Advanced Materials & Processes

NOV-DEC 2013

Covers developments in engineering materials selection, processing, fabrication, testing/characterization, materials engineering trends, and emerging technologies, industrial and consumer applications, as well as business and management trends

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LOW-TEMPERATURE SURFACE HARDENING OF STAINLESS STEEL PROVIDES THE REQUIRED PERFORMANCE PROPERTIES WITHOUT AFFECTING CORROSION RESISTANCE. Thomas L. Christiansen and Marcel A.J. Somers Technical University of Denmark, Lyngby Stainless steels rely on the presence of chromium in solid solution, which allows the development and maintenance of a passive layer at the surface. Nitriding, carburizing, and nitrocarburizing are generally not considered good practice, because processing in the conventional temperature range between 490° and 950°C leads to chromium nitride and chromium carbide precipitation. While this provides a hardening effect, it is highly detrimental to corrosion properties. Since the mid1980s, several processes were developed that enable low-temperature surface hardening of stainless steel at temperatures below 440°C. The first deliberate surface hardening of stainless steel was achieved by a process known today as Kolsterizing[1], a method ostensibly inspired by corrosion phenomena observed in liquid-metal fast breeder reactors[2, 3]. About the same time, seminal work by Zhang and Bell[4] on plasma nitriding of stainless steel was published. Throughout the 80s and 90s, the development of low-temperature surface hardening of stainless steel relied largely on plasma-based techniques, while in the past 10 years in particular, gaseous processing was developed and commercialized. This article describes fundamental and technological aspects of low-temperature surface hardening (LTSH) of stainless steel. The results shown are taken from the authors' research during the past 15 years. LTSH principles The TTT diagram in Fig. 1 demonstrates the combination of allowable treatment time at low temperature before precipitation of Cr-based nitrides or carbides occurs. In this temperature range, interstitially dissolved nitrogen and carbon can diffuse over a relatively long distance, while substitutional dissolved metallic elements can be considered stationary. Consequently, nitride or carbide development 52 High-temperature solution nitriding/carburizing T HTPRO 16 LOW-TEMPERATURE SURFACE HARDENING OF STAINLESS STEEL Chromium nitrides/carbides Austenite Low temperature surface hardening log t Fig. 1 — TTT diagram of austenite with a high nitrogen or carbon content. A low-temperature treatment of long duration or a high temperature treatment (>1050°C) combined with fast cooling can be applied to prevent development of chromium nitrides/carbides. proceeds so slowly that a nitrogen or carbon rich case free of chromiumnitrides/carbides develops. In the early days of LTSH, the case produced was considered a new phase, dubbed the S phase[4]. Recent research shows that no new phase develops, but rather LTSH of austenitic stainless steels produces a case that is essentially a solid solution of high amounts of nitrogen and/or carbon in austenite where interstitial atoms group around chromium atoms[5–7]. Therefore, it is incorrect to refer to the case produced as S phase; expanded austenite is preferred. The hardening effect that occurs by dissolving nitrogen and carbon at low temperature in stainless steel is not due to nitride or carbide formation. Rather, solution of high amounts of interstitial atoms in the austenite lattice provides effective hardening. Process technology and applications Plasma processes, apart from the proprietary Kolsterizing process, have a unique advantage over gaseous processing, because surface activation (removal of the passive film through sputtering) is an inherent step of such treatments. The (temporary) removal of the passive layer is ADVANCED MATERIALS & PROCESSES • NOVEMBER-DECEMBER 2013 necessary to allow surface penetration of nitrogen and carbon from the nitriding/ carburizing atmosphere. Gaseous processing enables the highest flexibility, as well as straightforward monitoring and control. For a long time, it appeared that gaseous processing of stainless steel was possible only by in-situ removal of the passive layer in aggressive halogenides[8, 9], or after ex-situ deposition of a metal layer promoting dissociation of the gas components and protecting the surface against (re)passivation during storage and treatment[10, 11]. Later, robust gaseous treatments were developed based on gas mixtures that can both remove the passive layer and provide the nitrogen/carbon to the stainless steel surface[12–14]. Expanite, a company co-founded by the authors and Thomas Strabo Hummelshøj, works exclusively with gas mixtures that have this dual ability. Figure 2a shows the case produced during gaseous nitriding of austenitic stainless steel. The corresponding nitrogen content profile, hardness, and residual stress level are shown in Fig. 3. Dissolution of a huge amount of nitrogen leads to an appreciable increase in surface hardness and high compressive residual stresses, which arise due to austenite lattice expansion in the nitrided case. High surface hardness contributes to improved wear and galling performance, while residual stress enhances fatigue performance. During nitriding of austenitic stainless steel, an almost featureless case develops at the surface (Fig. 2a), indicating that the zone is more difficult to dissolve by the etching reagent than the unaffected austenite. Similar results are obtained with carburizing (Fig. 2b), although less carbon can be dissolved resulting in lower increase in hardness and residual stress. The choice of nitriding or carburizing depends on the application, as both processes have advan-

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