Microstructure Variation of Ultrasuper Critical 11Cr Ferritic Steel Under Long-Term Thermal Ageing and Creep

The microstructure variation of ultrasuper critical 11Cr ferritic steel was quantitatively investigated under long-term thermal ageing and creep, with the emphasis on the size of precipitates, martensite lath width, and dislocation density. The tests were interrupted at different deformation stages. The precipitates on prior austenite grain and martensite lath boundaries became obscure due to coarsening of secondary particles. The Laves phase grew faster than M23C6 carbides in both creep and thermal ageing. The dislocation density rapidly decreased at the initial stage, while the martensite lath width increased with ageing and creep duration. The microstructure variations under creep were more pronounced than under thermal ageing, indicating that stress exerts an important effect on microstructure degradation.