Higher Nitriding Efficiency in Ultrafine-Grained Iron Processed by Ultrasonic Nanocrystal Surface Modification

In this work, the nitriding efficiency in an ultrafine-grained iron was investigated. Prior to nitriding, the ultrasonic nanocrystalline surface modification (UNSM) technique was used to process the coarse-grained iron to obtain ultrafine-grained iron. Gas nitriding was conducted for the UNSM-treated iron and the coarse-grained iron at temperatures from 300 °C to 560 °C. The results indicate that the combination of UNSM and nitriding provide much higher efficiency for hardening than the sum of the efficiencies of UNSM alone and nitriding alone. Nitride nucleation was confirmed in the diffusion layer of the UNSM-treated iron following the nitriding process, enhancing the hardness after nitriding. X-ray diffraction patterns were obtained for both the UNSM-treated iron subjected to nitriding as well as iron that underwent only nitriding at different temperatures. It was also found that the minimum temperatures required to form nitrides, including γ′-Fe4N, ε-Fe₃N, ε-Fe₂N, and FeN, were reduced because of the high chemical potential at the grain boundaries. The theoretical analysis suggests that heterogeneous nitride nucleation at grain boundaries is the key factor that contributes to the enhancement in hardness in the nitriding layer in ultrafine-grained metals and nanocrystalline metals.