Development of advanced chromium steels with respect to microstructure and structural stability

Abstract

The really promising way how to improve the creep resistance of chromium steels was found in the early 80th and was based on increasing nitrogen content in the steel up to 0.07 %. Vanadium nitride precipitating especially on dislocations inside subgrains during creep exposure plays the decisive role in the excellent creep strength of modified 9%Cr-1%Mo steel of the P91 type. The beneficial effect of tungsten on solid solution strengthening as well as on the creep resistance was appreciated especially in Japan and steel P 92 has been developed. Nevertheless, high creep resistance of P 92 steel is not only the result of increased tungsten content but boron addition to steel P 92 has also the significant effect. It must be realized that the increasing of Mo and/or W content in the steel should be limited with respect to Laves phase precipitation (about 1%Mo or 2%W) with high coarsening rate and therefore limited effect on the precipitation strengthening. The precipitation strengthening of these steels is ruled by M23C6 and especially by vanadium nitride. Further increasing of CRS is possible by raising nitrogen content.