Effect of the thermal ageing on the tensile and impact properties of a 18%Cr ODS ferritic steel

The effects of the thermal ageing at 400 °C, 500 °C and 600 °C during 5000 h on the mechanical properties of a 18%Cr ODS ferritic steel are investigated. A hardening effect is observed after ageing at 400 °C and 500 °C, probably due to the presence of chromium rich α′ particles as suggested by the literature. The impact resistance and the ductility of the material are strongly lowered by the ageing at 600 °C. This embrittlement is characterized on the fracture surfaces by the presence of cleavage facets on the whole range of testing temperatures. The intermetallic σ phase is found to be responsible for the occurrence of cleavage fracture on the material aged at 600 °C, and thus for the significant embrittlement of this material. M23C6 carbides are also observed before and after thermal ageing. The lattice parameters of the σ phase and the M23C6 carbides observed in this 18%Cr ODS steel aged at 600 °C during 5000 h are measured.