A study on Z-phase nucleation in martensitic chromium steels

9-12% chromium martensitic steels are liable to the precipitation of Z-phase, Cr(V,Nb)N, after long time exposure at 550-650 °C. This complex nitride consumes vanadium nitrides and causes the creep strength of the material to fall drastically after several thousand hours of exposure. In this work, initial stages of precipitation of Z-phase have been studied and characterized using energy-filtered transmission electron microscopy (EFTEM). Vanadium nitrides were found to provide the most suitable nucleation site for Z-phase, since the misfit between the (0 0 1) planes of VN and Z-phase is very small. Furthermore, such a nucleation site would provide vanadium and nitrogen for the growth of Z-phase. The presence of niobium carbide has also been observed close to Z-phase nucleation sites, indicating niobium to be important for the nucleation and growth of Z-phase.