From Austenitic Stainless Steel to Expanded Austenite-S Phase: Formation, Characteristics and Properties of an Elusive Metastable Phase

Abstract

Austenitic stainless steels are employed in many industrial fields, due to their excellent corrosion resistance, easy formability and weldability. However, their low hardness, poor tribological properties and the possibility of localized corrosion in specific environments may limit their use. Conventional thermochemical surface treatments, such as nitriding or carburizing, are able to enhance surface hardness, but at the expense of corrosion resistance, owing to the formation of chromium-containing precipitates. An effective alternative is the so called low temperature treatments, which are performed with nitrogen- and/or carbon-containing media at temperatures, at which chromium mobility is low and the formation of precipitates is hindered. As a consequence, interstitial atoms are retained in solid solution in austenite, and a metastable supersaturated phase forms, named expanded austenite or S phase. Since the first studies, dating 1980s, the S phase has demonstrated to have high hardness and good corrosion resistance, but also other interesting properties and an elusive structure. In this review the main studies on the formation and characteristics of S phase are summarized and the results of the more recent research are also discussed. Together with mechanical, fatigue, tribological and corrosion resistance properties of this phase, electric and magnetic properties, wettability and biocompatibility are overviewed.

Microstructure, Mechanical and Tribological Behaviour of Aisi 316l Stainless Steel During Salt Bath Nitriding

“…The crystal structure of the S-phase also has a face-centered cubic structure, the S-phase was formed due to the incorporation of the nitrogen atoms in the interstitial positions and gaps of the austenite structure. Overall these findings are following results reported by many studies [55,56].…”

“…It can be considered a nitrocarburizing treatment since the salts used often typically contribute carbon to the surface of the workpiece, and the two elements generally permeate into the surface of the industrial parts [48]. There have been researched studies of the effects of nitriding treatments on the behavior of surface microstructure and characteristics of austenitic stainless steel such as AISI 304 [49,50], AISI 304L [51], AISI 321 [52], AISI 201 [53], AISI 316 [54], AISI 316L [55,56], AISI 202 [57], AISI 316LN [58] and AISI 316LVM [59], AISI 310[60], AISI 303 [61], AISI 204 [62]. However, there is a lack of research about the surface microstructural and tribological behavior of AISI 316L stainless steel during salt bath nitriding by the Tenifer process (TF1) at 580 °C.…”

Surface Modification and Hardness Behavior of AISI 304 as an Artificial Hip Joint using Ammonia and Scallop Shell Powder as a Nitriding Agent

“…Figure 5 shows the untreated EDS, T500W3, T550W7, and T600W7 specimens. The most strategic areas to be observed were those closer to the surface, which had the highest number of N atoms detected 24) . Furthermore, no N atoms were detected on untreated specimens.…”

Formation and Properties of Nitrocarburizing S-Phase on AISI 316L Stainless Steel-Based WC Composite Layers by Low-Temperature Plasma Nitriding

“…Low-temperature plasma nitriding and carburizing at temperatures less than 450 • C for austenitic stainless steels can improve their wear resistance. Rather than producing a nitride or carbide, this treatment produces supersaturated solid solution of nitrogen or carbon in the face-centered cubic lattice, which is known as the S-phase (or the expanded austenite) [27][28][29][30][31][32][33]. Accordingly, the corrosion resistance does not degrade because the ability to form a passive film is maintained after plasma treatment [34,35].…”