Stress Values for Incoloy 800H

[RooRoo]

I have a situation where a furnace is operating at 1922F with 3/4" Sch40 tubes. The Boiler & Pressure Vessel Code publishes allowable stress values up to 1650F. Does anyone have a recommendation for finding allowable stress values above 1650F for SB-407 N08810 pipe?

 

[TGS4]

Hwat design Code/Standard is the furnace designed to? I would look there first.