Gaseous Nitriding Process of Surface Nanocrystallized (SNCed) Steel

The behavior of gaseous nitriding on the surface nanocrystallized (SNCed) steel was investigated. The mild steel discs were SNCed on one side by using the method of ultrasonic shot peening. The opposite side of the discs maintained the original coarse-grained condition. The gaseous nitriding was subsequently carried out at three different temperatures: 460°C, 500°C and 560°C. The compound layer growth and diffusion behaviors were then studied. It is revealed that SNC pretreatment greatly enhances both diffusion coefficient D and surface reaction rate. As a result, the nitriding time could be reduced to half. It is also found that the growth of compound layer with the nitriding time conforms with the parabolic relationship from the start of nitriding process in the SNCed samples, the ultrasonic shot peening can accelerate the surface reaction rate greatly and the whole nitriding process in SNCed samples can be controlled by the nitrogen diffusion.