Study On Active Screen Plasma Nitriding Of Austenitic Stainless Steel

Active screen plasma nitriding (ASPN) is a novel nitriding technology, whichovercomes many inherent problems associated with the conventional DC plasmanitriding (DCPN), such as difficulties in maintaining a uniform chamber temperatureand measuring the nitriding temperature, arcing surface damage, the edging effects,hollow-cathode effects and so on. ACPN can achieve a similar nitriding effect asDCPN for both low alloy steel and stainless steel. It's very suitable for the nitriding ofaustenitic stainless steel, especially for the austenitic stainless steel parts, complicatedin shape and different in size.In this dissertation, the ASPN of AISI 316L austenitic stainless steel at lowtemperature was studied systemtically. Nitrided layers were observed and analyzed byfield emission scanning electron microscope (SEM), X-ray diffraction (XRD), opticalmicroscope (OM), microhardness tester and the electrochemical testing technique. Itwas discussed about the effect of nitriding temperature, nitriding time, potentials of thesamples, and materials of the active screen on the surface morphologies, phasestructure, microstructure, microhardness and corrosion resistance of the nitrided layer.The main experimental results are as follows: both the nitriding temperature andthe nitriding time are very important for the result of ASPN. At low temperature and inshort time, a precipitation-free nitrided layer of S phase structure can be produced,which exhibits not only high hardness but also very good corrosion resistance. Athigher temperatures and with prolonged nitriding time, the depth of nitrided layerincreases, but the CrN precipition appeared in the nitrided layer, which reducedcorrosion resistance of the nitrided layer. It was found that the ASPN should be a process of adsorption and desorption. Theneutral particles of S phase sputtered from the active screen by positive ions are theprimary nitrogen carrier from atmosphere to matrix surface in the nitriding process.Adsorbed physically and adsorbed by the paticles in nanometer scale, some activenitrogen atoms were transferred from the active screen space to the substrate surface,and then diffused into the substrate surface under a certain temperature. In theatmosphere of less oxygen and more hydrogen in the vacuum chamber, the surfacepassive film of stainless steel can be cleaned out by many kinds of particles bumpingits surface, so the nitrided layer was produced successfully.A lot of austenitic stainless steel valve parts offered by the manufacturers, werenitrided by ASPN at low temperatures, and gained the uniform nitrided layers, whichsatisfied the manuactures. It's testified the advantage of ASPN in this field.