Microstructures and tensile properties of 304 steel with dual nanocrystalline and microcrystalline austenite content prepared by aluminothermic reaction casting

The microstructures of 304 stainless steels with different amounts of nanocrystalline and microcrystalline austenite prepared by an aluminothermic reaction casting, without and with annealing at 1073?K for 8?h, have been investigated by X-ray diffraction, an electron probe micro-analyser, a transmission electron microscope and a scanning electron microscope. The steels, both without and with annealing, consisted of different dual nanocrystalline and microcrystalline austenite combinations and a little nanocrystalline δ ferrite, while the average grain size of the nanocrystalline austenite increased from 19 to 26?nm and volume fraction of the microcrystalline austenite increased from 17 to 30% after annealing. The tensile strength of the steel was dramatically increased from 500 to 1000?MPa and the tensile elongation ratio increased from 8 to 12% after annealing. However, the tensile strength was decreased to 600?MPa and the tensile elongation ratio increased from 12 to 22% after an annealing at 1273?K. The combination of dual nanocrystalline and microcrystalline austenite obtained after the annealing at 1073?K results in the best tensile properties.