Effects of Nb and Y on the Solidification Microstructure and Mechanical Properties of S321 Austenitic Stainless Steel

delta-Ferrite plays a controversial role in austenitic stainless steel and controlling its morphology and distribution is essential for the optimization of the properties of stainless steel. Herein, the solidification microstructure of S321 stainless steel with the addition of Nb and Y is studied by electron probe microanalysis, nanoindentation, and transmission electron microscopy. The results show that the addition of Nb promotes the precipitation of (Nb, Ti)(C, N) and coarsens the grain boundary carbides in S321 stainless steel, resulting in a decrease in the solute carbon content near the grain boundaries and a delay in the delta -> gamma phase transformation. The addition of Y inhibits the segregation of Ni along the original austenite grain boundaries and promotes the delta & RARR; gamma phase transformation. Furthermore, the morphology of delta-ferrite changes from lathy delta to skeletal delta and the grain boundary carbide network is broken. The higher density dislocation due to the phase transformation leads to an increase in the hardness of S321 steel containing Y.