New elevated temperature design criteria are based on time-dependent material deformation behavior. These criteria establish time-dependent allowable stresses which are evaluated from a consideration of three quantities:

  1. the stress to produce rupture
  2. the stress to produce the onset of tertiary creep
  3. the stress to produce 1% total strain.
These quantities must be determined over a time range from 10 to 200 000 h for temperatures from 800 to 1300°F. Allowable stress values for Types 304 and 316 stainless steel are derived from an analysis of experimental creep data. Stress quantities associated with rupture and tertiary creep are largely determined from existing data and analysis methods, while new data and analysis procedures are employed to evaluate the stress to produce 1% strain. An analytical formulation describing the transient and steady-state creep strain response observed in constant load, constant temperature tests is developed for use in structural analysis. This formulation is used with an equation-of-state approach based on strain hardening for application to variable load or temperature conditions.