Abstract
Plasma nitriding is a surface modification process that uses glow discharge to diffuse nitrogen atoms into the metallic matrix of different materials. Among the many possible parameters of the process, the gas mixture composition plays an important role, as it impacts directly the formed layer's microstructure. In this work an AISI 4140 steel was plasma nitrided under five different gas compositions. The plasma nitriding samples were characterized using optical and scanning electron microscopy, microhardness test, X-ray diffraction and GDOES. The results showed that there are significant microstructural and morphological differences on the formed layers depending on the quantity of nitrogen and methane added to the plasma nitriding atmosphere. Thicknesses of 10, 5 and 2.5 µm were obtained when the nitrogen content of the gas mixtures were varied. The possibility to obtain a compound layer formed mainly by γ'-Fe4N nitrides was also shown. For all studied plasma nitriding conditions, the presence of a compound layer was recognized as being the responsible to hinder the decarburization on the steel surface. The highest value of surface hardness - 1277HV - were measured in the sample which were nitrided with 3vol.% of CH4.
Highlights
Nitriding is a very useful method to improve tribological and corrosive properties of iron-alloy components[1,2]
Once plasma nitriding allows the use of different kind of gas mixtures, this parameter should be explored as an alternative to achieve different properties
The results presented in this investigation have shown that by varying gas mixtures is possible to obtain compound layers for an AISI 4140 steel with significant different microstructural and mechanical properties
Summary
Nitriding is a very useful method to improve tribological and corrosive properties of iron-alloy components[1,2]. The aim of the nitriding processes applied to iron-based alloys is to introduce and diffuse atomic nitrogen into the surface of these metallic materials to improve wear and corrosion resistance. For example, exhibit high hardness due to dispersed alloy nitrides in the matrix and high surface residual stresses are generated in the compound and diffusion zones for these steels, which are the result of chemical composition gradients, stress fields around precipitates, volume changes and thermal effects[4]. Once plasma nitriding allows the use of different kind of gas mixtures, this parameter should be explored as an alternative to achieve different properties In this investigation AISI 4140 steel samples were plasma nitrided under different gas compositions (as given in Table 2) with a constant temperature of 500 °C. The samples were characterized concerning the chemical composition, roughness, microstructure and hardness to evaluate the response of the steel to the applied treatment
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