Characterization of nanostructured high nitrogen Fe–18Cr–xMn–4Mo austenitic stainless steel prepared by mechanical alloying

In this research, production of high nitrogen austenitic stainless steel (Fe–18Cr–xMn–4Mo) by mechanical alloying (MA) and its characterization were investigated. For this purpose powder mixtures with different chemical compositions (Fe–18Cr–11Mn–4Mo, Fe–18Cr–8Mn–4Mo and Fe–18Cr–7Mn–4Mo) were used. MA was performed under nitrogen atmosphere, using a high-energy planetary ball mill for different time periods. X-ray diffraction (XRD) patterns showed that reducing the Mn content from 11 to 7 wt% led to decrease the α to γ transformation rate. However, nitrogen analysis and XRD patterns indicated that by decreasing the Mn content, the amount of nitrogen absorbed in milled powder increased leading to austenite phase formation. Also, nitrogen analysis revealed that nitrogen solubility in milled powders increased significantly by increasing the milling time. It is believed that the increase in lattice defects and development of nanostructure through MA are responsible for higher solubility. Variations in grain size and internal lattice strain versus milling time in both cases showed that the critical ferrite grain size for austenite nucleation was less than 10 nm. Moreover, the results obtained showed that the PREN value could be increased from 30.92 to 41.16 due to decrease in the amount of Mn and increase in N content.