Abstract: In this paper, a commercial grade 304 SS has been processed by high-frequency pulse forging - rotary swaging (RS) to different strains up to 1.5. The average grain size, volume fraction of martensite and total dislocation density of ∼80 nm, 68 vol.% and 2.15 × 10¹⁵ m^-2, respectively, are achieved in the 304 steel at an equivalent strain of ∼1.5, granting the steel withan ultra-high yield strength of 2235 MPa. Subsequent annealing at 450 °C for 10 min, led to the unexpected formation of high-density B2 nanoprecipitates for the first time in the 304 steel and an increased volume fraction of nanostructured martensite, and thus further increased the yield strength to a new record of 2375 MPa. Annealing the RS steel at 700 °C for 10 min, led to the heterostructure consisting of coarse-grained austenite, nanostructured austenite and nanostructured martensite. Semi-in-situ X-ray diffraction tensile tests, in-situ digital image correlation analysis and ex-situ microstructural characterization together reveal that the heterostructured 304 steel deforms via synergistic deformation mechanisms involving martensitic transformation, deformation twinning, dislocation-precipitate interactions, thus demonstrating the excellent yield strength of 1290 MPa and uniform elongation of 20.1 %.

Key words: Austenitic stainless steel, Rotary swaging, Martensite transformation, Annealing hardening, Nanoscale precipitates