Creep properties of advanced heat-resistant martensitic steels strengthened by L10 type ordered intermetallic phase

Creep properties of advanced heat-resistant martensitic steels with the compositions of 0.08C-0.3Si-0.5Mn-9Cr-3.3W-(0/3)Pd-0.2V-0.05Nb-0.05N-Fe (in mass%) have been studied to explore the fundamental guiding principles for the development of new steels with improved creep strength at higher temperatures over 650 degrees C. The steel with 3%Pd exhibits higher creep strength than that of the base steel, in particular, at higher temperatures. The microstructure of the steel with Pd is characterized by fine precipitation of a FePd based Ll(0) type ordered phase, alpha " phase. The alpha " phase forms in the tempered martensitic matrix, alpha, along with the existing phases such as MX (M; V, Nb, X; C, N) carbonitride and M23C6 (M; Cr, Fe, W) carbide after normalizing at 1100 degrees C and tempering at 770 degrees C. A detailed TEM observation has shown that the alpha " phase precipitates in a plate-shaped with the habit plane parallel to {001}(alpha) and with a crystallographic orientation relationship with the matrix, {001}(alpha) // {001}(alpha) and < 100 >(alpha) // < 110 >(alpha "). The alpha " phase is found to be a most effective precipitate to increase creep resistance of the steel in terms of the inter-particle spacing estimated. The alpha " phase is stable even after aging at 700 degrees C and hence is more useful at higher temperatures. It is thus concluded that the creep resistance of the ferritic steels can be increased further by optimizing the combination of fine precipitation such as the alpha " phase and MX carbonitride.