Effect of Si content on the microstructure and mechanical properties of 9Cr-ferritic/martensitic steels

9Cr-F/M-xSi (x = 0-1.0 wt%) steels were fabricated through vacuum induction melting technique and processed by hot forging, hot rolling, normalizing and tempering, subsequently. Their microstructure and mechanical properties were characterized using OM, SEM, TEM, Vickers hardness tester and tensile tester. The steel has a typical ferrite/martensitic structure, with M23C6 and MX phases precipitated at the martensite lath boundary or in the lath. With Si content increasing, the average ultimate tensile strength (UTS) and hardness of 9Cr-F/M-xSi increase simultaneously from 678 MPa and 235 HV for 9Cr-F/M-0Si steel to 788 MPa and 265 HV for 9Cr-F/ M-1.0Si steel, respectively, while the total elongation kept almost constant at 22.5%. The main strengthening mechanism is the solid solution strengthening due to silicon addition and the change in the carbide precipitation behavior caused no remarkable hardening contribution. These results can provide a reference for the composi-tion design of structural materials for nuclear reactors.