Gas Mixture and Current Density Variation and Their Effect on White-Layer Properties of Nitrided Steel

Abstract

The effect of current density and gas mixture on surface properties of low alloy steel processed in a pulsed plasma nitriding system under strictly controlled conditions is evaluated in this work. As current density is a parameter that has not been particularly studied, this study seeks to demonstrate that current density and gas mixture have an important influence on the process and an adequate combination of these two variables is essential to obtain the desired surface properties: hardness, white layer thickness (or absence), and crystalline phases of the modified layer. In this sense, samples processed at low current densities with low nitrogen content in the plasma atmosphere show a diffusive nitrides region which increases the hardness from 250 HV (untreated reference sample) to 400 HV without the presence of the typical white layer that is sometimes undesired due to its fragility and tendency to break. On the other hand, treatment at high current densities and high nitrogen concentrations leads to obtain pieces with a homogeneous single-phase layer (γ-Fe4N) with an outstanding corrosion performance, diminishing the corrosion rate from 0.160 μm per year in the untreated sample up to 0.050 μm per year.