Internal carburization and scale formation on austenitic steels in supercritical carbon dioxide

ABSTRACT

Direct-fired supercritical CO₂ (sCO₂) power cycles are being commercialised to revolutionise fossil energy as a low-emission power source. To lower the cost of this technology, less expensive steels are needed in the lower temperature segments of the cycle. However, there are concerns about internal carburisation of steels in sCO₂.¹ A consistent observation is that thin, Cr-rich oxides appear to reduce C ingress compared to thick Fe-rich oxides formed on 9–12% Cr ferritic-martensitic steels. Advanced austenitic stainless steels (SS) like alloy 709 (20Cr-25Ni) are able to continue to form Cr-rich oxides at 650°C, while a conventional type 316 H SS formed a Fe-rich scale. The C diffusion profiles in SS specimens were quantified at 550°C–650°C using glow discharge optical emission spectroscopy and electron probe microanalysis. Analytical transmission electron microscopy was used to compare the thin protective Cr-rich oxide formed on alloy 709 in sCO₂ at 650°C to that formed in ambient air.

KEYWORDS:

• austenitic stainless steels
• alloy 709
• supercritical carbon dioxide
• analytical transmission electron microscopy

Acknowledgments

The authors are grateful to Tenaris (T91), EPRI (VM12), Special Metals (740 H) and Sam Sham at INL (US heat of alloy 709) for supplying the materials. The experimental work was conducted by B. Johnston, T. Lowe, V. Cox and D. Newberry at ORNL. M. Romedenne and S. Dryepondt provided useful comments on the manuscript. The STEM work was supported by the Office of Nuclear Energy, Fuel Cycle R&D Program and the Nuclear Science User Facilities.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

1. This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide licence to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

Additional information

Funding

The work was supported by the U.S. Department of Energy, Office of Fossil Energy and Carbon Management, Advanced Materials Program [Field Work Proposal FEAA144]