Precipitation behavior of carbonitrides in type 347 stainless steels with various C and N contents
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Precipitation Behavior of Carbonitrides in Type 347 Stainless Steels with Various C and N Contents YONG-JUN OH, BYUNG JU LEE, ONE YOO, BONG SANG LEE, and JUN HWA HONG Type 347 austenitic stainless steel (SS) stabilized by niobium has good mechanical properties and corrosion resistance. Niobium in the steel combines with carbon and nitrogen to form carbonitrides and reduces a susceptibility to intergranular stress corrosion cracking (IGSCC) through reducing chromium carbide precipitation in grain boundaries.[1,2,3] In regard to mechanical properties, niobium strengthens the steel through precipitating fine carbonitrides in the matrix and inhibiting grain growth during annealing.[4] But, unfortunately, it also precipitates coarse carbonitride particles in the normal concentration range of niobium and carbon present. Such coarse particles are known to drastically reduce fracture resistance by acting as potential sites to initiate ductile voids.[5–8] Accordingly, in order to optimize corrosion resistance and mechanical properties of type 347 SS, the understanding of the carbonitride precipitation behavior with different carbon and nitrogen concentrations is of primary concern. Some previous investigations have revealed that niobium precipitates two different types of carbonitrides—Nb(CN) and NbCr(CN)—and lower carbon and nitrogen concentrations increase the tendency toward NbCr(CN) precipitation in type 347 SS.[1,2] But, these investigations were confined to a qualitative analysis of the carbonitride behavior, and their size and shape changes, which affect the mechanical properties, were not dealt with. In the present study, as a part of compositional optimization research for the best corrosion and fracture resistance of type 347 SS, the change of carbonitride precipitation behavior was quantitatively examined as a function of carbon and nitrogen concentrations. Additionally, this study presents the effect of carbon and nitrogen concentrations on coarse particle formation, which is responsible for poor fracture
YONG-JUN OH, Assistant Professor, is with the Advanced Materials Eng. Div., Hanbat National Univ., Taejon 305-719, Republic of Korea. Contact e-mail: [email protected] BONG SANG LEE, Project Manager, and JUN HWA HONG, Division Head, are with the Korea Atomic Energy Research Institute, Taejon 305-600, Republic of Korea. BYUNG-JU LEE, Principal Researcher, is with the Materials Evaluation Center, Korea Research Institute of Standards and Science, Taejon 305-600, Republic of Korea, ONE YOO, Senior Researcher, is with the Mechanical System Engineering Department, Korea Power Engineering Company, Inc., Taejon Republic of Korea. Manuscript submitted July 2, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
resistance, through the thermodynamic calculation as well as experimental observation. Two different series of type 347 stainless steels were used in this study. The two series were designed to contain a constant nitrogen level of 0.11 and 0.04 wt pct for seriesN and series-P, respectively, with the carbon concentra
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