Microstructure and high-temperature strength of high Cr ODS tempered martensitic steels

11-12Cr oxide dispersion strengthened (ODS) tempered martensitic steels underwent manufacturing tests and their ferritic–martensitic duplex structures were quantitatively evaluated by three methods: high-temperature X-ray diffraction (XRD), electron probe microanalyzer (EPMA), and metallography. It was demonstrated that excessive formation of residual-α ferrite, due to increasing Cr content, could be suppressed by appropriately controlling the concentration of the ferrite-forming and austenite-forming elements on the basis of the parameter “chemical driving force of α to γ reverse transformation. 11Cr-ODS steel containing a small portion of residual-α ferrite was successfully manufactured. In the as-received condition, this 11Cr-ODS steel was shown to have satisfactory creep strength and ductility, both as high as those of the 9Cr-ODS steel, while its 0.2% proof strength at 973 K was lower than in the 9Cr-ODS steel.