Isothermal Decomposition of Ferrite in a High-Nitrogen, Nickel-Free Duplex Stainless Steel
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DUPLEX stainless steels (DSS), possessing a balanced microstructure with approximately equal proportions of ferrite (a) and austenite (c), offer an attractive combination of properties including high strength, excellent resistance to stress corrosion cracking, and good weldability.[1–3] The Ni, which improves both c stability and formability, is contained in currently popular DSS in the range from 4 to 7 wt pct. Reducing the use of Ni in the fabrication of DSS not only provides substantial economical savings but also decreases the hazard that nickel could bring to allergic reaction in human skin. Recently, considerable efforts have been devoted to the development of cost-effective DSS containing low Ni content (termed as lean DSS). Some commercial grades of lean DSS such as UNS 32101 are now available and have a good balance of mechanical and corrosion properties: The strength of lean DSS is comparable with conventional DSS such as UNS S32205, and the corrosion properties are in general better than those of austenitic UNS S30400.[4,5] Precipitation behavior has been regarded as one of the most important features of DSS because their mechanical and corrosion properties are profoundly dependent on the formation of second phases.[1–3,6–10] Compared with TAE-HO LEE and SUNG-JOON KIM, Principal Researchers, HEON-YOUNG HA and BYOUNGCHUL HWANG, Senior Researchers, are with the Ferrous Alloys Group, Korea Institute of Materials Science, 531 Changwondaero, Changwon 642-831, South Korea. Contact e-mail: [email protected] Manuscript submitted June 8, 2011. Article published online October 12, 2011 822—VOLUME 43A, MARCH 2012
austenitic stainless steels, the precipitation reaction of DSS has been considered to be more complicated in that (1) the nucleation sites of the second phases are diverse: a/c phase boundaries, grain boundaries of both phases and interior of grains, and (2) significant amounts of Cr and Mo, which is the potent elements for the formation of the second phases, are contained for stabilization of a matrix of DSS.[1–3] A consensus exists that isothermal aging in conventional UNS S31803 and S32750 induces the decomposition of the supersaturated a matrix and produces various second phases involving secondary austenite (c2), intermetallic sigma (r), chi (v), R phase, and several types of carbide or nitride.[1–3] Despite many investigations on the precipitation behavior of commercial grades of DSS, little information on precipitation reaction of high-nitrogen, nickel-free DSS is available.[2,9,10] Ramirez et al.[9] reported that intergranular and intragranular Cr2N were observed in aged UNS S32304 and they served as sites for heterogeneous nucleation of intragranular c2. Recently, Calliari et al.[10] showed that the precipitation of second phases in lean DSS was much more sluggish compared with conventional UNS S31803 and S32750: In UNS 32101, only Cr-rich nitride was detected, whereas no second phase was identified in UNS S32404 even after prolonged aging at 1023 K (750 C) for 750 hours. Granting that the precipitation
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