Low temperature nitriding of AISI 300 and 200 series austenitic stainless steels
Introduction
Austenitic stainless steels are employed in many industrial fields thanks to their corrosion resistance, ease of formability and weldability. Their very good corrosion resistance in many aggressive environments is due to the presence of a surface passive film, mainly consisting of chromium oxide, which is self healing if damaged and protects the alloy from the surrounding medium [1]. The face centred cubic structure of these alloys is maintained owing to the presence of austenite stabilizing elements, like nickel, manganese and nitrogen. Fairly high amounts of nickel (usually 8 wt% or higher) are present in the so called AISI 300 series stainless steels, which are widely used. Nickel is partly substituted by manganese and nitrogen in the cheaper AISI 200 series stainless steels, which have higher strength; however, these alloys may show lower corrosion resistance than AISI 300 series steels, due to the lower chromium content [2].
The industrial applications of austenitic stainless steels may be limited by their low hardness and poor tribological properties, which may compromise the performance of components subjected to wear. Moreover, in spite of the very good resistance to general corrosion, these alloys suffer localised corrosion in specific environments, especially in chloride-ion containing solutions. Among the surface engineering techniques, employed to improve wear and corrosion resistance properties of austenitic stainless steels, low temperature nitriding has received increasing attention in the last years [1], [3]. Treatment temperatures lower than 450 °C allow to avoid the precipitation of large amounts of chromium nitrides and the consequent decrease of corrosion resistance [4]. In this manner, a supersaturated solid solution of nitrogen in the expanded and distorted austenite lattice is produced, known as S phase [1], [3], [4] or expanded austenite [1], [3], [5], which has high hardness and improved corrosion resistance in chloride-ion containing solutions [3], [4]. Many researches were devoted to study the structure [3], [4], [6], [7], [8], [9], [10], [11] and the properties [3], [4], [5], [12], [13], [14], [15] of this phase, and the treatment techniques able to produce it [3], [5], [16], [17], [18]. These studies were usually carried out using a single steel type, in order to put in evidence the effects of different nitriding techniques [3], [16], [17], [18] or treatment parameters [3], [13], [14], [18], [19], [20]. The effect of steel composition was investigated especially regarding the influence of nickel [3], [21], [22], [23], molybdenum [24], [25], [26], and chromium [3], [21], [23], [27]. Recent studies also regarded steels with low amounts of alloy elements like copper and niobium [26], or the high nitrogen and nickel-free stainless steels with improved biocompatibility [28], [29]. The analysis of the scientific literature shows that the used steels are mainly of the 300 series [3], [5], [6], [25], [26]; limited studies regarded low-nickel alloys, as 17Cr–15Mn–4N [30] and X50CrNiNbN 21 9 [31], or the nickel-free F2581 [29], which have a niche use. The direct comparison of the effects of nitriding on AISI 300 and 200 stainless steels has received little attention. Even if a comparison could be performed using the results obtained by different researches, it is hindered by the fact that different experimental conditions, and, in particular, nitriding parameters, were usually used.
In the present research a direct comparison of the effects of low temperature nitriding on the characteristics of the modified surface layers produced on AISI 300 and 200 series austenitic stainless steels was performed. Three steels of large commercial use were chosen, CrNi grades AISI 304L and AISI 316L, and the low-nickel CrMn grade AISI 202. Glow-discharge nitriding technique was used to perform nitriding treatments. The microstructure, phase composition, hardness and corrosion resistance characteristics of the nitrided samples were investigated and they were compared with those of the untreated alloys.
Section snippets
Experimental procedure
Three austenitic stainless steels were used, AISI 202, AISI 304L and AISI 316L, supplied by ThyssenKrupp Acciai Speciali Terni (Italy); the chemical composition of the steels is reported in Table 1. The steels, in the form of cold rolled, annealed and pickled plates, were cut in order to obtain prismatic samples (40 × 17 × 0.7 mm). The surface finishing of the samples was classified as 2D according to the EN 10088-2:2005 norm [32]. The surfaces, which had to be treated, were not ground and
Morphology and microstructure
The untreated samples have an austenitic microstructure. X-ray diffraction analysis shows that, beyond austenite, γ-Fe (f.c.c.), small peaks of α-Fe (b.c.c.) are present in the steels.
The nitriding treatments produce modified surface layers, the characteristics of which depend on the treatment temperature and the used alloy; the thickness of the modified layers is reported in Table 2.
When the steels were nitrided at 400 °C, the modified surface layers have a double layer microstructure, as
Discussion
Low temperature nitriding is recognised as an effective surface engineering technique for improving surface hardness and corrosion resistance of austenitic stainless steels. It is a general agreement that this improvement is due to the formation of the supersaturated solid solution of nitrogen in the expanded and distorted austenite lattice, named S phase, and that the treatment temperature should not exceed about 450 °C in order to avoid the precipitation of large amounts of chromium nitrides.
Conclusions
Low temperature glow-discharge nitriding, carried out at 400 and 430 °C, at 1000 Pa for 5 h on AISI 202, AISI 304L and AISI 316L austenitic stainless steel samples produces modified surface layers, the characteristics of which depend on both the treatment parameters and alloy composition.
The modified surface layers of all the sample types have a double layer microstructure, with an outer layer, mainly consisting of S phase, and an inner layer, in which a solid solution of interstitial atoms in
Acknowledgments
This study was supported by grants from MIUR (years 2012, 2014) and a donation of Ente Cassa di Risparmio di Firenze (2012.0175).
Dr. C. Rocchi and Dr. D. Sciaboletta (ThyssenKrupp Acciai Speciali Terni (Terni, Italy)) are acknowledged for providing austenitic stainless steels.
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