Trattamenti di post-ossidazione in scarica ionica dell’acciaio AISI H13

The corrosion resistance of nitrided steel components can be improved by post-oxidising treatments. The glow-discharge process appears particularly suitable, since it allows to perform the post-oxidising treatment subsequently after the nitriding treatment, simply varying the treatment atmosphere and working parameters. In the present research the possibility of using plane air as treatment atmosphere of glow-discharge post-oxidising treatments, performed on AISI H13 (UN1 X40CrMoV5 1 1 KU) steel samples subsequently after the nitriding treatments, has been studied. Glow-discharge nitriding treatments were carried out at 500° C, at 10 mbar, for 5 h, using a gas composition of 80 vol. % N2 and 20 vol. % H2; post-oxidising treatments were performed subsequently after the nitriding process at two different temperatures, 350 and 500°C, at 10 mbar, for 1 h. All the treatment types produce modified surface layers, consisting of an outer compound layer and an inner diffusion layer. When the nitriding treatment is performed, the compound layer consists of iron (ε-Fe2-3N and γ'-Fe4N) and chromium (CrN) nitrides. The nitriding + post-oxidising treatments produce a compound layer consisting of a surface oxide layer on the top of the nitride layer. The 350-°C post-oxidised samples show a thin (< 0.5 μm) oxide layer, consisting essentially of magnetite, Fe3O4, while on 500-°C post-oxidised samples a thicker (∼ 1.7 μm) oxide layer is formed, consisting mainly of hematite, Fe2O3. In the inner nitride layer iron (ε-Fe2-3N and γ'-Fe4N) and chromium (CrN) nitrides are present. The diffusion layer consists of nitride precipitates dispersed in the ferritic matrix enriched in nitrogen. In 350-°C post-oxidised samples the diffusion layer thickness is about 115 μm and is comparable with the one of nitrided samples; after a nitriding + 500-°C post-oxidising treatment the thickness of this layer slightly increases up to ∼ 125 μm, owing to the higher nitrogen diffusion rate at higher temperatures. For all the treated samples the modified surface layers have high hardness values (up to ∼ 1300 HK 0.025), which decrease to matrix values (∼ 300 HK 0.025). The case depth of 350-°C post-oxidised samples is comparable with the one of nitrided samples (∼ 120 μm), while for 500-°C post-oxidised samples a slight increase is observed (case depth: ∼ 130 μm), in accordance with morphological observations. Moreover, a decrease of surface microhardness values is observed after the post-oxidising treatments, in respect of nitriding, and this effect is greater as the oxide layer is thicker, i.e. when the oxidising temperature is higher. The potentiodynamic tests, performed in a 5 % NaCl aerated solution, show that both nitriding and nitriding + postoxidising treatments are able to improve the corrosion resistance of the untreated AISI H13 steel: all the treated sample types are more resistant to general corrosion but are subjected to pitting corrosion. When the post-oxidising treatment is performed at 350°C the samples show higher corrosion potential and lower anodic current values in respect of nitrided samples. The 350-°C post-oxidised samples show a quasi-perfect passive behaviour with a corrosion potential of about - 180 mV (Ag/AgCl) and anodic current values of about 1 + 3 μA cm^-2. Moreover, after the tests the samples show the presence of fewer and smaller pits, in comparison with the other treated samples, presumably owing to the high protective magnetite oxide layer. Thus, the 350-°C post-oxidising treatment allows to significantly improve the corrosion resistance of AISI H13 steel samples in comparison with both untreated and nitrided samples.