Austenitic stainless steels are selected as candidate materials for in-core and out-of-core components of Generation-IV fast reactors due to their excellent operating experience in light-water reactors over several decades. However, the performance of conventional austenitic stainless steels proves to be inadequate through operation feedback in fast reactors. To withstand the demands for material performance exposure to the extreme operating environment of fast reactors, modified austenitic stainless steels for in-core and out-of-core components have been developed from the first-generation 300-series steels. The design of an appropriate microstructure becomes a top priority for improving material performance, and key metallurgical features including δ-ferrite content, grain size and secondary phase precipitation pertinent to austenitic stainless steel are focused on in this paper. δ-ferrite content and grain size are closely correlated with the fabrication program and their effects on mechanical properties, especially creep and fatigue properties are critically assessed. Moreover, the impacts of some major elements including nitrogen, stabilization elements (Nb, Ti, V), phosphorus and boron on secondary phase precipitation behaviors during aging or creep are reviewed in detail. Based on the role of the aforementioned metallurgical features, the recommended specification of nitrogen content, stabilization ratio, phosphorus content, boron content, δ-ferrite content and grain size are put forward to guarantee the best-expected performance, which could provide reactors designers with attractive options to optimize fast reactor systems.

The microstructure of solution-annealed and aged tensile properties and the stress rupture properties of 15Cr–15Ni titanium-modified austenitic stainless steel with different phosphorus contents were investigated using OM, SEM and TEM. The results showed that two phosphide morphologies were observed after long-term isothermal aging at 850 °C for 1000 h. One was the needle-like M₂P distributed within the grain. The other was the blocky M₃P distributed at the grain boundaries and twins. The tensile properties of the alloy were unaffected by the phosphorus content, but the stress rupture properties were significantly impacted. With the increase in the phosphorus content from 70 ppm to 250 ppm, the stress rupture life increased from 148 to 269.7 h. Since the strengthening effect of phosphides within the grain or at the grain boundary has been shown to improve the stress rupture properties of alloys, many nanosized granular precipitates, such as the sigma phase, carbides and phosphides, have been observed at the grain boundary, capable of alleviating the stress concentration and limit the crack propagation between two phases, improving the strength of the grain boundary. Intragranular needle-like phosphides can hinder dislocation movements effectively, which improves the intragranular strength of alloys.