Without current ASME Code books, I do not have a specific answer about the required changes for re-rating.  However, 316ss should not be too brittle or weak at -80 degrees F.  The vessel does need to be rechecked by an ASME Code shop and re-inspected by an ASME Code inspector, with a new nameplate reflecting the lower temperature for it to be insured and used at the lower temperature.  Unless the vessel is a very marginal design or has been modified, most of the re-rating should be paperwork and inspection.

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