20th International Forgemasters Meeting - IFM 2017

The most significant outcome of the European COST programs (1983 – 2009) was the development of creep resistant 9-10%-Cr-steels for 600°C and 620°C application temperature. COST FB2 steel alloyed with boron is currently the best available material for turbine shafts subjected to steam temperatures up to 620°C and meanwhile introduced in production for application in commercial power plants. Currently several development programs are running to develop materials for further increase of application temperature up to 650°C. For realization of a 325 bar/650ºC power plant doubling of strength compared to FB2 steel is necessary, but also resistance against steam oxidation must be improved by increase of Cr content up to 11-12%. In the past all attempts to develop stable creep resistant martensitic 11-12% Cr steels for 650°C have failed due to breakdown in long-term creep strength. Therefore new alloy concepts have been developed by replacing the fine (V, Nb) Carbonitrides strengthening particles by controlled and accelerated precipitation of the thermodynamically stable Z phase (Cr(V, Nb)N). Within the 7th Framework program of the European Union the project “Z-Ultra” was launched for further development and manufacturing of large components in this new alloy type. Saarschmiede has manufactured a full scale turbine rotor forging from the Z-phase steel ZU1 using an ESR ingot of ca. 23 t. The ingot was forged, machined and heat treated to a rotor part of 1100mm final diameter and a weight of ca. 12t. The paper reports on remelting, forging and heat treatment as well as on testing and achieved properties of the full scale forging.