As a rough guide, I can tell you that for boiler and pressure vessel service, the maximum service temperature for copper and copper alloy tubing, and fittings is 400 deg F.
Up to 300F the stress rating is the same as for room temp.
Going from 300 to 350 the reduction is 50%.
From 350 to 400 the allowed stress drops to about 20% of the RT values.
For any temp above 300F they warn that creep will be a factor.
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Rust never sleeps
Neither should your protection