Influence of Plastic Deformation on Low-Temperature Surface Hardening of Austenitic Stainless Steel by Gaseous Nitriding
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STAINLESS steels are extensively used in structural applications where corrosion resistance is of crucial importance. Austenitic stainless steels of the Ni-containing 300 series are the most widely applied due to their excellent corrosion resistance, mechanical properties as well as formability and weldability.[1–3] In recent years, an increasing interest in alloys of the 200 series has arisen, because of their improved mechanical properties[3] and lower cost,[4] as compared with the 300 series steels. In the stainless steels of the 200 series, the expensive nickel is replaced with other austenite stabilizing elements, in particular manganese and nitrogen. Despite the wide use of the two classes of stainless steels due to their high mechanical properties and excellent corrosion resistance, the 200 and 300 series stainless steels suffer from poor wear resistance, particularly galling. Major improvement of the galling resistance can be achieved by dissolving high amounts of nitrogen and/or carbon in the austenite phase through low-temperature thermochemical processing, e.g., nitriding, carburizing, or nitrocarburizing. Through the years, plasma and implantation technologies have provided the possibility of low-temperature surface hardening of stainless steels.[1,5–8] Plasma/Ion-based surface engineering removes the passive layer by sputtering and incorporates N and/or C interstitials in FEDERICO BOTTOLI, Ph.D. Student, GRETHE WINTHER, Associate Professor, THOMAS L. CHRISTIANSEN, Senior Researcher, and MARCEL A.J. SOMERS, Section Head, Professor, are with the Technical University of Denmark, Department of Mechanical Engineering, Produktionstorvet b.425, 2800 Kgs. Lyngby, Denmark. Contact e-mail: [email protected]; [email protected] Manuscript submitted September 10, 2014. Article published online March 12, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
austenite, thus leading to the formation of expanded austenite,[9–11] also referred as to S-phase.[8] In the last 15 years, gaseous processes for nitriding and carburizing of stainless steel were developed, matured, and commercialized.[9,12] Gaseous processes provide a substantial advantage over plasma and implantation processes, in particular with respect to process control, materials handling, and geometrical constraints. Expanded austenite is not a new phase, as S-phase would suggest, but is merely a supersaturated solidsolution of nitrogen/carbon in austenite. Its supersaturated nature implies that expanded austenite is metastable and is kinetically stabilized by choosing a process temperature that does not allow the precipitation of Cr-based nitrides and/or carbides during processing. The change in the surface hardness and the incorporation of composition-induced compressive residual stress, which both are associated with the dissolution of high contents of interstitials, are beneficial for the wear and fatigue performance of stainless steel, without affecting the resistance against general corrosion.[6,13–15] Furthermore, the incorporation of a high amount o
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