Precipitation in AISI 316L(N) during creep tests at 550 and 600 °C up to 10 years A.F. Padilha a , D.M. Escriba a , E. Materna-Morris b, * , M. Rieth b , M. Klimenkov b a University of S. Paulo, Department of Metallurgical and Materials Engineering, Av. Prof. Mello Moraes 2463, CEP 05508-900, S. Paulo, Brazil b Forschungszentrum Karlsruhe, Institut fu ¨ r Materialforschung I, P.O. Box 3640, 76021 Karlsruhe, Germany Received 9 November 2006; accepted 13 December 2006 Abstract The precipitation behaviour in the gauge lengths and in the heads of initially solution annealed type 316L(N) austenitic stainless steel specimens tested in creep at 550 and 600 °C for periods of up to 85 000 h has been studied using several metal- lographic techniques. Three phases were detected: M 23 C 6 , Laves, and sigma phase. The volume fraction of the precipitated sigma phase was significantly higher than that of carbides and the Laves phase. M 23 C 6 carbide precipitation occurred very rapidly and was followed by the sigma and Laves phases formation in the delta ferrite islands. Sigma and Laves phases precipitated at grain boundaries after longer times. Two different mechanisms of sigma phase precipitation have been proposed, one for delta ferrite decomposition and another for grain boundary precipitation. Small quantities of the Laves phase were detected in delta ferrite, at grain boundaries and inside the grains. Ó 2007 Elsevier B.V. All rights reserved. PACS: 61.82.Bg; 68.37.Lp; 68.37.Hk 1. Introduction The sigma phase (r) is probably the most studied intermetallic phase in stainless steels. Already in 1907, even before the discovery of stainless steels, Treitschke and Tammann [1] studied the Fe–Cr sys- tem and proposed the existence of an intermetallic compound containing 30–50 wt% of chromium. In 1927, Bain and Griffiths [2] studied the Fe–Cr–Ni system and observed a hard and fragile phase, which they called constituent B, denoting ‘brittle’. In 1936, Jett and Foote [3] called it sigma phase and in 1951, Bergmann and Shoemaker [4] determined through crystallography its structure in the Fe–Cr system. Precipitation of the sigma phase in stainless steels may occur in the austenitic, ferritic, and ferritic– austenitic with a duplex structure type. The precip- itation of the sigma phase does not only cause losses of ductility and toughness of the steel, but also reduces its corrosion resistance by removing chro- mium and molybdenum from the austenitic matrix. In the austenitic stainless steels [5–11], sigma phase precipitation generally requires hundreds or 0022-3115/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2006.12.027 * Corresponding author. Tel.: +49 7247 82 2162; fax: +49 7247 82 4567. E-mail address: materna-morris@imf.fzk.de (E. Materna- Morris). Journal of Nuclear Materials 362 (2007) 132–138 www.elsevier.com/locate/jnucmat