Abstract
Creep strength of ferritic and austenitic steels has been investigated on the correlation between stress vs. creep rupture life curve and 50% of 0.2% offset yield stress (half yield) at the temperatures. Inflection of stress vs. creep rupture life curve was recognized on ferritic creep resistant steels with martensitic or bainitic microstructure. However, no identifiable correlation was observed on ferritic steels with ferrite and pearlite microstructure, as well as austenitic steels and superalloys except for several alloys. Ferritic steel with martensitic or bainitic microstructure indicates softening during creep exposure, however, hardening due to precipitation takes place in the ferritic steel with ferrite and pearlite microstructure and austenitic steels. This difference in microstructural evolution is associated with indication of inflection at half yield. Stress range of half yield in the stress vs. creep life diagram of creep strength enhanced ferritic steels is wider than that of conventional ferritic creep resistant steels with martensitic or bainitic microstructure. As a result of wide stress range of boundary condition, risk of overestimation of long-term creep rupture strength by extrapolating the data in high-stress regime to low-stress regime is considered to be high for creep strength enhanced ferritic steels.
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