Effect of Si Content on Microstructure and Mechanical Property of 9%Cr Ferrite Martensitic Steel

The 9%-12%Cr ferritic Martensitic (F/M) steel has the advantages of excellent creep performance at high temperature, remarkable irradiation resistance, high thermal conductivity, small thermal expansion coefficient, good economy and mature engineering application. The 9%12%Cr F/M steel is one of the main candidate materials for leadcooled fast reactor core. The corrosion resistances of liquid Pb and Bi were improved by increasing Si content, and the microstructure and mechanical properties of F/M steel, especially the plasticity of F/M steel were affected simultaneously. The 5 types of 9%Cr F/M steels with Si content (mass fractions of 0.34%, 0.61%, 0.80%, 0.98% and 1.20%) were designed and prepared to develop the mechanical properties and corrosion resistance of the high Si content F/M steel. The samples of 9%Cr F/M steels were named Z1Z5 according to different Si contents. And the sheet metal samples with thickness of 8 mm were prepared by vacuum melting, forging, hot rolling and final heat treatment. Optical microscope, scanning electron microscope, transmission electron microscope and energy spectrometer were used to characterize the microstructure of F/M steel matrix, M23C6 and Laves phase. The room temperature tensile properties and room temperature impact properties of F/M steel were tested in accordance with national standards of China. In result, the microstructure of F/M steel prepared by normalizing (1 050 ℃, 0.5 h, air cooling) and subsequently heated (760 ℃, 1.5 h, air cooling) at room temperature is all Martensitic. The grain size and Martensite lath width are not affected by the Si content. The yield strength and tensile strength of F/M steel at room temperature both increase as the increase of Si content. The influence of Si content on the structure and room temperature impact properties of F/M steel is divided into two stages. When the Si content in the steel is 0.34%0.80% (mass fraction), the structure and room temperature impact properties of the steel change little. When the Si content in the steel is 0.98%-1.20%, the size and quantity of the second phase in the steel increase, and a small amount of Laves phase is precipitated along the Martensite lath interface, and the room temperature impact performance of the steel decreases. In this study, the optimal addition amount of Si element in the 9%Cr F/M steel is less than 0.8%, so that the steel has remarkable properties while maintaining high strength.