Tensile Strength of Austenitic Stainless Steels with Grain Refinement by Mechanical Milling

Abstract

Grain refinement of 304L stainless steel was examined by mechanical milling. Fine austenitic microstructure materials with grain sizes smaller than 1μm were obtained by consolidating the mechanically milled powder with nano-meter-sized martensitic microstructure and reversing to austenite during the hot isostatic pressing (HIP). It is important for grain refinement to form finer martensitic microstructure with random crystallographic orientations before the reversion treatment. The strongest material showed the microstructure with the average grain size of 0.3μm and 3 times of 0.2% proof stress and 1.5 times of tensile strength compared with conventional steel. Hot forging after HIP treatment was effective to improve the ductility because the interfaces between powders were strengthened. The forged materials had elongation over 30% and reduction of area over 50%. The strengthening of as-HIP materials is mainly effected by grain refinement, because the dependence of proof stress on grain size agreed with the Hall-Petch relationship. M₂₃C₆ on grain boundaries and nano-meter-sized precipitates in grains, identified as Cr-carbite or Si-oxide, were observed in the microstructure. These precipitates might be related not only to suppressing the coarsening but also to strengthening. The dislocation structure was observed in the microstructure of the forged materials. The strengthening of the forged materials is probably controlled by work-hardening and recovery during forging process.