Effects of pulse length on low frequency plasma nitrided 316L steels

Abstract

The influence of pulse time (tp) variation on hardness, crystalline phases and electrochemical corrosion susceptibility of plasma nitriding 316L was investigated. Hardness test, X-ray diffraction, scanning electron microscopy and electrochemical polarisation tests let to identify the best processing conditions. Nitriding at tp = 5 ms in a 100 Hz discharge increased the 316L hardness around four times and decreased its corrosion susceptibility one order of magnitude. The X-ray diffraction analysis showed the shifted reflections for the expanded austenite and let to identify an anisotropic strain with expansion lattice around 7·5 and 9% for (111) and (200) orientations respectively. For the evaluated tp range, the increase in the pulse length has not beneficial influence on surface hardness; on the contrary, there is a decrease in its value when tp grew from 5 to 8 ms. Corrosion and hardness performance has been related to the strain level and to the nitrogen content.