Influence of Oxidation on Long-Term Creep Strength of ASME Grade 23 Steel

Creep test of ASME Grade 23 steel has been conducted at 625 and 650°C in helium gas atmosphere. Long-term creep strength of the steel in helium gas was compared with that in air and the influence of oxidation on long-term creep strength was investigated. Creep rupture strength drop was observed in the long-term at 625 and 650°C in air, and the same creep rupture strength drop was observed also in helium gas at 625°C. On the other hand, although creep rupture strength drop was observed in the long-term at 650°C in helium gas, creep rupture life in the long-term in helium gas was slightly longer than that in air at 650°C. Creep rupture life in the long-term at 650°C in air is reduced by not only degradation due to microstructural change, but also marked oxidation, however, that at 625°C is considered to be shortened mainly by a degradation caused by microstructural change. Long-term creep strength of ASME Grade 23 steel at 600°C and above should be reevaluated in consideration of strength drop due to microstructural change.