TZM for High Strength Bolting for 1500F Vacuum

Q:
I’m looking into a bolt which will be used to create vacuum seal with graphite and hold vacuum of 1 Pa. I’ve tried Inconel 718 but they seem to fail loosen at high temperatures, and my system requires 1500 F. Because I am trying to seal a vacuum system, it requires very high internal strength to compress the gasket. Ideas? The bolts are inside process / Vacuum an not on the outside.
A:
Inconel 718 bolts are age hardened, so at 1300 F, they will be re-annealed and lose the strength from the hardening treatment. For the highest strength at 1500 F, in vacuum, I would recommend TZM alloy screws or perhaps tungsten. They should have excellent room temperature strength as well as high strength at 1500 F. As long as there is NO oxygen in the system this would be a good choice.
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