Cast iron plasma nitriding in N2 /H2 /Ar working gas: the role of auxiliary gases (H2 /Ar) in the growing kinetics of compound layers

Abstract

Present paper sets out to investigate the role of N2/H2/Ar working gas in plasma nitriding of nodular cast iron. The proportion of N2/H2/Ar proved to be a relevant parameter in the growing kinetics control of the γ’-Fe4N e ε-Fe3N compound layers. X-ray diffraction and optical microscopy were used in order to characterize the compound layers. Results indicate that there are no generation of compound layers if the treatment is performed through plasma of pure N2. However, even small percentages (~10%) of auxiliary gases (H2/Ar), added to the working gas, are enough to generate compound layers as thick as 10μm (at 500 °C, and 3,0 h). In addition, it is possible to obtain γ’-Fe4N monophasic compound layer when the nitriding process is carried out in low proportion of nitrogen in the N2/H2/Ar working gas mixture.