The Significance of the Tertiary Creep Criterion in Setting Time-Dependent Allowable Stresses

Abstract

The database for Alloy 800H was reviewed with respect to the initiation of tertiary creep and subsequent rupture. Data covered temperatures in the range of 600 to 1000°C and times to 100,000 hours. The establishment of the time to initiate tertiary creep presented a problem because of the non-classical behavior of creep often exhibited in the creep curves. Stress values based on the current tertiary creep and rupture criteria were compared for several heats of material considering both classical and non-classical creep data. Fits to tertiary creep showed more scatter and it was found that 80% of the minimum stress for tertiary creep was generally lower than 67% of the minimum stress to rupture. Using only data from classical curves did produce stress values higher than those from the rupture criterion.