bfrost]

Thanks guys. Unfortunately we didn't have a lot of options in terms of getting the tubes since we wanted small quantity and most places have large minimum orders for 430. A Chinese manufacturer had some in stock so sent us samples. I didn't realize even within the standard of "430" there could be so many differences in permeability that cannot be fixed by annealing? Isn't part of the 430 material definition the maximum amount of C, Cr, etc. it can have?

 

[EdStainless]

Yes, but it is like 304 and 304L.
All 304L is both 304L and 304, not not the other way around.
And 430 comes as both martensitic (higher C with no minimum) or 430S (for special) that is ferritic (lower C).
What spec did you order to?
What UNS number was called out?
To make it more confusing there are also stabilized versions of 430 with Ti and/or Nb in them.

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P.E. Metallurgy, Plymouth Tube

 

[djhurayt]

We use a lot of 430FR in solenoids.
Our recipe is:
anneal at 800-850°C for 1 hour in a dry hydrogen atmosphere
cool at 100°C per hour(max)rate to 500°C, any cooling rate thereafter

 

[Bfrost]

Oh man...didn't know all these details since I'm not a metallurgist. I found the attached curve in a paper, realized it would work, and ordered 430 tubes. If we need to be specific about type of metal (like UNS number or other spect), any help would be much appreciated. Also, cost is a concern, so something too specialized or very expensive wouldn't do. Thanks!

 

[EdStainless]

We make welded tubing here in the US from 430S.
Quantities are dictated by the coil sizes that the mills will sell us.
We deliberately buy the C in the 0.05-0.09% range, it still forms and welds easily but also anneals as ferritic.

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P.E. Metallurgy, Plymouth Tube

 

[Bfrost]

So I've been doing a bunch of reading, and looks like permeability is affected by a number of factors.

If I'm looking for material with permeability above a certain amount, what's the best way of specifying that to a supplier? I imagine most manufacturers don't measure permeability? Are there specific codes I should mention? Should I specify some form of treatment? Or would it be normal to specify magnetic permeability? Thanks!

 

[MagMike]

What minimum value of permeability do you require?

 

[EdStainless]

Unless you are buying it as magnetic material from someone who does this for a living then you will get no help.
If someone asked us to work to a perm we would ask them to spec chemistry and validate a heat treatment, we wouldn't guarantee perm.

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P.E. Metallurgy, Plymouth Tube

 

[Bfrost]

MagMike, We would be looking for relative magnetic permeability of 600+ (this is what I saw on the paper).

EdStainless, based on that, any thoughts on chemistry / heat treatment that should deliver that or above?

 

[MagMike]

A magnetic permeability of 600+ in 430 is reasonable although I agree with others who have commented: It'll be very difficult to find a vendor willing to control/guarantee a permeability value. You'll likely have to assume the risks.

I'll defer to EdStainless for a recommendation of heat treatment.

 

[MagBen]

Specify 430 solenoid quality, you will get a max perm > 600 easily. Solenoid quality assure consistently high perm by controlling chemistry and processes. Normally a max coercivity is specified and accepted, but not for a min permeability, which, however, can be tested and certified.
For a maximized perm, you should at least get a ferritic structure with an anneal HT slow cool. The max carbon is normally <.065%. The material you got possibly has a high carbon content, fast cool (water, air cool) further helped to get a martensitic structure, otherwise, 600+ perm is not so difficult to get for a ferrite SS.

 

[dgallup]

You do NOT want 430F or 430FR for welded tubing. The F stands for free machining which means they have added sulfur to make the chips break. Unfortunately, the sulfur also makes the welds break. I am not familiar with the 430S that Ed stainless mentioned, I have never seen that in a standard specification. We had to write our own spec for non-free machining 430. For solenoid quality free machining 430F/FR you can use ASTM A 838 which lets you specify magnetic properties.

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