Effects of Isothermal Aging on the Microstructure Evolution and Pitting Corrosion Resistance of Lean Duplex Stainless St
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INTRODUCTION
DUPLEX stainless steels (DSSs) are alloys that contain both ferrite phase and austenite phase in the microstructure with an optimum volume ratio of 50:50. The ferrite phase has body-centered cubic (BCC) crystal structure, while the austenite phase has a face-centered cubic (FCC) crystal structure. The ferrite phase has high tensile strength and fatigue strength and the austenite phase provides good formability and high toughness. The combination of the two phases gives the DSSs both high mechanical strength and excellent corrosion and stress corrosion cracking resistance compared with austenitic and ferritic stainless steels. Because of these features, DSSs have wide applications in the pulp and paper industry, chemical process industry, oil and gas industry, pharmaceutical industry, and as structural materials. As mentioned above, the optimum performance of DSSs is achieved when the ferrite phase and austenite phase are balanced. However, processes such as
LIANG HE, LOVELYN WIRIAN, and PREET M. SINGH are with the School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30318. Contact e-mail: [email protected] Manuscript submitted October 23, 2018. Article published online March 15, 2019 METALLURGICAL AND MATERIALS TRANSACTIONS A
welding,[1–4] annealing treatment[5–7] as well as aging[8–17] can change the ferrite-to-austenite ratio, phase composition or lead to the formation of secondary phases including intermetallic phases or carbide and nitride precipitates, and thus further change the performance of duplex stainless steels. Precipitation of r phase and v phase in standard UNS S31803 (DSS 2205) and super DSSs has been widely investigated.[8–14] Both r phase and v phase are Cr and Mo enriched, but the Cr content is higher in r phase than that in v phase while the Mo content is higher in v phase.[8–10] r phase forms within the temperature range of around 923 K to 1173 K (650 C to 900 C). In the DSS 2205 as well as in super DSSs, it is precipitated within minutes of aging at the ferrite/austenite boundaries and grows into the ferrite phase because of the high content of Cr and Mo and higher diffusion rate in the ferrite phase.[9,12,13] Different from the fast formation of r phase and v phase in standard and super DSSs, the precipitation of intermetallic phases is sluggish in the lower-alloyed lean duplex stainless steels (LDSSs) UNS S32101 and UNS S32304 because of their reduced Cr and Mo content. Cr2N and M23C6 were found to precipitate after minutes of aging in LDSS 2101 along the ferrite/austenite interface and the ferrite/ferrite boundaries while the r phase were detected after 24 hours of aging at 963 K (690 C).[15] The nucleation and growth of precipitates and intermetallic phases at the ferrite/austenite boundaries also leads to the formation of the secondary austenite in the ferrite phase, which is depleted in Cr and VOLUME 50A, MAY 2019—2103
Mo.[8,15] Cr2N can also precipitate within the ferrite phase during fast cooling from high annealing tempe
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