Abstract
The development of new ferritic-martensitic steels for rotor applications was a primary focus of the joint research projects COST 501 and COST 522. During COST 501, multiple trial compositions of 9-10% chromium steels underwent comprehensive testing, with the COST 522 project ultimately selecting the most promising candidate, FB2, a 10% Cr steel containing cobalt and boron additions, notably without tungsten. Società delle Fucine (SdF) successfully produced an FB2 prototype rotor using a conventional manufacturing process involving ladle furnace and vacuum degassing techniques. A comprehensive creep test program was initiated to characterize the full-size component's properties, with results demonstrating consistency with laboratory material performance in both creep resistance and ductility. The extensive testing, which exceeded 30,000 hours, aimed to achieve a 15-20 MPa improvement over Grade 92, targeting 100,000 creep hours at 600°C. Complementing the mechanical testing, a parallel microstructural investigation program was launched to evaluate structural evolution and gain deeper insights into boron's role as a creep-strengthening element in advanced ferritic-martensitic steels.
