Z-ultra · Z phase strengthened steels for ultra-supercritical power plants

The aim is to develop a heat resistant steel with a 100 000 hour creep strength of 100 MPa at 650°C. This allows increasing the thermal efficiency of fossil power plants to over 50%, which is 30% higher than the present standard in most existing power plants. The CO2 emissions are reduced accordingly. The consortium combines the expertise of a steelmaker, two utility companies, an engineering consultant company from Ukraine and eight research organizations and universities from EU and Eastern Partnership countries.The idea is to exploit the Z phase as a thermodynamically stable strengthening agent in martensitic creep resistant steels. Previously the Z phase has been considered as detrimental, since the coarse Z-phase particles that develop during long time service in high-chromium steels hardly contribute to the strength while growing at the expense of the fine, strengthening nitride particles. However, high chromium contents around 12% are needed, since the current 9% chromium steels do not provide sufficient oxidation resistance at 650°C. Hence the challenge is to control the precipitation of the Z phase in 12% Cr steels such that fine Z particles are formed, which are stable for long times. An important aspect is that the beneficial microstructure should be established also by the post-weld heat treatment.To achieve the goal, test melts containing different alloying elements are prepared and subjected to different heat treatments. Welding consumables and processes are also tested. Since a purely empirical alloy development is time consuming and costly, the process is supported by microstructural investigations on an atomic scale. Similarly, multiscale modelling methods are developed and applied to Z-phase strengthened steels. Modelling not only enhances the fundamental understanding of the strengthening and degradation mechanisms, but also provides design tools and lifetime estimation methods for the safe and reliable operation of future power plants.