Decomposition and Precipitation Process During Thermo-mechanical Fatigue of Duplex Stainless Steel
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INTRODUCTION
FERRITIC-AUSTENITIC duplex stainless steels (DSS) are widely used in industry due to their excellent corrosion resistance and concurrently high strength, ductility, and good fatigue behavior. Thus, DSS are used in offshore applications, chemical industry, and power plants. Especially, the application in power plants gains high demands mainly on the thermal stability, which is still the limiting factor for applications at high temperature. Essential drawback limiting the application of DSS at higher temperatures [523 K to 823 K (250 C to 550 C)] is the so-called 748 K (475 C) embrittlement, which is directly related to the miscibility gap in the Fe-Cr system.[1,2] As a consequence, the ferritic phase (a) decomposes to an iron-rich ferritic phase (a) and a chromium-rich ferritic phase (a¢).[1,2] According to literature, the decomposition can occur either by spinodal decomposition (SD)[3] or by nucleation and growth
ANJA WEIDNER, Senior Researcher, is with the Institute of Materials Engineering, Technische Universita¨t Bergakademie Freiberg, Freiberg, Germany. Contact e-mail: [email protected] ROMAN KOLMORGEN, formerly Scientific Assistant with the Institute of Materials Engineering, Technische Universita¨t Bergakademie Freiberg, is now Staff Member with the Development Department, KOKI Technik Transmission Systems GmbH, Niederwu¨rschnitz, Germany. IVO KUBENA, Scientific Staff, and TOMAS KRUML, Professor, are with the Institute of Physics of Materials, Czech Academy of Sciences. DIRK KULAWINSKI, Scientific Staff, is with the Institute of Materials Engineering, Technische Universita¨t Bergakademie Freiberg. HORST BIERMANN, Professor, is with the Institute of Materials Engineering, Technische Universita¨t Bergakademie Freiberg. Manuscript submitted October 2, 2015. Article published online February 22, 2016 2112—VOLUME 47A, MAY 2016
(NG) process[1,2] depending on the chemical composition and the temperature. In the past, the decomposition paths in terms of NG processes or SD have been studied intensively in the Fe-Cr system using Mo¨ssbauer Spectroscopy,[1–3] Small-Angle Neutron Scattering,[4,5] or Transmission Electron Microscopy (TEM) investigations.[6,7] An excellent method in order to investigate the formation of a/a¢ by spinodal decomposition is atom probe field ion microscopy (APFIM), which allows to separate Fe-rich and Cr-rich domains due to its excellent lateral and mass resolution.[8] Numerous investigations using APFIM were performed both on the binary Fe-Cr system (e.g., References 9 through 11) as well as on different types of DSS after different aging treatments (e.g., References 12 through 15). These investigations[9–15] led to the conclusion that the a/a¢ decomposition is the major microstructural process in Fe-Cr model alloys as well as in industrial alloys like DSS after long-term aging treatments at intermediate temperatures. However, especially, the additional alloying elements present in DSS like Ni, Mo, Mn, Cu, have a significant influence on the decomposition and precipitation behav
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