Design of a new 11Cr martensitic steel and evaluation of its long-term creep rupture strengths

A new grade of 11 wt% Cr steel was designed in which no nitrogen was added to minimise the driving force of the Z-phase formation and hence to maximise the stability of the fine MX precipitate particles. But, 6 wt% Co was added to completely suppress the δ-ferrite formation in the matrix. The creep data were measured in the range of 650–725 °C at different stresses and rationalised on the basis of a new creep model, which revealed that the effect of stress on creep was equivalent to that of temperature. The model parameters so determined was used to predict the 100,000 h creep rupture strengths. The results showed that these long-term creep rupture strengths were higher than those for the P92 Steel, which is a 9 wt% Cr and the strongest steel grade among commercial 9–12 wt% Cr steels at present. The reliability of the long-term creep rupture strength predictions for the new steel is also analysed.