The depth of the diffusion zone and the thickness of the compound layer of the ion-softnitrided S15C steel were investigated by optical microscopy by varying the nitriding temperature (450∼600°C) and time (0.25∼30 h).
\ oindentThe main results were as follows.
(1) The growth rate of the diffusion zone followed the parabolic law. In the diffusion zone, the activation energy for diffusion of nitrogen was calculated to be 7.53×10⁴ J/mol (1.80×10⁴ cal/mol) by assuming the temperature dependency of the nitrogen concentration at the diffusion zone-compound layer interface.
(2) The concentration of the nitrogen at the top surface of the compound layer was concluded as 11.35 wt% which corresponded to that of the ζ-Fe₂N phase.
(3) The growth rate of the compound layer followed the linear law during the first 30 min, after which it approximately followed the parabolic law. In the compound layer, the activation energy for diffusion of nitrogen was calculated as 8.46×10⁴ J/mol (2.02×10⁴ cal/mol) by assuming the top surface nitrogen concentration to be 11.35 wt% which corresponded to the ζ-Fe₂N phase and the temperature dependency of the nitrogen concentration at the diffusion zone-compound layer interface.
(4) The diffusion coefficient of nitrogen in the compoud layer was very small compared to that in the diffusion zone.