Two-Dimensional and Three-Dimensional Analyses of Sigma Precipitates and Porosity in a Superaustenitic Stainless Steel
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I.
INTRODUCTION
THE alloy AL-6XN is often referred to as a ‘‘superaustenitic’’ stainless steel due to both its composition and its properties. Conventional austenitic stainless steels exhibit an excellent combination of high strength, toughness, ductility, and formability. In addition, those steels typically exhibit a corrosion resistance superior to that of martensitic or ferritic steels, and can retain their excellent strength and corrosion resistance at elevated temperatures. AL-6XN has a similar (20 to 22 wt pct) chromium content to other austenitic stainless steels, but differs from those steels with a high content (~24 wt pct) of nickel and a significant amount (~6 wt pct) of molybdenum[1] (Table I). These compositional changes provide a better resistance to corrosion (particularly pitting corrosion, crevice corrosion, and stress corrosion cracking), an improved yield strength, and a higher elevated temperature strength than exhibited by the 300 series stainless steels, while maintaining good ductility and toughness.[1,2] As is well known, during casting (including the continuous casting process typically used during fabrication of AL-6XN), alloying elements segregate into the interdendritic regions. Because solidification proceeds from the plate surface to the center, rejection of solute into the interdendritic regions results in an increasing solute concentration in the interdendritic regions toward the center of the plate. Subsequent hot rolling flattens and elongates these solute-enriched regions into compositional bands. When these bands contain sufficient levels of chromium and molybdenum, they are suscepR.W. FONDA and G. SPANOS, Heads, are with the Naval Research Laboratory, Washington, DC, 20375. Contact e-mail: [email protected] E.M. LAURIDSEN, Senior Scientist, is with the Center for Fundamental Research: Metal Structures in 4D, Risø National Laboratory, Frederiksborgvej 399, P.O. 49, DK-4000, Roskilde, Denmark. W. LUDWIG, Researcher, is with Lab. Mateis, INSA-Lyon, CNRS, UMR 5510, Lyon, France. P. TAFFOREAU, formerly Postdoctoral Fellow, now Thematic Scientist, is with the European Synchrotron Research Facility, BP 220, 38043, Grenoble cedex, France. Manuscript submitted August 1, 2006. Article published online September 15, 2007.
METALLURGICAL AND MATERIALS TRANSACTIONS A
tible to the formation of the sigma phase.[3,4] Thus, the maximum susceptibility to sigma formation is usually observed near the plate centerline. It has been shown that the presence of sigma precipitates can be quite detrimental to the mechanical properties of alloys. In particular, sigma is a hard and brittle phase, and is therefore notorious for reducing toughness.[3–6] Brittle particles such as sigma may fracture during deformation, potentially initiating cracks or widening into interparticle voids during continued deformation. Sigma precipitation also removes chromium and molybdenum from the austenite matrix, reducing the corrosion resistance of the alloy.[1,7] The initial purpose of the current research was to use bo
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