Stress to produce a minimum creep rate of 10 −5%/h and stress to cause rupture at 10 5 h for ferritic and austenitic steels and superalloys

Abstract

Long-term creep and rupture data of NIMS Creep Data Sheets show that the time to rupture at a minimum creep rate of 10 −5%/h is longer than 10 5 h for most ferritic steels, while it is shorter than 10 5 h for most austenitic steels, Ni base and Co base superalloys. This is correlated with the larger total elongation in ferritic steels than in austenitic steels and in Ni base and Co base superalloys at a minimum creep rate of 10 −5%/h. However, 67% of the average stress to cause rupture at the end of 10 5 h is lower than the stress to produce a minimum creep rate of 10 −5%/h for all the materials examined. This indicates that the ASME allowable stress in a high-temperature creep region is determined by creep rupture data but not by creep strain data. The creep and creep rate curves are compared between 2.25Cr–1Mo steel and 18Cr–8Ni austenitic steel at approximately the same minimum creep rate. 18Cr–8Ni austenitic steel reaches a minimum creep rate in shorter time and spends shorter time in the acceleration creep before creep rupture than 2.25Cr–1Mo steel at low stress and long time conditions. This is correlated with different microstructure evolution during creep.