Microstructure and corrosion property evolution of a surface-nanostructured 15-15Ti austenitic steel during immersion in liquid LBE at 550 °C

This study investigated the compatibility of surface-nanostructured 15-15Ti austenitic steel in 550 degrees C LBE with an oxygen concentration of 5 x 10(-7) wt.% for various exposure durations (759, 1638, 2404, and 3012 h). The results demonstrate that the grain size was reduced from 33.50 mu m to the nano-scale after shot-peening (SP), achieving 17.62, 15.44, and 14.25 nm under SP pressures of 0.06, 0.15 and 0.25 MPa, respectively. The untreated steel experienced severe oxidation and dissolution corrosion, whereas the surface-nanostructured steel exhibited only mild oxidation and was resistant to dissolution corrosion. The enhanced corrosion resistance of surface-nanostructured steel is attributed to the higher protectiveness of the Cr-rich spinel layer and the less defective Ni-rich layer beneath it. Recrystallization occurred exclusively in the Ni-rich region, while the deformed steel underwent recovery during exposure. The thickness of the recrystallization layer was 2.9 mu m at 759 h, increased to 8 mu m at 1638 h, and remained stable thereafter. The size of recrystallized grains in SP-samples processed under pressure of 0.06 MPa and 0.15 MPa was approximately 2.92 mu m, whereas it was about 1.32 mu m for 0.25 MPa processed sample.