Program Background and Project Benefits
The Department of Energy (DOE) pursues research to develop materials that can withstand high-temperature, high-pressure, and corrosive operating conditions in order to increase the efficiency of coal-fired power plants. The tendency of materials to deform increases as temperature increases. Metals and alloys that resist this deformation are said to have good creep strength. Creep strength-enhanced ferritic (CSEF) steels, such as alloys containing nine percent chromium (9 Cr steel), are good candidates for use in high-temperature processes. These alloys have been found to withstand increased temperatures and pressures and/or permit the use of decreased tube wall thicknesses, at a cost that is significantly lower than that of austenitic (non-magnetic, chromium-nickel) steels of equivalent strength. The use of CSEF steels has led to the increased efficiency of fossil power plants.
CSEF steels are used up to approximately 600 degrees Celsius (°C), and are increasingly being specified and used for superheater tubing and main steam piping in coal-fired steam boilers, as well as in heat-recovery steam generators used in combined cycle plants. This has been done to try to eliminate the need for austenitic steels and the problems associated with the performance of austenitic steel-to-ferritic steel weld joints. Until recently, the use of more expensive nickel (Ni)-based superalloys has been reserved for components in planned advanced steam cycles expected to reach temperatures of 650°C or higher. This led to the common practice of expecting components made of CSEF steels to function at temperatures above 600°C.
The performance of CSEF steels, however, does not always meet this expectation. There have been reports of numerous failures of CSEF steels after only a few years in service. This unacceptable behavior appears to result from two main causes: (1) long-term properties that are not in accord with the projections made from the measurements used to qualify the alloys; and (2) an inability to attain the alloy microstructures required to achieve the desired properties in structures that have experienced certain fabrication or repair procedures. An alloy’s microstructure is its fine-scale structure that can influence physical properties such as strength, ductility, and oxidation resistance.
The implications of such failures include electrical supply disruptions, increased cost of electricity, and the potential for catastrophic failure endangering the safety of power plant personnel. The National Energy Technology Laboratory (NETL) is partnering with Oak Ridge National Laboratory (ORNL) to better understand how components made from these materials fail. This project will serve as a basis for devising materials solutions not only to improve component performance, but also to exploit in the search for economical materials capable of operating at higher temperatures.
This project will extend our understanding of (1) an important failure mode for high-performance steels, which are widely used in the power industry, (2) the conditions that lead to such failures, and (3) a means to avoid those conditions. This knowledge may be used to help increase the steels’ effective operating temperatures without resorting to far more expensive alloys. The resulting improvements in materials specification and fabrication and component lifetime will increase plant safety, and reduce costs and downtime. Additionally, this project may also enable the realization of longer-term targets for power plant efficiency at reasonable cost which will contribute to better management of energy resources and the environment.