Resistenza ad usura di acciai inossidabili austenitici nitrurati in scarica ionica

Austenitic stainless steels are largely employed in many industrial fields due to their good general corrosion resistance. Nevertheless they can suffer pitting or crevice corrosion in chloride rich solutions. Moreover their low hardness and low wear resistance can limit the possible industrial applications. Traditional nitriding treatments, usually performed at temperatures higher than 500°C, can improve the surface hardness and wear resistance of stainless steels, but generally decrease their corrosion resistance properties. This effect is due to the very hard CrN chromium nitride formation. At the typical nitriding temperatures the Cr mobility is high enough to enable CrN chromium nitride precipitation in the nitrided layer; the Cr-depleted matrix cannot form a protective passive film and the material is subjected to active corrosion. Glow-discharge nitriding treatments of austenitic stainless steels can be carried out at relatively low temperatures, not only avoiding a decrease of corrosion resistance, due to chromium nitride formation, but also promoting the formation of the so called S-phase, which shows high hardness and good pitting and crevice corrosion resistance. S-phase is supposed to be a metastable nitrogen super-saturated austenite characterized by variable reticular parameter values depending on the nitrogen content. In this paper the microstructural, morphological, mechanical and wear resistance characterization of A1S1 316L stainless steel samples glow-discharge nitrided at temperatures in the range of 400-500°C for 5 hours was carried out. If the nitriding temperature is equal or lower than 430°C, the modified layers are essentially composed by S-phase. If the nitriding temperature are equal or higher than 470°C in the surface layer is present S-phase and sensible amount of CrN and gamma'-Fe4N precipitates. The surface microhardness values and the thickness of the hardened layers increase as the nitriding temperature increases. Wear resistance tests were carried out by means of a block-on-ring tribometer employing as a lubricant and aggressive environment a 5% NaCl aqueous solution. The ring was realized by AISI 316L stainless steel. The sliding speed and the coupling load were respectively fixed at 0.38 m/s and 15 N. The sliding distances, depending on the different wear resistance properties of the various kinds of samples, ranged from 500 to 20000 m. Wear tests were performed on nitrided and untreated samples in order to make a comparison. Untreated AISI 316L samples show low wear resistance. Samples nitrided at the higher temperatures (≥470°C) show a very high increase of the wear resistance, in comparison with the untreated ones, due to the presence of very hard nitrides in the thick modified surface layers. Nevertheless these kinds of samples, as it is well known in literature, are characterized by very low corrosion resistance properties. Samples nitrided at lower temperatures (≥430°C) show higher wear resistance in comparison with untreated samples. Moreover, as long as the modified layers, composed essentially by S-phase, are not completely removed, the wear resistance properties of these kinds of samples are comparable with those of the samples nitrided at higher temperatures. The higher wear resistance of samples nitrided at higher temperature seems to be ascribable firstly to their larger thickness and only secondly to their higher hardness values. Low temperature glow-discharge nitriding process is able to increase the hardness, wear and corrosion resistance properties of AISI 316L stainless steel. Moreover, increasing the nitriding time and consequently the thickness of the S-phase layers, it would seem possible to produce samples showing wear resistance properties comparable with those of samples nitrided at higher temperatures and localized corrosion resistance properties even higher than those of untreated stainless steel.