CCS and Power Systems
Crosscutting Research - Plant Optimization Technologies
Joining of Advanced High-Temperature Materials
Performer: PNNL - Pacific Northwest National Laboratory
Project No: FWP-12461
Program Background and Project Benefits
To remain economically competitive, the coal-fired power generation industry needs to increase system efficiency, improve component and system reliability, and meet ever tightening environmental standards. In particular, cost-effective improvements in thermal efficiency are particularly attractive because they offer two potential benefits: (1) lower variable operating cost via increased fuel utilization (fuel costs represent over 70 percent of the variable operating cost of a fossil fuel-fired power plant) and (2) an economical means of reducing carbon dioxide (CO2) and other emissions.
To achieve meaningful gains, steam pressure and temperature must be increased to advanced ultrasupercritical (A-USC) conditions; that is, operating at temperatures above 760 degrees Celsius (°C) and pressures above 35 megapascals (MPa). The upper bounds of operating pressure and temperature are limited by the properties of the current set of materials employed in the boiler components. Key concerns are creep resistance, corrosion resistance, and cost effectiveness of the materials for critical pressureboundary omponents, such as headers, piping, and superheater/reheater tubes.
Materials for boiler components can be divided into three general categories: (1) ferritic steels, (2) austenitic steels, and (3) nickel (Ni)-based superalloys. These materials are listed in order of increasing temperature of effective resistance to both creep and corrosion, as well as increasing cost and difficulty of working. In general, the major performance drivers for heavy section components such as headers and pipes are to minimize thermal fatigue while achieving high creep strength (resistance to deformation at higher temperatures). Historically, materials selection for these components has focused on the ferritic steels. These alloys display greater thermal conductivities and lower coefficients of thermal expansion (CTE) than do the austenitic steels, making them less susceptible to thermal fatigue cracking.
However, at temperatures higher than 620 °C, the ferritic steels are prone to corrosion. This can be overcome to some extent by increasing the chromium content of the steel, but at levels greater than 10 percent in ferritic steels, chromium can reduce creep strength. The primary material issues driving the materials selection process for superheater and reheater tubes is the resistance to steamside oxidation and to ireside corrosion, considerations which typically necessitate the use of austenitic steels. Although Ni-based superalloys meet the creep- and oxidation/corrosion-resistance requirements of the various boiler components, they tend to be cost prohibitive in terms of raw material cost and processibility (e.g., casting and welding).
Development of effective joining methods to maintain the material performance of high-performance alloys will enable their use in high-temperature, high-pressure, corrosive environments, including A-USC steam turbines and boilers. As USC power plants are being developed to reduce carbon dioxide emissions and increase fuel efficiency, this project will contribute to more efficient use of fossil fuels, which simultaneously leads to lower emissions of greenhouse gases and better management of the subsequent long-term effects of global climate change.
Goals and Objectives
The goal of this project is to contribute to the development of cost-effective methods of joining high-performance alloys for use in advanced coal-fired power generation plants. The project will initially focus on ODS steels and on Ni-based superalloys that are susceptible to sensitization upon fusion welding, with the objective of achieving joints that exhibit high-temperature strength, creep resistance, and corrosion resistance properties equivalent to the base material. Specifically, researchers will develop linear and rotary friction-stir welding processes to meet this objective.