Impact of Y and Nb on 321 Stainless Steel in Supercritical Water

Four different steels are designed and fabricated, denoted as 321, 321-Y, 321-Nb, and 321-NbY, to investigate the effects of 0.5wt%Nb and 0.015wt%Y on the microstructure and mechanical properties. The results show that the addition of Y and Nb promotes the spheroidization of square-shaped TiN/Ti(C, N). Heterogeneous nucleation is induced by Y through inclusions formed by Y, O, and S, whereas heterogeneous nucleation is induced by Nb through the NbC wrapping effect. The addition of Nb promotes the precipitation of small-sized (Ti, Nb)(C, N) particles. Nb promotes the formation of δ-ferrite, while Y inhibits its formation. Compared to 321 steel, Y has little effect on the strength of steel, while the addition of Nb decreases the tensile strength from 301 to 250 MPa and increases the yield strength from 192 to 210 MPa. Both Nb and Y enhance the steel's reduction of area. Compared to 321 steel, the co-addition of Nb and Y increases the reduction of area from 70.2 to 89.2%.