Q: We are having issues with a high temperature flanged joint where the bolts are continually loose when they cool down the system. The issue to me sounds like the bolts are losing tension due to the temps being too high and softening the bolts so they are plastically deforming. Using 718 bolts at temps are around 950C. Any recommendations?
A: Looking at your design parameters and making some assumptions it looks like you may be relying on the Inconel 718 bolts for its strength. However the problem you have is that Inconel 718 is an age hardened alloy. The means it gets its strength from a specific heat treating process at about 700C. At your process temps of 950C you are undoing the aging process and essentially left with annealed Inconel 718 (Which would be similar to an Inconel 600 in strength).
You may be better off starting with Inconel 625 bolts, this way the material is torqued properly and will not lose so much strength upon heating. Although at those temps I would not be surprised if you had to re-tighten from time to time.