This project will initially focus on three main tasks: (1) a parametric study to determine process parameters (i.e., spindle rotational and travel speeds, plunge force, and pin diameter/shoulder geometry) that yield defect-free welds in ODS and superalloy materials, (2) mechanical and creep testing of FSW samples welded under optimized conditions, and (3) oxidation and corrosion of FSW material. As part of the latter two tasks, the research team will also perform tests on an equivalent series of specimens fabricated from the unwelded base material. Tensile testing will be conducted over a range of temperatures, from room temperature to 800 °C, initially under inert gas conditions, and creep testing will be conducted at 650, 725, and 800 °C under inert gas conditions. Comparative microstructural analysis will be conducted on a set of witness specimens, as well as select mechanical test specimens, to identify mechanisms responsible for any potential changes in mechanical behavior. If the welding conditions truly have been optimized, there will be little difference in mechanical behavior between the unwelded base material and the FSW region. PNNL investigators will work with researchers at ORNL to devise the appropriate test conditions for oxidation and corrosion testing. The team will experimentally produce highquality welds at various amounts of heat input during the FSW process (controlled via the various tool parameters or secondary induction heating) and the resulting specimens will undergo subsequent oxidation/corrosion testing to determine whether FSW alters the high-temperature corrosion resistance relative to the base material. Select corrosion specimens will be chosen for subsequent tensile, creep, and fracture testing.
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.
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