Stress relief of 302 C.R. stainless steel, 0.004" thick
Stress relief of 302 C.R. stainless steel, 0.004" thick
(OP)
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
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
RE: Stress relief of 302 C.R. stainless steel, 0.004" thick
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
RE: Stress relief of 302 C.R. stainless steel, 0.004" thick
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."
RE: Stress relief of 302 C.R. stainless steel, 0.004" thick
Michael McGuire
RE: Stress relief of 302 C.R. stainless steel, 0.004" thick
RE: Stress relief of 302 C.R. stainless steel, 0.004" thick
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
RE: Stress relief of 302 C.R. stainless steel, 0.004" thick
@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.