This project will address two broad technical issues: (1) controlling mechanical properties and microstructure, and their dependence on chemical compositions and heat treatments; and (2) understanding the causes of Type IV failure and developing strategies to minimize or eliminate it in these steels. The overall project approach will rely on fundamental and applied studies of the effects of heat treatment, welding, and process control on microstructural evolution and material properties in CSEF steels.
For microstructure control in CSEF steels in general, two conditions are critical: (1) heat treating temperatures should not exceed the so-called A1 lower critical temperature which can cause hard, brittle, untempered alloys; and (2) the retention of primary ferrite must be avoided because it reduces strength and ductility which can compromise both fabrication processes and performance in service. For this project, researchers will build on exploring the limits for Grade 91 and similar types of steels through an analysis of chemical composition specifications using computational thermodynamics. Microstructure analysis and strength testing will also be conducted to verify thermodynamic predictions and to confirm any unusual effects on mechanical properties. It is anticipated that this information eventually will be used to consider restrictions on ASTM-specified chemical compositions with the development of a more reliable and predictable use of these advanced alloys in power generation equipment. Additionally, this work could establish a basis for identifying alloy compositions with the potential for reaching service temperatures beyond the current limits of commercial alloys. This activity will involve significant interaction with ASME Boiler and Pressure Vessel Code committees and major U.S boiler manufacturers.
Researchers will also study the effects of chemical composition and microstructure on Type IV failure in 9 Cr steels, mainly Grade 91. Past studies suggest that modifying the ASME code-specified post-weld heat treatment of Grade 91 may lead to minimizing the microstructure features associated with Type IV failure. Guided by previous studies, this possibility will be studied in detail by producing tempered plates, welding the plates using a conventional technique, and then further exploring a wider-thanusual range of post-weld heat-treatment temperatures. Weldments will be subjected to characterization of both microstructures and hardness distributions. Conditions leading to microstructures favorable for minimizing the Type IV cracking behavior will be further tested using aging treatments, as well as tensile and creep testing to determine both short- and longterm properties. Post-test microstructural evaluations will be conducted to determine the relationship(s) among alloy/heat affected zones (HAZ) microstructure.
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.
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