B31.1 Power Piping Creep Calculations

Hi all, 

I'm currently working on the design of a Power Plant's main steam line. During normal operation temperature is 485°C @ 66 barg while the design conditions are 510°C @ 79 barg. I'd like to know how does the yield strength of the material changes with the different temperatures. I know that for EN 13480 you can select the amount of ours to be considered for creep. However, I can't seem to find this for the B31.1 Code. 

Obviously the design conditions for Temperature comes for a particular situation where the steam temperature goes up (510°C is the Temperature Switch for the Boiler). However, from experience I know that this won't be a common situation (Less than 80hrs a Year) so designing for the sustained design condition doens't seem realistic. 

I've been reading the Code and it says that you should design the pipe for the nominal condition + a little margin determined by the manufacturer of the boiler (hence the 485 + 25°C) but given that the pipe will rarely ever see this condition I don't consider I should be taking into acount creep for 510°C. 

To sum up: I'd like to know how AutoPipe calculates creep stress (for B31.1) and whether I should design for the nominal or design conditions. 

Thanks.