Surface Properties of Fe4N Compounds Layer on AISI 4340 Steel Modified by Pulsed Plasma Nitriding

In this work, the effect of nitriding current density on hardness, crystalline phase composition, layer thickness and corrosion rate of AISI 4340 steel has been studied. X-ray diffraction analysis shows that thin layers formed during nitriding process are constituted of gamma-Fe4N for samples processed between 1 and 2.5 mA/cm(2). Thickness of nitrided layer increases proportionally to current density (0 mu m for 0.5 mA/cm(2) to 15 mu m for 2.5 mA/cm(2)). Plasma nitriding increased the surface hardness from 300 HV50g for untreated sample, to around 800HV(50g) for nitrided samples at 1 mA/cm(2). While the untreated samples exhibited a corrosion rate of 0.153 mm per year, the corrosion performance was improved up to 0.03 mm per year at current densities above 1 mA/cm(2), which is about one fifth of the corrosion rate of the untreated sample.