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INTRODUCTION

DUPLEX stainless steels (DSSs) based on Fe-Cr-Ni have both an excellent corrosion resistance and good mechanical properties as a consequence of their twophase microstructure, i.e., the tough austenite (c) and the harder ferrite (d), which improves their mechanical and welding characteristics. Their properties also depend heavily on the precipitation of secondary phases, such as the sigma r, phase, which is an intermetallic compound (FeNi)x(CrMo)y with a tetragonal crystal structure; chromium nitride Cr2N; and the chi phases v. The temperature range of precipitation for each phase depends on the workpiece’s chemical composition, its thermomechanical history, and its cooling rate (where higher rates considerably inhibit precipitation, etc.). A higher fraction of d-ferrite promotes r-phase precipitation during cooling in the temperature range between 873 K and 1273 K (600 C and 1000 C) because of the decomposition of the ferrite via the eutectoid reaction d fi c + r, which can reach more than 50 wt pct at equilibrium. The r phase usually precipitates at carbides, as nucleation sites, on the d/c grain boundaries and grows into untransformed ferrite grains, and because it is brittle and not resistant to corrosion, it has the most detrimental influence on the properties of the duplex stainless steels. Carbides of the type M23C6 and MC usually start to precipitate on the d/c grain MATEVZ FAZARINC, Researcher, MILAN TERCˇELJ, Associate Professor, DAVID BOMBACˇ, Researcher, and GORAN KUGLER, Doctor, are with the Department of Material Science and Metallurgy, Faculty of Natural Sciences and Engineering, University of Ljubljana, Askerceva Cesta 12, SI-1000 Ljubljana, Slovenia. Contact e-mail: [email protected] Manuscript submitted November 13, 2009. Article published online May 26, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A

boundaries and on other incoherent boundaries at temperatures above 1223 K (950 C).[1–7] To influence the ratio between the c and d phases, the ferritestabilizer elements, like Cr, Mo, Si, etc., and the austenite-stabilizer elements, like Ni, C, Mn, N, etc., have to be selected carefully. However, Ni, as an austenite-stabilizing element, despite the acceleration of the kinetics of precipitation of the r phase, also decreases its equilibrium volume fraction. Thus, the precipitation processes influence the properties of duplex stainless steels in a complex way, especially the possible increase of hardness, and the decrease of strength, ductility, toughness, corrosion resistance, etc. Consequently, to obtain the desired microstructure and properties of the duplex stainless steel, the parameters of the manufacturing process should be selected and controlled carefully, considering the precipitation and transformation conditions.[4–14] Many publications discuss the precipitation processes in duplex stainless steels that were previously aged at elevated temperatures for longer aging times (approximately 30 minutes to 70 hours).[8,15,16] In addition, many publications deal with the precip