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steels (HNS) are a family of stainless steel grades combining excellent corrosion resistance with desirable mechanical properties. Owing to the progress in processing technologies, some commercial grades of HNS are available for applications in power generation and oil field industries as well as biomaterials.[1,2] However, the following factors have raised obstacles to wider application of HNS: the need for high-pressure melting because of the low solubility of nitrogen in iron melts and the unusual brittle fracture at high-nitrogen contents above 0.7 wt pct.[3] A new alloy system, using carbon (C) as a major alloying element together with nitrogen (N), has been proposed to overcome the aforementioned difficulties and to obtain the excellent properties in a cost-effective way.[3–5] With regards to the alloying of C in austenitic stainless steels, many researchers[1] have pointed out the drawbacks associated with the degradation in the corrosion TAE-HO LEE, Principal Researcher, HEON-YOUNG HA, Senior Researcher, and SUNG-JOON KIM, Principal Researcher, are with the Ferrous Alloys Group, Korea Institute of Materials Science, Changwon 642-831, South Korea. Contact e-mail: [email protected] Manuscript submitted June 19, 2011. Article published online October 12, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A

resistance resulting from the formation of harmful carbides. Recent investigations, however, have shown that simultaneous alloying of N and C (N + C) not only increases the strength level effectively[3,4] but also improves the resistance to localized corrosion.[5] Precipitation behavior has been regarded as an important feature of austenitic stainless steels, because their mechanical and corrosion properties are profoundly dependent on the formation of second phases.[6,7] During isothermal aging in the range of 973 K to 1273 K (700 C to 1000 C), several types of carbides, intermetallic compounds, and nitrides tend to precipitate on grain boundaries (GB) as well as on the grain interior of an austenite (c) matrix.[6,7] In commercial austenitic stainless steel such as AISI 304, the major precipitates are known as M23C6-type carbide (M contains metal atoms such as Cr, Mo, and Fe) and intermetallic sigma (r) phase, and extensive literature is available that is associated with their formation,[6–8] crystallography,[9] and related property change, especially sensitization and deterioration in mechanical properties.[7] Precipitation characteristics, however, have been considered to be a complicated phenomenon in that small changes in the chemical composition, thermomechanical processing, and aging conditions can significantly influence the formation of second phases, and in some cases, it is not easy to distinguish one type of precipitate from another by only their morphologies.[7,9] In austenitic HNS, the precipitation behavior has been studied with a focus on the effect of nitrogen addition on the precipitation reaction of the second phases,[9,10] and the precipitation of M2X-type (M = Cr, Mo, Fe, and Mn; X = N, C) nitride, sim