AbstractThis study was carried out to clarify the effects of phosphorus and boron on cryogenic mechanical properties of an aged austenitic stainless steel strengthened by nitrogen because they are used for Nb3Sn magnets and sometimes suffer aging before cryogenic use. These minor elements affected precipitation at grain boundaries during aging at about 700°C. CrNbN was mainly observed in both matrix and grain boundaries. Cr2N and Nb(CN) were observed at grain boundaries too. Phosphorus promotes the precipitation of Fe2Mo at the grain boundary. The 4 K tensile properties were not affected by aging. Fracture toughness at 4 K, however, was deteriorated by aging. The degradation seemed to be connected with Fe2Mo at grain boundaries. This suggests that phosphorus leads to intergranular fracture at 4 K, consequently to the poor fracture toughness. But boron proved to suppress the phosphorus effect and to improve the fracture toughness.