Creep modeling and strengthening mechanism of tungsten alloyed 9-12% Cr steels for power applications

Recently, high strength, tungsten (W) alloyed steels have been developed for use in power plants with higher steam conditions resulting in improved thermal efficiency, lower fuel costs, and lower emissions. In order to establish a creep modeling of high strength martensitic steel and to understand the basic role of W in tungsten alloyed 9-12Cr steels, the conventional martensitic steels (X20CrMoV121, X20CrMoWV121 and Mod.9Cr-1Mo) and tungsten alloyed steels (NF616 and HCM12A) were subjected to creep tests and the data were analysed by the Omega method.(1) The proposed creep model, which takes into account both primary and tertiary creep, satisfactorily described the creep curves and accurately predicted creep Life, as martensitic steel undergoes a relatively large amount of primary creep of up to approximately 30% over normal Life. The tungsten alloyed steels exhibited a smaller minimum creep rate and a larger stress exponent compared with the conventional steels. Also, in tungsten alloyed steel the Omega value features strong stress dependence such that creep life is prolonged at lower stresses due to high Omega values. The importance of the Omega value from the standpoint of creep strengthening in primary and tertiary creep is discussed.