The Expanded Phases Formed in Stainless Steels by Means of Low-Temperature Thermochemical Treatments: A Corrosion Perspective

• Published in: Metals (Basel ) Vol. 14; no. 12; p. 1371
• Main Author: Borgioli, Francesca
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 01.12.2024
• Subjects: Alloys; Austenitic stainless steels; carburizing; Chromium compounds; Corrosion; Corrosion fatigue; Corrosion resistance; Corrosion resistant alloys; Corrosion resistant steels; Corrosion tests; Corrosive wear; expanded austenite; Fatigue strength; Fatigue tests; Ferritic stainless steels; Hardness; Iron compounds; Low temperature; low-temperature treatments; Martensite; Mechanical properties; Metal industry; nitriding; Phase composition; Phases; S-phase; Solid solutions; Stainless steel; stainless steels; Steel; Steel, Stainless; Surface hardness; Surface layers; Temperature; Wear resistance; United States; United Kingdom; Minnesota
• ISSN: 2075-4701; 2075-4701
• DOI: 10.3390/met14121371

Surface engineering of stainless steels using thermochemical treatments at low temperatures has been the subject of intensive research for enhancing the surface hardness of these alloys without impairing their corrosion resistance. By using treatment media rich in nitrogen and/or carbon, it is possible to inhibit chromium compound formation and obtain supersaturated solid solutions, known as expanded phases, such as expanded austenite or S-phase in austenitic stainless steels, expanded ferrite in ferritic grades, and expanded martensite in martensitic grades. These low-temperature treatments produce a significant increase in surface hardness, which improves wear and fatigue resistance. However, the corrosion behavior of the modified surface layers remains of paramount importance. In the international literature, many studies on this topic are reported, but the results are not always univocal, and there are still open questions. In this review, the corrosion behavior of the expanded phases and the modified layers in which they are present is critically analyzed and discussed. The relationships between the phase composition and the microstructure of the modified layers and the corrosion resistance are highlighted while also considering the different test conditions. Furthermore, corrosion test methods are discussed, and suggestions are given for improving the measurements. Finally, perspectives on future directions for investigation are suggested for encouraging further research.