Microstructural Evolution of Cast Austenitic Stainless Steels Under Accelerated Thermal Aging

  • Timothy G. LachEmail author
  • Thak Sang Byun
Conference paper
Part of the The Minerals, Metals & Materials Series book series (MMMS)


Thermal aging degradation of cast austenitic stainless steels (CASS) was studied by electron microscopy to understand the mechanisms for thermal embrittlement potentially experienced during extended operations of light water reactor coolant systems. Four CASS alloys—CF3, CF3M, CF8, and CF8M—were thermally aged up 1500 h at 330 and 400 °C, and the microstructural evolution of the material was characterized by analytical aberration-corrected scanning transmission electron microscopy. The primary microstructural and compositional changes during thermal aging were spinodal decomposition of the δ-ferrite into α/α′, precipitation of G-phase in the δ-ferrite, segregation of solute to the austenite/ferrite interphase boundary, and growth of M23C6 carbides on the austenite/ferrite interphase boundary. These changes were shown to be highly dependent on aging temperature and chemical composition, particularly the amount of C and Mo. A comprehensive model is being developed to correlate the microstructural evolution with mechanical behavior and simulation.


Thermal aging degradation Duplex stainless steel Spinodal decomposition Solute segregation G-phase precipitation 



This research was sponsored by U.S. Department of Energy/Office of Nuclear Energy through Light Water Reactor Sustainability R&D Program and International Nuclear Energy Research Initiative (I-NERI) Program. Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the U.S. Department of Energy under Contract No. DEAC05-76RL01830. APT and FIB/SEM were performed at PNNL’s Environmental Molecular Sciences Laboratory, a Department of Energy—Office of Biological & Environmental Research national scientific user facility.


  1. 1.
    O.K. Chopra, Effects of Thermal Aging and Neutron Irradiation on Crack Growth Rate and Fracture Toughness of Cast Stainless Steels and Austenitic Stainless Steel Welds (NUREG/CR-7185, 2014)Google Scholar
  2. 2.
    J.D. Tucker, M.K. Miller, G.A. Young, Assessment of thermal embrittlement in duplex stainless steels 2003 and 2205 for nuclear power applications. Acta Mater. 87, 15 (2015)CrossRefGoogle Scholar
  3. 3.
    Y.H. Yao, J.F. Wei, Z.P. Wang, Effect of long-term thermal aging on the mechanical properties of cast duplex stainless steels. Mater. Sci. Eng., A 551, 116 (2012)CrossRefGoogle Scholar
  4. 4.
    R.P. Kolli et al., Characterization of element partitioning at the austenite/ferrite interface of as cast CF-3 and CF-8 duplex stainless steels. Microsc. Microanal. 21, 365 (2015)CrossRefGoogle Scholar
  5. 5.
    J.T. Busby, P.G. Oberson, C.E. Carpenter, M. Srinivasan, Expanded materials degradation assessment (EMDA)-Vol. 2: aging of core internals and piping systems, NUREG/CR-7153, Vol. 2, ORNL/TM-2013/532, Oct 2014Google Scholar
  6. 6.
    R. Dyle, Materials degradation matrix and issue management tables overview-LTO update (The 2nd Workshop on U.S. Nuclear Power Plant Life Extension, Washington, D.C, 2011)Google Scholar
  7. 7.
    O.K. Chopra, A. Sather, Initial assessment of the mechanisms and significance of low-T embrittlement of cast stainless steels in LWR systems. NUREG/CR–5385 (1990)Google Scholar
  8. 8.
    T.S. Byun, J.T. Busby, Cast Stainless Steel Aging Research Plan. ORNL/LTR-2012/440Google Scholar
  9. 9.
    T.S. Byun, Y. Yang, N.R. Overman, J.T. Busby, Thermal aging phenomena in cast duplex stainless steels. J. Met. 68, 507 (2016)Google Scholar
  10. 10.
    T.S. Byun, N.R. Overman, T.G. Lach, Mechanical properties of model cast austenitic stainless steels after thermal aging for 1500 h. LWRS Report, PNNL-25377, Apr 2016Google Scholar
  11. 11.
    S. Mburu et al., Effect of aging temperature on phase decomposition and mechanical properties in cast duplex stainless steels. Mater. Sci. Eng., A 690, 365 (2017)CrossRefGoogle Scholar
  12. 12.
    C. Pareige et al., Kinetics of G-phase precipitation and spinodal decomposition in very long aged ferrite of a Mo-free duplex stainless steel. J. Nucl. Mater. 465, 383 (2015)CrossRefGoogle Scholar
  13. 13.
    W. Liu et al., Phase-field simulation of the separation kinetics of a nanoscale phase in a Fe-Cr alloy. J. Mater. Eng. Perform. 25, 1924 (2016)CrossRefGoogle Scholar
  14. 14.
    N. Pettersson et al., Nanostructure evolution and mechanical property changes during aging of a super duplex stainless steel at 300C. Mater. Sci. Eng., A 647, 241 (2015)CrossRefGoogle Scholar
  15. 15.
    J. Zhou et al., Concurrent phase separation and clustering in the ferrite phase during low temperature stress aging of duplex stainless steel weldments. Acta Mater. 60, 5818 (2012)CrossRefGoogle Scholar
  16. 16.
    T.S. Byun, T.G. Lach, Y. Yang, C. Jang, Influence of δ-ferrite content on thermal aging induced mechanical property degradation in cast stainless steels, in Proceedings of the 18th International Conference on Environmental Degradation of Materials in Nuclear Power Systems—Water Reactors (2017) SubmittedGoogle Scholar
  17. 17.
    B. Myers, TEM sample preparation with the FIB/SEM. NUANCE Center. Northwestern University (2009)Google Scholar
  18. 18.
    A. Devaraj, et al., Three-dimensional nanoscale characterization of materials by atom probe tomography. Int Mater Rev 1 (2017)Google Scholar
  19. 19.
    N. Shigenaka, T. Hashimoto, M. Fuse, Effects of alloying elements (Mo, Si) in an austenitic stainless steel on dislocation loop nucleation under ion irradiation. J. Nucl. Mater. 207, 46 (1993)CrossRefGoogle Scholar

Copyright information

© The Minerals, Metals & Materials Society 2019

Authors and Affiliations

  1. 1.Nuclear Sciences Division, Energy and Environment DirectoratePacific Northwest National LaboratoryRichlandUSA

Personalised recommendations