Surface engineering of IN-718 by low-temperature carburisation: properties and thermal stability

ABSTRACT

IN-718 was surface-engineered by low-temperature gas-phase carburisation, generating a highly concentrated solid solution of interstitial carbon with an average depth of $10$  μm and a carbon fraction up to 0.18, exceeding equilibrium solubility by orders of magnitude. Here, we investigate the stability of this highly supersaturated material against heat exposure and proton irradiation. We found: (i) Short-term exposure to high temperature causes de-carburisation. Long-term exposure to moderate (reactor operating) temperature does not. (ii) Exposure to MeV proton irradiation at elevated temperature – as well as high-temperature only – causes partial de-carburisation and precipitation of nanometre-sized Cr-rich carbides. This reduces Cr loss by evaporation. Physical models are proposed to explain the observed phenomena in terms of microscopic mechanisms. In conclusion, low-temperature gas-phase carburisation can significantly improve the properties of IN-718 for technical applications, including nuclear reactors.

KEYWORDS:

• IN-718
• high-temperature alloys
• surface engineering
• mechanical properties
• corrosion resistance
• ion irradiation

Acknowledgments

We thank G Bracker and H Kahn for help with the experimental work and S Collins, P Williams, and S Marx for helpful discussions. Authors have been ordered following the ‘first–last author emphasis’ convention.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

R.S. Lillard http://orcid.org/0000-0002-1313-1849 F. Ernst http://orcid.org/0000-0002-4823-2041

Additional information

Funding

This work was financially supported by the NEUP (Nuclear Energy University Program) of the DOE (Department of Energy) as project 12-3451 through Batelle Energy Alliance, LLC, under contract # 00128081.