Cast alloys for advanced ultra supercritical steam turbines

The proposed steam inlet temperature in the Advanced Ultra Supercritical (A-USC) steam turbine is high enough (760°C) that traditional turbine casing and valve body materials such as ferritic/martensitic steels will not suffice due to temperature limitations of this class of materials. Cast versions of three traditionally wrought Ni-based superalloys (Haynes 263, Haynes 282, and Nimonic 105) were evaluated for use as casing or valve components for the next generation of industrial steam turbines. The full size castings are substantial: 2-5,000 kg each half and on the order of 100 mm thick. Experimental castings were quite a bit smaller, but section size was retained and cooling rate controlled to produce equivalent microstructures. A multi-step homogenization heat treatment was developed to better disperse the alloy constituents. These castings were subsequently evaluated by characterizing their microstructure as well as their steam oxidation resistance (at 760 and 800°C). (AU) © 2010 by The Materials, Metals, & Materials Society. All rights reserved.