Nano-crystalline structure formation in mechanically milled 304L steel by reversion of deformation-induced martensite

Grain refinement by heavy cold working of mechanical milling was examined for gas atomized powder with the composition of 304L steel. Furthermore, microstructural changes of the milled powder were investigated during heating which simulated the sintering process. Also RT tensile properties of the sintered materials by HIP were investigated. About 90 vol.% of gamma-phase transformed to alpha'-phase during milling by the deformation-induced-martensite process. The microstructure contained grains with sizes from 50 to 200 nm, in which subgrains or dislocation cells with sizes from 10 to 20 nm were present. According to the XRD results, reversion to gamma-phase began under 773 K and was completed between 923 and 973 K. Microstructural changes were investigated, in-situ, by TEM with temperature control. Sintered materials had 0.2% proof strength of 757 MPa and tensile strength of 865 MPa.