KINETICS OF GROWTH OF PLASMA NITRIDED LAYERS ON A TITANIUM MODIFIED STAINLESS STEEL

The kinetics of growth of the subsurface layer containing CrN in plasma nitrided titanium modified stainless steel D–9 in the temperature range 773–923 K have been studied using the techniques of microhardness measurements, X-ray diffraction, and electron probe microanalysis. The hardness profile of D–9 exhibits a sharp boundary between the nitrided layer and the bulk and consequently, it has been analysed as one in which strong nitrogen-chromium interaction prevails. The kinetics of nitriding has been discussed according to a rate equation derived from an internal nitriding approximation. The growth of the thickness of the nitrided layer has been found to be parabolic with nitriding time in the temperature range of investigation. The calculated diffusion coefficients of nitrogen in the nitrided layer have been found to be larger than the reported diffusion rates in the austenite phase. The nitrogen concentration profiles have been explained on the basis of differing diffusion rates of nitrogen in the nitrided layer and the bulk.