Behavior of the S-phase of plasma nitrocarburized 316L austenitic stainless steel on changing pulse frequency and discharge voltage at fixed pulse-off time

Abstract

Abstract A plasma nitrocarburizing process was carried out in a high pulse frequency regime, like 25 kHz and higher on AISI 316L austenitic stainless steel at different conditions, to observe the effect on S-phase layer formation with changing pulse frequency and duty factor. The S-phase layer thickness and hardness increase slightly with increasing the pulse frequency. And they also change slightly with increasing the duty factor up to 80%, but beyond 80%, their values remains almost the same. Considering the results of these experiments, further investigation was done with fixing the pulse-off time in order to explore the behavior of the nitrogen- and carbon-enriched layer formation. So, by fixing pulse-off time, different experiments were conducted with changing the pulse frequency in the high-frequency region and discharge voltage to analyze the behavior of the nitrogen- and carbon-enriched layers of plasma nitrocarburized 316L austenitic stainless steel. After treatment, the behavior of the nitrogen- and carbon-enriched layers were investigated by optical microscopy, micro-Vickers hardness testing, roughness testing, X-ray diffraction and potentiodynamic polarization testing. At a fixed pulse-off time, there is a slight increase in surface hardness along with carbon-enriched layer thickness and nitrogen-enriched layer thickness with increasing pulse frequency. However, the surface roughness does not increase very much with increasing the pulse frequency. The hardness difference and layer thickness difference between high and low pulse frequency, decreases with increasing the pulse-off time because of the minimizing effect of the duty factor. On the other hand, at a fixed pulse-off time, the thickness of the carbon-enriched layer increases sharply with increasing discharge voltage, though the nitrogen-enriched layer thickness does not change much. Hardness also increases sharply with increasing discharge voltage. Moreover, corrosion resistance decreases with increasing discharge voltage due to chromium-nitride formation.