Metallurgy and creep properties of new 9-12%Cr steels

Current understanding of the creep strengthening mechanisms in newly developed 9-12%Cr creep resistant steels is discussed. It is demonstrated how new computer programs for calculation of equilibrium phases in the steels can contribute to increased understanding of the microstructure of these steels. The calculations are useful for, e.g. estimation of the risk of delta-ferrite formation, and of balancing of the addition of V, Nb and N so M(2)X formation can be avoided. Combination of the equilibrium calculations with kinetic models and experiment has led to a description of the precipitation of Laves phase in W-alloyed 9%Cr steels. Combination of these results with creep tests indicated that Laves phases have a creep strengthening effect through precipitation hardening, and that any solid solution strengthening effect of W is small.