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Microstructural analysis of steam oxidation of IN617 for use in ultra-supercritical steam plants

Posted on 4. July, 2012.

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A major step towards the UK’s goals of reducing carbon emissions can be made by improving the efficiency of the steam driven power plant.

One route to achieving an improvement in efficiency involves increasing the operating conditions of the working steam. The typical operating conditions in coal fired power plants in the hottest regions of the subcritical steam cycle are ~540OC/~175 bar. State-of the- art ultra-supercritical (275 bar/600/620oC) are offered today where the required material properties are already well established. Further efficiency increases of 8 –10% points may be possible with the adoption of cost effective and robust materials available for long term use capable of withstanding temperatures and pressures in excess of 700oC and 300 bar. One possible avenue to achieve this is to use nickel based alloys, which are well known for their high creep strength and resistance to corrosion at high temperatures. One such candidate material is IN617, which is a well established alloy which already finds use in numerous applications in the nuclear, waste incineration and gas turbine industries, all of which require excellent mechanical properties and very good oxidation/corrosion resistance at elevated temperatures.

The microstructural evolution of IN617 subjected to oxidising atmospheres of 100% steam, 50% steam/argon and air in the temperature range 700 – 750oC for exposures times up to 4000 h at atmospheric pressure has been investigated using a range of analytical electron microscopy techniques. It has been found that for this alloy the presence of steam in the atmosphere has an effect on the oxidation kinetics, and influences the nature of the scale. It has also been shown that there are differences in the volume and nature of voids formed, and that the voids are often associated with an internal structure of alumina. Significant internal oxidation was observed, particularly in the presence of steam, and a 3D reconstruction of the microstructure using FIBSEM techniques showed that this comprised interconnected alumina plates that followed the grain boundaries into the substrate.

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Keywords: microstructural analysis, steam oxidation, IN617, ultra-supercritical steam plants

Doi: 10.3184/096034012X13313071300779

D.M. Gormana *, R.L. Higginsona, Hailiang Dub, Gordon McColvin(b), A.T. Fry(c) and R.C. Thomson(a)
 (a)Department of Materials, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK
 (b)Alstom Power Ltd., Newbold Road, Rugby, Warwickshire CV21 2NH, UK
 (c)National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, UK

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