Surface properties of nitrided layer on AISI 316L austenitic stainless steel produced by high temperature plasma nitriding in short time
Graphical abstract
Introduction
The austenitic stainless steels are widely used in many industrial fields because of their very high general corrosion resistance [1], [2], [3]. Unfortunately, their low hardness and poor wear resistance seriously limit these applications [4], [5], [6], [7], [8], [9]. Low temperature nitriding can improve the hardness and wear resistance of austenitic stainless steels without losing corrosion resistance by producing a layer of supersaturated nitrogen solid solution phase which is usually called ‘expanded austenite’ γN, or S-phase [2], [10], [11], [12], [13], [14], [15], [16], [17]. Many nitriding techniques were used to produce this phase layer: glow discharges plasma nitriding, RF plasma nitriding, plasma immersion ion implantation, plasma-based low-energy ion implantation, and active screen plasma nitriding [4], [12], [18], [19]. Table 1 is data on the low temperature nitriding of austenitic stainless steels collected from the literature and including some important experiments.
In order to avoid the drop in corrosion resistance of austenitic stainless steels, these nitriding techniques are characterized by treatment temperatures (<480 °C). At temperatures above 480 °C, hardness continues to increase but corrosion resistance is affected, due to the mobilization of Cr and formation of CrN precipitates. Li [42] also stated that the precipitation of CrN occurs, above the nitriding treatment temperatures of 470–490 °C for the AISI 316L steels.
In most of the published results, long nitriding times are necessary to obtain the sufficient thickness γN phase layers for low temperature nitriding techniques. From Table 1, it can be seen that the formation of about 5–6 μm thickness γN layer commonly requires 2 h or longer. We report a study of high temperature (520–560 °C) nitriding of AISI 316L austenitic stainless steel carried out in the plasma atmosphere enclosed by bilayer active screen. The results show that high temperature nitriding can also nitrogen expanded austenite layer with the thickness of 6 μm within 60 min. The aim of this work is to study the influence of high temperature nitrided on the microstructure, morphology, hardness and the corrosion behaviour in NaCl aqueous solutions of the AISI 316L austenitic stainless steel.
Section snippets
Experiments
The samples used in this work were AISI 316L austenitic stainless steel with the following chemical compositions (wt.%.): Cr (17.10–17.80), Ni (10.10–11.20), Mo (2.16–2.30), C (<0.03), Si (<0.80), and Fe balance. Samples (15 mm × 15 mm × 4 mm) were cut from a hot rolling plate, ground and mirror polished then cleaned with acetone before nitriding.
The nitriding was carried out in an ion nitriding furnace and the details of which had been described elsewhere [43]. The discharge current and voltage
Results and discussion
Fig. 1 presents the cross-section optical micrographs of the samples nitrided at various temperatures and times. For all samples nitrided at 520 °C and 540 °C, it could be seen that the nitrided layers are resistant to the etching reagent, so that it appears ‘bright’ under an optical microscope. The thickness of the ‘bright’ layer ranged from1.5 to 10 μm, depending on processing temperature and time. With increasing temperature at 540 °C, some dark spots became visible in the nitrided layer. It can
Conclusions
The nitrided layer mainly composed of nitrogen expanded austenite phase was formed on the surface of AISI 316L austenitic stainless steel by plasma nitriding at high temperatures (>520 °C) in short time (15–60 min). In case of the sample nitrided at 540 °C for 60 min, the nitrided layer with a thickness of 8 μm was produced in the surface. The surface microhardness was about 980 HV0.05, nearly 4.5 times higher in compared with the untreated substrate.
All the nitrided samples showed better corrosion
Acknowledgment
The authors gratefully acknowledge the National Natural Science Foundation of China (Nos. 51301149 and 51179017) for financial support of this research work.
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