In this study, an innovative process called nanocrystallization-assisted nitriding was used to process 4140 steels. First, a nanocrystalline surface layer was induced in 4140 steel by ultrasonic nanocrystal surface modification (UNSM). The abundant nanoscale grain boundaries provide micro-channels for efficient nitrogen diffusion during nitriding at relatively low temperature (450 °C) and short duration (4 hours). The samples were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The hardness and corrosion resistance were examined and compared for samples after different processing conditions. It has been demonstrated that the sample processed by nanocrystallization-assisted nitriding has much higher hardness and corrosion resistance compared with the samples processed by nitriding only.

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