Evaluation of mechanical properties and nano-meso structures of 9–11%Cr ODS steels

• We successfully manufactured 11Cr-ODS steels with residual α-ferrite controlled.
• Dispersion conditions of nano oxide particles were quantitatively characterized.
• Tungsten solid solution could improve only tensile strength of ODS steels at 973 K.
• Oxide dispersion strengthening was dominant in creep strength of ODS steels at 973 K.

This study carried out mechanical tests and microstructural characterizations of several 9Cr and 11Cr-ODS tempered martensitic steels. From those results, the appropriate chemical composition range of 11Cr-ODS tempered martensitic steel was discussed from the viewpoint of high temperature strength improvement. It was shown that the residual α-ferrite fraction in 11Cr-ODS steel was successfully controlled to the same level as the 9Cr-ODS steel, which has excellent high temperature strength, by selecting the chemical compositions on the basis of the multi-component phase diagram. The tensile strength decreased with decreasing W content from 2.0 to 1.4 wt%. On the other hand, creep strength at 973 K did not degrade by the decreasing W content. Both tensile strength and creep strength increased with increasing population of the nano-sized oxide particles. Small angle X-ray scattering analysis revealed that titanium and excess oxygen contents were key parameters in order to improve the dispersion conditions of nano-sized oxide particles.