INFLUENCE OF WORKING ATMOSPHERE COMPOSITION ON PLASMA NITRIDED LAYER CONSTITUTION

Two steels belonging to the "Steels for quenching and tempering"class (EN 10083): C45 and 42CrMo4, were plasma nitrided under identical temperature conditions (T-N = 570 degrees C) and several atmosphere chemical compositions. The structural investigation was carried out by optical microscopy, scanning electron microscopy and X-ray diffraction, while the chemical composition on the layer depth was conducted by Glow Discharge Mass Spectrometry. Low nitrogen content in the working chamber (5%N-2 +95%H-2) completely suppresses the compounds layer formation, while an average nitrogen content (20...30%N-2 + 80 ... 70%H-2) facilitates the formation of a mainly gamma' structure. Contrariwise, a high nitrogen content (80%N-2+20%H-2) allows a biphasic epsilon + gamma' compound layer (prevailingly epsilon) to form. One may observe that by plasma nitriding a layer formed exclusively by epsilon phase can never be obtained. Adding to the first atmosphere a small proportion of carbon component gas (80%N-2 + 19.5%H-2 + 0.5%CH4) makes possible to obtain a layer consisting in two subzones, the external one as an epsilon and the internal one as a gamma' structure. A complete epsilon structure of the layer could be achieved only by plasma nitrocarburizing, but it is necessary to carefully measure the optimal methane content. The diffusion zone is less influenced by the nitriding technique (plasma or gaseous), being formed under constant nitrogen activity conditions, directly dependent on its diffusive transfer.