New high-temperature alloy
Thermal power generation will continue in service for a long time, but maximum efficiency requires the highest possible temperatures. High temperatures mean accelerated metal degradation, which can be resolved with new steel alloys. The EU-funded Z-ULTRA (Z phase strengthened steels for ultra-supercritical power plants) project developed such an alloy. Previous alloys used for the purpose contained 9 % chromium, which had a thermal limit of 615 °C, beyond which such alloys suffered corrosion. The new alloy contains 12 % chromium, achieved through exploiting the otherwise undesirable Z-phase rather than trying to eliminate it. The study had to find chemical compositions and heat treatments leading to controlled nucleation of finely dispersed Z-phase particles. The team developed seven new test alloys. The alloys' creep strength was 30 % better than the best existing 9 % chromium steels. Oxidation resistance was also excellent. Researchers determined that further improvements to creep strength were possible, thanks to the project-developed tools. For example, the multi-scale modelling toolbox supported development of Z-phase-strengthened alloys by detailing pathways to Z-phase formation. Researchers also developed new welding methods for the new steels. No cracks or other faults were detected. The consortium scaled up the production process to include 12 tonne forging and large-section welds. Again, no severe cracks were detected. The resulting material showed excellent impact toughness, although the yield stress was lower than expected for turbine-rotor applications. Investigators installed demonstration tubes into boilers at two Ukrainian power plants. The group assessed the alloys' corrosion-resistance properties, confirming that the oxidation rate was very slow and that the oxide layer was thin and homogeneous. Corrosion layers were substantially thicker on the flue gas side of the tube surface. The corrosion properties were acceptable up to 647 °C. The Z-ULTRA project's new high-chromium alloys' corrosion- and heat-resistance properties proved suitable for application in high-temperature thermal power plants. The results mean more efficient operation of such plants, and reduced carbon dioxide emission.
Keywords
High-temperature alloy, power plants, chromium, Z-ULTRA, Z-phase
