The Role of Boron in the Long-Term Stability of a CrMoCoB (FB2) Steel for a Rotor Component

Abstract Research conducted under European COST programs has demonstrated the beneficial role of boron in enhancing the microstructural stability and creep performance of new martensitic steels. The FB2 steel (a 10%Cr steel containing Co and B, without W) emerged as the most promising candidate and was successfully scaled up to a full industrial rotor component by Società delle Fucine. Extensive creep testing, now reaching 50,000 hours, indicates an improvement of 15-20 MPa over Grade 92 at 600°C for 100,000 hours. STEM and X-ray analysis of long-term aged specimens confirmed that boron significantly enhances precipitate stability compared to Grade 91 and 92 steels, validating its role as a creep-strengthening element and stabilizer of carbides and martensitic structure.