ASTM Grade 23 is a 2.25Cr-0.3Mo-1.5W-V-Nb-B steel widely used for the fabrication of boiler components of the most recent ultra super critical power plants; it combines high creep resistance, enhanced oxidation and corrosion resistance and good weldability. Microstructural, mechanical, and creep properties of seamless tubes and pipes after normalizing and tempering heat treatment are compared with those obtained after cold bending and hot induction bending. The creep resistance is obtained through the precipitation of fine carbides after tempering. A broad program of TEM investigations on crept samples has been carried out in order to assess the evolution of the microstructure and its phases after long term high-temperature exposure, in terms of chemical composition, size and distribution of precipitates.

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.

TenarisDalmine, in collaboration with CSM, developed and characterized ASTM Grade 91 steel tubes and pipes for high-temperature boilers in ultrasupercritical power plants. This paper details the mechanical properties of both untreated and long-term exposed tubes. Extensive creep testing allowed extrapolation of stress values for rupture in 100,000 hours (93 MPa at 600°C) and 1% elongation in 250,000 hours (83 MPa at 600°C), satisfying ECCC and TÜV requirements. Additionally, STEM analysis investigated microstructure and precipitate evolution after long-term exposure, revealing changes in precipitate chemistry, size, and distribution.