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[1] Wright, I.G., et al. Materials Issues for Turbines for Operation in Ultra-Supercritical Steam. US Department of Energy.

[2] Viswanathan R. and W.T. Bakker. Materials for Ultra Supercritical Fossil Power Plants. EPRI, Palo Alto, CA: 2000.



[3]Viswanathan, Vis, Purget, Robert, and Patricia Rawls. Coal-Fired Power Materials. Advanced Materials & Processes. August, 2008.

[4] Viswanathan R. and John Shingledecker. Evaluating Materials Technology for Advanced Ultrasupercritical Coal-Fired Plants. Power: August 1, 2010.

http://www.powermag.com/issues/features/Evaluating-Materials-Technology-for-Advanced-Ultrasupercritical-Coal-Fired-Plants_2880.html



General criterion of an average stress of 100MPa to produce rupture in 100,000 hours can serve as a guideline to set the temperature or stress limits. Nickel based alloys Haynes 230, Inco 740, and CCA 617 are major selections for boilers in the elevated temperature of advanced ultra supercritical systems. [3]

The range of alloys in steam turbines is relatively small because of the need to match thermal properties, such as expansion and conductivity, and the need for high temperature strength at a reasonable cost. The alloys used are heavily dependent upon the design of the turbine which varies between manufacturers. The first thing it is important to do is to identify the conditions under which they will operate. The main components are the turbine casing/shell, cylinders and valve bodies, bolting, turbine rotors or discs, and vanes and blades. [1]

Important material properties for HP/IP Rotors are creep strength, low cycle fatigue strength, and fracture toughness. High creep strength is needed to resist deformation and crack initiation in the bore or blade attachment areas. Low cycle fatigue strength is needed to prevent cracking from thermal stresses due to cycling, and fracture toughness is needed to contain the possibility of brittle fracture during transient conditions such as startup or shutdown.

The critical material properties for blades designed for high temperatures are creep rupture strength, thermal expansion coefficient (to match rotor), and ductility. As a first step selection, therefore, candidate superalloys must have expansion coefficients close to that of the rotor material (thermal expansion coefficient ratio between bucket and rotor less than 1.2).

Bolting material and bucket materials have similar features in their selection process. Bolting materials must possess high mechanical strength, creep strength, freedom from notch sensibility, resistance to stress relaxation (high creep resistance), and a coefficient of thermal expansion compatible with the casings. Bolt must remain tight between schedule outages (between 20,00 and 50,000 hours). [2]

According to research all candidate alloys for turbine blades in AUSC applications showed a low oxidation rate in the long-term (16 thousand hours) at a-USC temperatures. Metal samples in these tests were coated with 12 commerical coatings for solid particle erosion resistance and tested at 760C. One coating performed particularly well, but the others showed some erosion resistance coatings and do not have acceptable oxidation behavior. As far as non-welded rotors go, five alloys were identified as candidate materials, and three of those were selected for further evaluation. The latter are Nimonic 105, Haynes 282, and Waspaloy. The Haynes 282 possesses the greatest flexibility in terms of processing, heat treatment, and welding capability. Nimonic 105 and Waspaloy are also 1,400F capable alloys and can be used for components that need higher strength. Rotor samples are being scaled up to near full-size forgings for continued testing. For welded rotors in a-USC plants, designs may include welding nickel-base alloys but could use ferritic steel to minimize use of the expensive nickel-base alloy. This could also avoid difficulties in producing a large enough ingot for forging monoblock nickel-base alloy rotors. Three types of weld trials have been conducted: welding Inconel 617 to creep-strength-enhanced-ferritic steel, welding Nimonic 263 to Inconel 617, and welding Haynes 282 to Udimet 720Li. The welds are being evaluated for microstructure, mechanical properties, hardness, tensile properties, and impact strength. In the castings, significant challenges were identified with casting age-hardenable alloys due primarily to concerns of internal oxidation of aluminum and other hardening elements during typical air or protected air melting and pouring. The National Energy Technology Laboratory (NETL, Albany, Ore.) and Oak Ridge National Laboratory have initiated a project to address these fundamental issues and have identified a total of seven alloys for the initial trials. These trials were completed, and work is being done to scale up the best performers. In conclusion, for the future of A-USC steam turbines, the ability to produce alloys in steam turbine sizes will need to be proven, the supplier base of forgings and castings will need to be grown, and the lifetime material performance of alloys will have to be tested. [4]