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Low temperature impact on 304L permeability

Q. Hello, I am an engineer with admittedly little M&P knowledge (though its growing by the day!) working on a perplexing issue.

I have a small 304L stainless steel tube part that has been annealed in a vacuum for 2-4 hours at 2150 °F. It is then cooled at a rate of 100-200 °F/hour. This part is confirmed to have a relative permeability of 1.00, and then put through several thermal cycles. At approximately -15 °F, the permeability of the part begins to increase at a significant rate, reaching nearly 1.50 by the final temperature of -65 °F.

Interestingly, these same parts have been in this design for decades, and this phenomenon has seemingly not been an issue in the past. Additionally, by annealing these same parts using a cycle of 15 minutes at 1950 °F, followed by a gas quench, there is no increase in permeability at low temperature.

I have looked at a significant number of articles and journals, mostly revolving around the use of Austinitic stainless steels in cryogenic applications. There are definitely similar phenomenon at those extremely low temperatures, but they're so far out of the range of the relatively benign temperatures we're experiencing this at.

Does anyone have any ideas on what might be causing this increase in permeability? I have not found any process changes that line up with this sudden tendency to increase in permeability with response to cold temperatures, and I'd sincerely appreciate any additional places I can dig further into.

Please let me know if there is any additional information I can provide, we've been bashing our heads against this for weeks now, and we'd greatly appreciate the help!

Shawn Falzone
- Buffalo, New York
November 30, 2020