ABSTRACT

The development of new ferritic-martensitic steels for rotor applications was one of the main actions in the joint research projects COST 501 and COST 522. During COST 501 several trial compositions of 9-10%Cr steels were tested. In the COST 522 the best candidate, coded FB2 - a 10%Cr steel with additions of Co and B, without W - was selected for scale-up from laboratory trial to full industrial component. Società delle Fucine (SdF) produced a FB2 prototype rotor using a conventional process route, based on ladle furnace and vacuum degassing. Results of mechanical and low cycle fatigue tests of the trial rotor are presented and discussed. A large creep test programme has been launched to define the creep properties of the full size component and the results are in line with these from laboratory material, in terms of creep resistance as well as ductility. The tests reached more than 30.000 hours and the assessment of creep-rupture tests targeted an improvement of 15-20 MPa to obtain 100.000 creep hours at 600°C with respect to Grade 92. A programme of microstructural investigation has been recently launched to evaluate the evolution of the structure and to have better knowledge of the role of B as a creep strengthening element.