The effect of particle size of iron powder on α to γ transformation in the nanostructured high nitrogen Fe–Cr–Mn–Mo stainless steel produced by mechanical alloying

► High nitrogen austenitic Fe18Cr10Mn4Mo stainless steel can be produced by MA and without annealing. ► The initial powder particle size plays an important role on the rate of nitrogen absorption and consequently on phase transformation kinetics. ► Nitrogen content in the steel is the most effective parameter for α to γ kinetic enhancement. In this study, the effect of particle size of iron powder on α to γ transformation in the nanostructured high nitrogen Fe–18Cr–10Mn–4Mo stainless steel, produced by mechanical alloying (MA) was investigated. For this purpose iron powders with two different particle sizes were used. MA was performed under nitrogen atmosphere, using a high-energy planetary ball mill. X-ray diffraction (XRD) patterns and nitrogen analysis revealed that by decreasing the iron mean particle size, a higher transformation rate is obtained due to increase in the rate of nitrogen absorption. Moreover, nitrogen solubility in both milled samples was increased noticeably by increasing the milling time. This is believed to be due to the increase of lattice defects and development of nanostructure through MA. Variations of grain size and internal lattice strain versus milling time, for both iron particle sizes, showed that the critical ferrite grain size for austenite nucleation is less than 10 nm.