Kinetics of layer growth during plasma nitriding of nickel based alloy Inconel 600

Abstract

The response of nickel based superalloy, Inconel 600, to plasma nitriding has been investigated in the present work. Plasma nitriding experiments carried out at temperatures between 400 and 600 °C for times between 1 h and 40 h revealed that the growth of the nitrided layer varied with temperature in a way significantly deviating from the conventional diffusion law. For similar nitriding times, the maximum layer thickness was obtained at about 450 °C. A further increase in nitriding temperature resulted in a decrease in layer thickness. The growth of the nitrided layer with the square root of time followed two linear regimes, i.e. an initial fast growth regime and then a slow growth regime. The critical time for the transition from the fast growth regime to the slow growth regime decreased with increasing temperature, such that for prolonged nitriding times a thinner layer was produced at high temperatures than at low temperatures. In conjunction with structural and compositional analysis, the reasons for such an abnormality in nitriding kinetics have been analysed. It was found that the reduction in layer growth rate in the slow growth regime was associated with the formation of an external scale on top of the nitrided layer, which impeded the transfer of nitrogen from the plasma to the nitrided layer.