Recent Advances in Creep Resistant Steels for Power Plant Applications

The higher steam temperatures and pressures required to achieve an increase in the thermal efficiency of fossil fuel fired power generation plant necessitate the application of steels with improved creep rupture strength. The 9% chromium steels developed during the last three decades are of great interest for components of advanced, high efficiency power plant. In this Paper, the development of the steels P91, P92 and E911 are described. It is shown that the martensitic transformation in all three steels produces a high dislocation density that confers a high degree of transient hardening. However, the dislocation density decreases during exposure at service temperatures due to recovery effects and for the long-term creep strength the sub-grain structure that is produced by the martensitic transformation and the precipitation of carbides, nitrides and carbonitrides are the decisive microstructural features. The changes in the microstructure mean that great care is needed in the extrapolation of experimental data to obtain design values. Only data from tests with rupture times above 3000 h provide reasonable extrapolated values. It is further shown that for the 9% chromium steels, the oxidation resistance in steam is not sufficiently high for their use as thin-walled components at temperatures of 600°C and above. The potential for the development of steels of higher chromium content (above 11%) to give an improvement in steam oxidation resistance whilst maintaining creep resistance to the 9% chromium steels is discussed.