Abstract
When low temperature nitriding of austenitic stainless steels is carried out, it is very important to remove the surface passive layer for obtaining homogeneous incorporation of nitrogen. In the glow-discharge nitriding technique this surface activation is performed by cathodic sputtering pre-treatment, which can heat also the samples up to nitriding temperature. This preliminary study investigates the possibility of producing modified surface layers on austenitic stainless steels by performing low pressure glow-discharge treatments with nitrogen, similar to cathodic sputtering, so that surface activation, heating and nitrogen incorporation can occur in a single step having a short duration (up to about 10 min). Depending on treatment parameters, it is possible to produce different types of modified surface layers. One type, similar to that obtained with low temperature nitriding, consists mainly of S phase and it shows improved surface hardness and corrosion resistance in 5% NaCl solution in comparison with the untreated steel. Another type has large amounts of chromium nitride precipitates, which cause a marked hardness increase but a poor corrosion resistance. These surface treatments influence also water wetting properties, so that the apparent contact angle values become >90°, indicating a hydrophobic behavior.
Highlights
In the last few years low temperature nitriding has received increasing attention for the surface modification of austenitic stainless steels
It may be supposed that, in these conditions, efficient surface activation and heating of the samples can occur, and more active nitrogen atoms are obtained, which can diffuse into the substrate and produce modified surface layers in shorter times than those usually employed in nitriding treatments
Modified surface layers could be produced, on AISI 202 austenitic stainless steel, by means of treatments performed with the glow-discharge technique at a pressure of 130 Pa in a 80 vol.% N2 and
Summary
In the last few years low temperature nitriding has received increasing attention for the surface modification of austenitic stainless steels. When the treatments are performed at temperatures lower than 450 ◦ C, the precipitation of large amounts of chromium nitrides is usually avoided and modified surface layers are produced, which consist mainly of a supersaturated interstitial solid solution of nitrogen in the expanded and distorted γ-Fe f.c.c. lattice, known as S phase or expanded austenite [1,2] Due to this phase, having high hardness (up to 1500 HV [1]) and corrosion resistance, improvements of surface hardness, wear resistance and corrosion resistance in chloride-ion containing solutions are obtained [1,2]. An increase of nitriding efficiency was observed decreasing the treatment pressure, and a significant nitrogen incorporation was obtained when the process was performed in the range
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