Development of new 11%Cr heat resistant ferritic steels with enhanced creep resistance for steam power plants with operating steam temperatures up to 650 °C

The goal of developing new heat resistant 11%Cr ferritic-martensitic steels with sufficient creep and oxidation resistance up to 650 °C was pursued within a joint project following an alloying concept based on physical metallurgy principles. The highest creep strength combined with good oxidation resistance was achieved for a Ta-alloyed test melt (11 wt.% Cr, W, Co, Mo, V, 0.09 wt.% Ta, relatively high contents of C and B). The microstructural evolution during creep was investigated by transmission electron microscopy for the Ta-alloyed melt in comparison to a sister melt where Ta was exchanged for 0.04%Nb. It is proposed that fine particles of types MX and M23C6 (M: metallic element, X: interstitial elements) are the cause of the outstanding creep resistance of the Ta-alloyed melt.