Definition of Solution Heat Treatment in Austentitic SS 1

[Sparweb]

I'm trying to pin down a specific process for this. Not coming up with a spec for it in the AMS or ASTM catalogues that I am searching.

For a long time I haven't worried about the terminology much. It seemed pretty obvious to me that "solution heat treated" CRES 321 tubing was equivalent to annealed, or close enough. Now I have a purchaser that questions me on the details, and it does make me wonder, is there a difference? Or is it just a semantic question? I can find detailed processes for solution heat-treatment of aluminum alloys, and for precipitation-hardened stainless alloys. Why am I missing a SHT process spec for austentitic alloys like 321?

I have read specs from AMS (such as AMS 5557) which call for "solution heat treatment" of the materials, and I've read through the heat treatment spec's like AMS 2759 (and the sub-chapter 2759/4 for austentitic steels) which don't say "solution heat treatment", but do define "annealing" in detail. AMS 5557 does not say to heat treat per AMS2759 - in fact is doesn't offer a specific process for SHT at all. So what is it, exactly?

Seems like they guys at SAE have missed something. They also don't offer any definition of "pickling" or "passivation" which really seems to leave the final condition of the material unclear, now.


STF

 

[metengr]

I have seen many times when solution annealing is used interchangeably with solution heat treatment. Actually, they are technically the same. The purpose for solution treatment or solution annealing is to remove forming strains and to dissolve all carbides or precipitates at an elevated temperature (specified) followed by water quenching. This results in maximum corrosion resistance for service.

Annealing by itself is a different heat treatment to remove all forming strains.

 

[EdStainless]

For this alloy, 321 you have to be careful.
Solution anneal infers removal of all secondary phases, including Ti carbo-nitrides.
This is not the optimal condition for this alloy.
Typically the final anneal on 321 will be high enough to remove carbides and forming strain, but leave some of the stabilization phase.
In 304 or 316 I would expect to see single phase only. And if the part was welded on residual delta ferrite either.

Yes there are a bunch of gaps in the specs, this is because of the wide variety of product forms and the differing requirements.
There are specs for pickling and passivation of stainless, though I don't recall them off the top of may head.

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P.E. Metallurgy, Plymouth Tube

 

[Sparweb]

My research has turned up exactly what you warn about, Ed. The advantage of 321 (and 347) at high temperature is dependent on what is called (in the ASM Handbook, for example) as a "stabilizging anneal". It consists of holding 1550F-1650F for up to 5 hours, which is cooler than the anneal temp but much longer. Somehow the inclusion of titanium or columbium in 321 prevents precipitation of carbides in this range.

Given how scrupulous the military is about every process in every material they procure, I would have thought that they would have pinned down a spec for solution heat treat, for themselves (as a MIL-H-###) at the very least, if not before turning the whole lot over to SAE when they developed the AMS system.

The parts in question are just hydraulic tubes. No welding. Just bending, flaring, swaging. I won't have to dig further into the heat-treatment business for now. If you find any hints about SHT in the future, please check back! I'd like to pin this down to a specific standard, instead of 3 indirect references and a musty old textbook.

STF

 

[EdStainless]

Hold on, never use 321 for hydraulic applications unless you need to do a lot of welding, and you are concerned about corrosion after the welding. I would not trust the fatigue properties in an alloy designed to have many inclusions.
There are numerous specs for 304 (both L and straight grade, both seamless and welded, and both annealed and cold worked) for hydraulic service. Many of these specs require some additional testing.

321 and 347 serve two uses. One is in higher C grades you can weld them without forming CrC (sensitization) in the HAZ. This use is actually sort of passe because most 321 and 347 today is supplied with very low C (0.015 or less) and actually will not sensitize anyway.

The other use is for service in the 1150F-1500F range where formation of massive grain boundary Cr carbides will occur. For this service you need to run the material hotter (either before installation or as part of startup), this is the stabilization where Ti combines with all available C and N and then prevents the subsequent formation of Cr carbides.

