August 1, 2010

Evaluating Materials Technology for Advanced Ultrasupercritical Coal-Fired Plants

By R. Viswanathan and John Shingledecker, Electric Power Research Institute and Robert Purgert, Energy Industries of Ohio

A-USC Steam Turbine Materials Research

For the steam turbine portion of the U.S. materials project, only the highest temperature components are of concern: the high-pressure/intermediate pressure (HP/IP) rotors and discs, HP/IP blades/buckets, and castings. The materials utilized and the design philosophy for steam turbines are unique to each manufacturer. Alloys are not subject to code approval and thus may or may not have internationally recognized material standards.

The U.S. project is focused on four turbine areas: oxidation and erosion resistance of turbine blades, nonwelded rotor materials, welded rotor materials, and castings.

Oxidation and Erosion Resistance of Turbine Blades. In research to date, all candidate alloys showed a rate of low oxidation in long-term (16,000 hour) testing at 1,290F, 1,400F, and 1,475F. Internal oxidation is the dominant oxidation mechanism. Base metal samples coated with 12 commercial coatings for solid particle erosion resistance were tested at 760C. One coating performed exceptionally well, and three other coatings showed good performance, but initial oxidation test results show some erosion resistance coatings do not have acceptable oxidation behavior.

Nonwelded Rotors. The material requirements for HP/IP rotors and discs are high creep strength with adequate fracture toughness. HP/IP rotors and discs are produced either by machining a single forging (monoblock, most commonly) or by welding discs together (welded rotor). The project team evaluated more than 25 possible nickel-base superalloys for a rotor forging, using minimum mechanical property criteria for yield strength and creep life.

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. In ongoing work, rotor samples are being scaled up to near full-size forgings for continued testing.

Welded Rotors. Rotors for A-USC designs may involve welding nickel-base alloys and ferritic steel to minimize use of the expensive nickel-base alloy, and due to 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.

Castings. Large-scale A-USC castings were explored through discussion with vendors, casting companies, and casting experts. 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. Under the direction of the consortium, 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 the consortium is working to scale up the best performers.

Looking Ahead

As the U.S. project continues materials testing for both boilers and turbines, it faces a number of challenges:

• For boilers, the supply base of large components will need to be built up to ensure that plant components are available for potential future plants.
• ASME code approval is needed for Inconel 740 and Haynes 282.
• For 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.

For the long term, it will be necessary to demonstrate A-USC technology at full scale. One proposal is to create a demonstration plant with shared ownership among several utilities to distribute the financial risk. The new high-efficiency plant could be located at an existing subcritical power plant site. This arrangement would minimize the plant cost through utilization of some of the existing infrastructure, reduce the permitting cycle, and upgrade the air quality of the local area. Because successful demonstration would be a significant step toward improved air quality for the entire country, government support along the lines provided for demonstration of the integrated gasification combined-cycle could appropriately be justified to expedite implementation.

The authors would like to acknowledge the encouragement and guidance of Mario Marrocco (OCDO), Jeff Phillips (EPRI), and Patricia Rawls and Robert Romanosky (NETL/DOE).