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.
Highlights
IntroductionStainless steels are an important class of iron (Fe)-based alloys, which owe their name to their capability to resist to general corrosion in many environments, so that staining or rusting is avoided
Stainless steels are an important class of iron (Fe)-based alloys, which owe their name to their capability to resist to general corrosion in many environments, so that staining or rusting is avoided.In order to obtain this high corrosion resistance chromium (Cr) must be added in a content of at least 10.5 wt.%, so that an adherent, self-healing Cr-rich oxide film is able to form on the steel surface [1]
Among the different classes of stainless steels, the largest one in term of alloys and market share is constituted by austenitic stainless steels, because they have excellent corrosion resistance, ease of formability and weldability, and they can be employed from cryogenic temperatures to elevated ones [1,2]
Summary
Stainless steels are an important class of iron (Fe)-based alloys, which owe their name to their capability to resist to general corrosion in many environments, so that staining or rusting is avoided. The beginning of low temperature carburizing is less uncertain, as recently reviewed by Christiansen and Somers [31], and it dates back to 1983, when Kolster reported, in a technical paper in German, a new process able to improve the wear and fatigue resistance of stainless steels without impairing the corrosion resistance [32] This process, which was the basis of the proprietary treatment known as Kolsterizing, was probably inspired by previous studies [33] regarding the carburizing reaction occurring between stainless steels and liquid sodium, used as cooling medium in liquid-metal fast-breeder reactors and containing small amounts of C, in which Kolster was involved since 1970s [34]. Regarding the properties of surface modified austenitic stainless steels, beyond the changes in hardness, tribological and fatigue properties and corrosion resistance, which are usually the most considered, this review reports the effects of the treatments on magnetic properties, electric properties, wetting behavior and biocompatibility
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