Have a look on these standards:

API RP 579-1 / ASME FFS-1: Fitness for Service (Part 10, including annexes)

API Std 530: Calculation of Heater-Tube Thickness in Petroleum Refineries

WRC 541 Bulletin: Evaluation of Material Strength Data for use in conjunction with API 530


As a personal suggestion, try to evaluate the allowable stress within creep range as function of 100000h Creep Rupture Strength (Rr/100000) and Creep Strain Stress Limit (R0.01/1000, R1.0/100000 etc.) as well.

For design lifetime amount different than standard values, use Larson-Miller Parameter approach to evaluate the actual Creep Rupture Strength.


There are many issues to be discussed here, but there is not the right place and the right time for that.
Read the Codes I've mentioned above, but do not exclude other "Mechanics of Materials" and/or "Pressure Vessels" text books.

Good luck!