Prospects for Martensitic 12 % Cr Steels for Advanced Steam Power Plants

Creep strength improvements of martensitic 9 % Cr steels have been obtained by controlled additions of V, Nb, N and B to the steels, which resulted in precipitation hardening by fine stable nitrides based on V and Nb as well as in stabilization of Cr carbides against coarsening. The best steels P92 and FB2 are now used in power plants up to 600-620 A degrees C steam temperature. For higher steam temperatures up to 650 A degrees C steels with 11-12 % Cr are needed for better resistance against steam oxidation. However, fine V and Nb based nitrides may transform to coarse Z-phase [Cr(V,Nb)N] nitrides in steels with such Cr contents, leading to a strength loss in long-term creep. The paper presents the status of development of new stable 12 Cr steels based on the concepts of either strengthening by fine Z-phase nitrides or on elimination of nitrides from the steels.