In both of these it isn't the Cr carbides themselves that are bad, but when they form (say at 1250F) the diffusion of Cr is very slow. The result is that it leaves a significant Cr depleted region next to the carbides. Since Cr is what gives the alloy corrosion resistance you end up with locations that are very susceptible to attack. If the bulk alloy is 18% Cr the regions next the carbides may be as low as 10%, and the HAZ gets ditched out.

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P.E. Metallurgy, Plymouth Tube

 

[Sparweb]

Ok, here's where the fun begins.
These aren't being used for hydraulics. They are in fact distributing Halon in a fire suppression system. The selection of material is mostly out of my control, because I am staying in step with the existing tubing in the aircraft. Which is 321 (with some exceptions). Ignoring these exceptions for a minute, I also had to use 321 because the aircraft OEM has laid out all of the necessary forming, swaging, and splicing practices that our crew must follow as we add to the system, for 321. Now back to the exceptions. Do you think I should have used titanium? Because that's the alternative. I told you it was going to be fun.
Anyway, I don't believe that cold-working (bending/swaging) will lead to increased susceptibility to corrosion. Since this fire extinguishing system gets used "never" in the life of the aircraft, except for the one time we test the whole system, it is actually long-term corrosion in the range of -50C to +40C and 0 to 100% humidity that drives the material selection.
Note: The AMS spec for this tube (and others like it, like AMS5561) does call it "hydraulic tube", though I think I understand your meaning.
Thank you for the added info.

STF

 

[EdStainless]

How light weight is this tube? very thin wall?
If not then going to a cold worked stainless would let you thin the walls more.
All of these materials (even the AMS 5561 at 150ksi) get bent and flared, there are ways to do it.
AMS 5561 with detailed UT and such is overkill for this, but AMS 5564 or 5566 may be a better option.
Class 2 is welded and drawn, 95ksi min UTS, and ET inspected (not UT). This would work very well for the application and allow very thin walls. This can be joined by brazing, welding, or mechanical fittings.

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P.E. Metallurgy, Plymouth Tube

 

[Sparweb]

Great suggestions, which bring me back full circle.
My choice of AMS 5557 was for formability and reliable swaging, and the walls will be thin enough that weight isn't a problem, nor is the system pressure (factor of about 5). My own (limited) shop experience with bending tubing is that there is a range that works best - too thick and it cracks, too thin and it wrinkles. No matter the material no matter the heat treatment, there will be a range where the tubing can be formed best, but outside that range, expect scrap. Or expect the guy to get out the blowtorch - I don't want to go down that road. So I selected the easiest to bend available in the OEM approved materials (welded AMS 5561 was in the list, but on the criteria I just set out, 5557 was better) to avoid fabrication problems, not knowing with certainty the capabilities of the people who will do the work. Last step in the selection process was to talk to the guy in my own shop who did this regularly and look at the stock on his shelf. Too bad he won't be making these lines. He told me himself that bending the annealed CRES was easiest, but when I started looking for "annealed" stainless tube, all I found was "solution heat treated". That's how I arrived at the game of words. I'm still satisfied with the material choice, but it's true that your suggestion, AMS 5566 may have been a better option, if it proves to be just as easily formed with the tools available as 5557. The only way to prove to myself in advance is to order small quantities of each, bend them to the same profile and swage with the same tools, and observe the results. Time doesn't permit many of my little science experiments - I've already taxed my management's understanding of preliminary tests on this project so far!


STF

 

[EdStainless]

We make 5561 (all welded an drawn, required for high fatigue resistance) in sizes from 0.250" x 0.016" to 1" x 0.088" (and some special cases larger).
All of the bends are made using internal mandrels, all bends have less than 3% ovality, and it gets flared.
For high fatigue applications 5561 is the best option, otherwise not worth the cost.
In general working with walls that are 5-10% of the OD for high strength and 10-15% for annealed grades will work out fine.
The rules do change with larger (over 1.5") tube.
5564 and 5566 are slightly harder than annealed tube, but not by much. They are easier to form without bucking and bit more tolerant of handling and such.

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P.E. Metallurgy, Plymouth Tube

 

[Sparweb]

Yeah it says so on your website
I've bought from Ply in the past (usually it was 4130 square structural, though.)

STF