The role of grain boundary ferrite evolution and thermal aging on creep cavitation of type 316H austenitic stainless steel

S He*, H Shang, A Fernández-Caballero, A D Warren, D M Knowles, P E J Flewitt, T L Martin

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

1 Citation (Scopus)

Abstract

To understand the interaction between microstructural evolution and creep cavitation during stress relaxation at an elevated temperature, an ex-service AISI type 316H stainless steel sample containing both weld metal and heat affected zone (HAZ) from an advanced gas-cooled reactor was studied. Multiple techniques that include secondary electron microscopy, electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and plasma focused ion beam tomograph were used for microstructure and creep cavities characterisation. Although no creep cavities were observed in the weld metal, the HAZ was extensively creep cavitated. At randomly oriented grain boundaries, creep cavities are present and closely linked with M23C6 and ferrite precipitates formed during thermal aging. Less precipitation (e.g. absence of ferrite) and less creep cavitation were observed at Σ3 coincidence site lattice boundaries. During in-service aging, at random grain boundaries, M23C6 formation and growth cause the local elemental depletion of γ stabilisers and promote a phase transformation from austenite to ferrite. The crystallographic relationship between ferrite and austenite were also studied by EBSD and TEM. Ferrite precipitates formed during aging often grow into the austenite grain not expected by traditional nucleation and growth theory, likely due to physical constraints by the existing carbides at the grain boundaries. The formation and growth of creep cavities is closely associated with the M23C6 and ferrite formed on grain boundaries. This study highlights the importance of considering the effect of thermal aging in accelerating creep cavitation.
Original languageEnglish
Article number140859
Number of pages18
JournalMaterials Science and Engineering: A
Volume807
Early online date2 Feb 2021
DOIs
Publication statusPublished - 11 Mar 2021

Structured keywords

  • Cabot Institute Low Carbon Energy Research

Keywords

  • Creep
  • thermal effects
  • ferrite
  • Austenitic Stainless Steel
  • Orientation Relationship

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