Stress relief of 302 C.R. stainless steel, 0.004" thick

[Werecow]

Some background: One of the products made by our company was designed almost 50 years ago. Needless to say, the original engineers are long gone, and downsizing over the years has eliminated almost all of those positions. One of their legacies is a stainless steel part spec that we're wondering about, and I'm just a lowly EE, so this is way out of my field.

Material description is: CR Stainless Steel, 0.004" thick; Temper: Full Hard; Hardness: VHN452 (C 46); Ultimate Tensile Strength: 203,000/205,000 PSI. Mat'l Specs: AMS-5906B, AMS-5913B, ASTM-A-666-15. The part is a flat spiral spring, acid etched, 0.995" dia., appearance very similar to the image below:

The problem is that the spec calls for stress relieving, 3 hours at 600F, air cooling. The supplier has tried this using the full 12" strip size, before etching, and has extremely low yield of the finished parts due to warping of the strip.

Our questions are: 1) What benefit, if any, is derived from stress relieving this part; and 2) If it is beneficial, should stress relief be done before etching, or after? (My novice brain tells me "after", but what do I know?) The springs are constantly subjected to axial deflection up to +/- 0.100" and temperature up to 500F in the final assemblies, if that affects the answers.

Thanks in advance for any guidance.

-Brian

 

[EdStainless]

SR will improve long term stability and fatigue resistance.
The SR temp does need to be above the max service temp.
The SR should be after all steps.
The distortion is likely due to uneven heating and cooling. Especially with thin parts this must be done very gently.
These could be put on a very flat SS plate, stacked up, and have another SS plate on the top to hold them flat.
Put into an oven at say 300F, let sit 30 min, and then gently ramp up the temp. This could take 3 hours to get to temp.
Make sure that they cool in still air.


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P.E. Metallurgy

 

EdS,

Does 600°F accomplish any degree of stress relief in austenitic SS (I believe this one is 302)?

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[mcguire]

You may do better getting the strip stretcher leveled to eliminate residual stress.

Michael McGuire

 

[TugboatEng]

This is a good example of pedigree. The guy that used to make the part was probably darned good at what he did and could crank these out without specification. Now that that guy is gone you don't have the luxury of simply asking for a part to be made. Now, you're going to need to spend the time and money to develop a specification. This may require quite a bit of trial and error as you've lost the person with the experience to do it without experiment.

 

[EdStainless]

You aren't looking for large scale SR, just enough to lower some of the peeks.
3hr at 600F will do this. Often coils springs are SR at 500-550F after coiling for the same reason.
The original strip stress could be an issue, but that is out of our control.
How the strip was flattened does matter. It would be worth knowing how it was processed.

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P.E. Metallurgy

 

[Werecow]

Thanks to everyone for the answers so far. This doesn't seem to be as simple a question as I'd thought! You have given me some excellent direction to figure out where things are going wrong, though.

@TugboatEng: Yes, a perfect example of pedigree, on several levels. Our instruction was inadequate from the beginning, and "the guy" here who could clarify it is gone. But our vendor, who has been making this part with no problem since 1986, figured out how to process the part correctly anyway. Unfortunately, "the guy" working for the vendor is now gone too, so his knowledge is also lost. Just goes to show that no matter how much you think you've improved your documentation to comply with current ISO standards, a legacy item like this can still come back to bite you.