Phase Transformations During the Low-Temperature Nitriding of AISI 2205 Duplex Stainless Steel
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INTRODUCTION
DUE to the excellent corrosion resistance and mechanical properties, duplex stainless steels (DSS) are widely used in the petrochemical, chemical, and food industries.[1,2] The microstructure of DSS consists of approximately equal proportions of austenite and ferrite balanced under solution-annealed condition, and is considered as an alternative to austenitic stainless steels, especially when high corrosion resistance is required.[1,2] However, the low surface hardness, poor wear resistance and poor fatigue resistance have restricted the applications of DSS. Nitriding is an effective process to improve the surface hardness and wear properties of duplex stainless steels.[1–7] Different nitriding and carburizing techniques have been explored in recent years: plasma nitriding,[6–8] gasphase nitriding,[9] and liquid nitriding (also called the JING YAN, Senior Engineer, is with the School of Manufacturing Science and Engineering, Sichuan University, Chengdu 610065, P.R. China and also with the Research Institute of Natural Gas, Southwest Oil and Gasfield Company, Chengdu 610023, P.R. China. TAN GU, Senior Engineer, is with the Research Institute of Natural Gas, Southwest Oil and Gasfield Company. SHAOYU QIU, Professor, is with the The Key Nuclear Fuel and Nuclear Materials Laboratory of China, Nuclear Power Institute of China, Chengdu 610041, P.R. China. JUN WANG, Associate Professor, and JI XIONG and HONGYUAN FAN, Professors, are with the School of Manufacturing Science and Engineering, Sichuan University. Contact e-mail: [email protected] Manuscript submitted September 18, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
‘‘salt-bath nitrocarburizing’’).[11–17] Although these methods are well established, gas-phase and plasma nitriding have some disadvantages from an engineering viewpoint. For example, they may require using rather complicated and/or expensive apparatus. Liquid nitriding has important advantages, such as, a broad sphere of application to product shapes that can be treated, high dimensional stability of product, process safety, and reproducibility.[18–21] Some concern has been expressed about possible pollution due to the use of liquid nitriding technology. However, the main components of the liquid nitriding medium are nontoxic; consisting of urea, M2CO3 (M denotes some elements of alkali), chloride, and some other trace components.[11] In the process of nitriding, some NH3 gas will be emitted. If the off gas and waste flax are treated appropriately, liquid nitriding can provide an alternative green technology option.[18] Previous reports have indicated that the traditional liquid nitriding techniques [above 853 K (580 C)] can enhance the micro-hardness and wear properties of stainless steels, but lose the essential good corrosion resistance of alloys.[4–6] So the treatment temperature must be below the critical temperature. It has been proposed[4–6] that the critical temperature is 723 K (450 C). When the temperature of thermochemical treatment is above this value, such improvements in har
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