Oxidation behaviour of cast Ni–Cr alloys in steam at 800°C

Abstract

To evaluate the steam oxidation resistance of cast Ni base alloy candidates for advanced steam turbine casings, laboratory experiments were conducted at 800°C. Alloys ranged from weaker, solid solution strengthened alloys 230 and 625 to stronger, precipitation strengthened alloys 105, 263 and 740, which are more difficult to fabricate and join. In general, these Ni–Cr based alloys exhibit low mass gains and form thin, protective Cr rich external oxides in 17 bar steam or 1 bar air. However, Al and Ti in these alloys internally oxidise in all cases. After 5000 h exposures, the average and maximum internal oxide penetration depths were measured, and the values were ranked based on the alloy Al+Ti contents. The middle range of Al+Ti compositions investigated, such as for alloys 617, 263, 282 and 740, showed the deepest penetrations. Further characterisation of the reaction products by electron microprobe showed a complex behaviour including significant Ti incorporation into the scale formed in both steam and air, and Ti rich oxide at both the gas and metal interfaces. Based on the Al and Ti contents, the internal oxidation observed in these alloys in steam was atypical.

Keywords:

• Steam oxidation
• Cast Ni base alloys
• Internal oxidation

This manuscript has been authored by UT-Battelle, LLC, under contract no. DE-AC05-00OR22725 with the US Department of Energy. The US Government retains and the publisher, by accepting the article for publication, acknowledges that the US Government retains a non-exclusive, paid-up, irrevocable, worldwide licence to publish or reproduce the published form of this manuscript, or allow others to do so, for US Government purposes.

The authors would like to thank G. Garner, T. Lowe, M. Howell, H. Longmire and H. Meyer for assistance with the experimental work. The alloys were cast at NETL-Albany by P. Jablonski. M. P. Brady and I. G. Wright provided many helpful comments on the manuscript. The research was sponsored by the US Department of Energy, Fossil Energy Advanced Research Materials Program.

The present paper is based on a presentation made at the 8th International Charles Parsons Turbine Conference organised by the Institute of Materials, Minerals and Mining at Portsmouth, UK on 5–8 September 2011.

Notes

This paper is part of a special issue featuring contributions from the 8th International Charles Parsons Turbine Conference