Microstructure Evolution During Steam Oxidation of a Nb Stabilized Austenitic Stainless Steel

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Abstract

The oxidation behaviour of TP 347H FG in mixtures of water, oxygen, and hydrogen was investigated at 500, 600, and 700 A degrees C for a fixed oxidation time of 336 h. The samples were characterised using X-ray diffraction, reflective light and electron microscopy methods. Thin discontinuous double-layered oxide scales developed during oxidation at 500 A degrees C, whereas continuous double-layered oxide scales covered the entire sample surface after oxidation at 600 and 700 A degrees C. The major part of the inner oxide layer developed within the former alloy grains, whereas a Fe-Cr spinel formed along the former alloy grain boundaries. Transmission electron microscopy and electron energy loss spectroscopy investigations revealed that the part of the scale that grows into the alloy grains consists of particles of Fe-Cr spinel embedded in a metallic Fe-Ni matrix, which indicates that this part of the scale grows by an internal oxidation mechanism. The thickness of the inner oxide zone at high humidity (46%) is not significantly affected by the type of carrier gas used, whereas this thickness at low humidity (8% H2O) is sensitive for the carrier gas and increases in the following order: air <Ar+7% H-2 <Ar, indicating that the presence of oxygen or hydrogen in addition to a relatively low content of water vapour counteracts the effect of water vapour on the development of the inner oxide zone.
Original languageEnglish
JournalOxidation of Metals
Volume73
Issue number1-2
Pages (from-to)289-309
ISSN0030-770X
DOIs
Publication statusPublished - 2010

Keywords

  • Internal oxidation
  • Grain boundaries
  • Diffusion
  • Austenitic stainless steel
  • Oxide scale
  • Water vapour oxidation

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