Effect of Si content on the mechanical behavior and microstructure of a 9Cr ferritic/martensitic steel

Si-containing ferritic/martensitic (F/M) steels are ideal candidate structural materials for the lead-cooled fast reactors on account of their good mechanical properties, corrosion resistance and irradiation resistance. In order to figure out the optimal Si content and the effect of Si-content on the microstructure and mechanical behavior of 9Cr-F/M steels, F/M steels with different Si addition of 0, 0.36, 0.68, and 0.93 % (wt.%) were prepared. It is found that with increasing Si content, both the room temperature ultimate tensile strength and the total elongation of the F/M steels are enhanced, but the yield strength has a little change. At elevated temperatures (500 and 800 C), the steels containing 0.68 % and 0.93 % Si show better comprehensive tensile properties. The addition of Si is overall beneficial to the impact toughness. Si addition also refines the prior austenite grains and the martensitic structure units (packets, blocks and laths). When the Si content exceeds 0.68 %, delta-ferrite is formed in the steels. No adverse effect from the delta-ferrite was found on the tensile properties. As the Si content increases, the MX precipitates are refined and their area fraction is reduced. However, the area fraction of the Crrich carbide precipitates increases with the increase of Si content. For the steels containing 0 % and 0.36 % Si, the Cr-rich precipitates are mainly M23C6, while for the steels with 0.68 % and 0.93 % Si, the Cr-rich precipitates are mainly M7C3.