SS316L diaphragm 2

[Pras]

Hi,
I need to know the stress relieving procedure (time/temp) for SS316L corrugated diaphragms (0.1mm thk, 48mm diameter).

Thanks in advance

Pras

 

[unclesyd]

What are you trying to accomplish with the stress relief? Are the parts too stiff, curling?
The reason I ask is that in working with very thing S/S I had to go almost to the annealing temperature to get the desired end product.

 

[BobUXL]

600-900F fot 4-6 hours should stress relieve 316

 

[Pras]

unclesyd / BobUXL,
Thanks for your reply.
unclehyd,
I need to use the said diaphragms for very sensitive pressure measurment application. Hence would like to ensure that all the residual stresses after cold working are effectively removed.

Pras

 

[unclesyd]

In my work with thin S/S diaphragms, I found that relatively high temperatures were required to remove stress imposed from forming thin parts. I derived my temperatures by experimentation as the tables values wouldn’t accomplish what I wanted. I used 1200°F for 30 minutes. You may be able to go to a lower temperature, but increase the time, 30 min/50°F. At this temperature there was considerable stress relief.
As you know forming or working Austenitic S/S results in a high degree of working hardening. To remove the effects of work hardening short of annealing required a higher temperature. If you would like the physical properties back to the approaching the starting material then use 1500°F for 5-10 minutes. Make sure you parts are clean and don’t allow the parts to stack. Do a few at a time I would try both temperatures to see what you need. One thing that bugged me for a while was getting variable results that urned out to be due to thickness variations in my starting material, shim stock, the tolerances were +/- 10%.
I know some people will come after me for this advice, but it worked. I made high pressure rupture disks for many years that were several orders of magnitude more accurate than anything available at the time.

 

[Pras]

unclesyd,
Do we need to worry about IGC while the diaphragms are being heated to 1200°F or so. Also do we need to have inert atmosphere while performing the stress relieving?

 

[mcguire]

No IGC problems with L grades. Protect from oxidation as much as possible, because any heat tint or heavier oxide must be removed.
Unclesyd is right on target as far as I am concerned. Stress relief is really just creep, so you reach zero residual stress asymptotically. You won't get zero unless you recrystallize, which requires more like 1800F.

 

[unclesyd]

Excuse this if it duplicates

Unfortunately my system wasn’t corrosive to S/S. I did think of carbide precipitation, that’s why I strived to keep the time at temperature as short as possible. Sensitization is fortunately also time dependent. My initial worry was forming a metallurgical notch, being such a thin section. These discs were used on systems with pulsating pressure and analysis of the initial failures showed that all failures were due to pure strain. Over pressure occurring at a very high strain rate.
As to the inerting of the furnace atmosphere I found it to be matter of aesthetics. You will have a heat tint or oxidation layer that is hard to remove. I was able to clean some of my discs and saw no difference in its’ properties functioning as a rupture disc. My cleaning procedure, considered pretty aggressive, would have identified any carbide precipitation.
Depending on the corrosive media the heat tint could be beneficial. Also being L grade helps. In your application do you see any effects of welding the 316L S/S material in other areas. My original discs were 18/8S/S (302). After the first batch we used 316 S/S material, not verified only ordered as such. We ordered several packages of the same lot to eliminate some of the variability in materials.
If the facilities are available inerting couldn’t hurt.
If you have the time I would run some tests on the material used in your disc. Try shorter times at temperature. Also don’t give up on the higher temperature at very short times.
Make sure you use a muffle furnace, no exposed elements.
Keep the parts clean and no fingerprints.
All testing will have to be on the same or very similar thickness parts. I found that data didn’t translate very well.
If this doesn’t help please respond as being retired and recovering from Prostate Cancer treatment. I seem to have a problem with my hydraulics. The computer is next to the W.C.

All my work was done under extreme time constraints and I didn’t have the time to do some of the testing I would have liked to. But through a few disappointments and cat-calls I was able to make a viable rupture discs.
If you think it will be helpful I can give you the exact procedure I used to make the rupture disc

 

[BobUXL]

In order to get rid of all the stresses in these diaphrams, you will need to fully anneal the parts. However, since the anneal also includes a water quench, you will severly deform the diaphram. I am not sure whether vacuum annealers have enough quenching capabilities to properly quench stainless.

 

[unclesyd]

BobUXL,
That is why I mentioned the higher temperature(1550°F) in my post. I could get rid of say 95% or better of the stress at this temperature. This was measured by the end result that is that my parts were very uniform batch to batch with different material batches. I think it if can't do what he needs to get at the lower temperature he can accomplish it at the higher one.

 

[BobUXL]

unclesyd,
I was more concerned about the keeping the flatness of the diaphram after quenching. Yes, you will get rid of the stresses, but if you have to restraighten or flatten the diaphram, you will reintroduce the stresses. The trick will be to maiuntain shape after annealing.

 

[unclesyd]

The parts are thin enough that they can air cool with no problem. As I mention they will have a heat tint after heat treatment. I was making rupture discs to operate at 6750 psig and saw no incident where this had any effect on the discs. I started out using a flat piece of shim stock and they stayed flat, that is I could see no problem in my jig.
They can be supported and taken out as group. I heat treated mine in baskets (20-25 discs at a time) made of 310 S/S perforated metal, just because it was there. Get a piece of your starting material wrinkle it up and do a little testing on it. If your furnace has ceramic fiber insulation take a test sample and treat it and then open the door and let it furnace cool. The reason in trying to hold the time at temperature down is the possibility of grain growth.

How many of these are you having to do?

If there are any questions please ask.

 

[Pras]

unclesyd / BobUXL / mcguire,
Very sorry for responding after a very long gap in this thread.
Let me put my requirement in simpler words :-

I need to use SS316L and also SS316 diaphragms in one of the pressure measurement applications. The thickness of the diaphragms would vary from 0.1mm to 0.4mm.

I would like to eliminate the residual stresses in the diaphragm to the best possible extent before putting them to use. Finally what I want is the linear curve of diaphragm deflection vs pressure. Tried stress relieving of 0.1mm thk diaphragm at 410°C-420°C for 6 hrs. Did not work as I could not get the linear relationship of deflection / pressure as mentioned above.

could you please advise further.

Thanks in advance.

Pras