Design of structural components for FE power plant components that operate in the creep regime of mechanical behavior (high applied stress and temperatures above about 40% of the alloy melting point in absolute temperature units) is normally done on the basis of a minimum 300,000 hours operating life. Most existing FE power plants in the United States (US) were designed for steady state operation in which creep effects alone dominate the mechanical design of high temperature stress bearing components. The average age of US coal fired power plants is over 30 years, and many coal fired plants will be operated in cycling mode with an expectation of another 30 years of life. Cyclic operation results in additional structural material degradation due to thermo-mechanical fatigue. Under certain conditions the combined effect of creep and fatigue can reduce the useful life of high temperature materials beyond the effect of either creep or fatigue by themselves. Weldments of high temperature creep resistant materials are also susceptible to premature failure when subject to the combined effects of creep and thermo-mechanical fatigue.

Advanced FE power generation cycles will operate at temperature and pressure/stress conditions that exceed current commercially operating FE power plants. Examples of advanced FE power generation cycles include, but are not limited to, Advanced Ultrasupercritical (>1300 deg F and >4000 psi steam temperature and pressure) air and oxygen fired pulverized coal power plants, chemical looping combustion, and supercritical CO2. Advanced FE power generation cycles will utilize advanced structural alloys, such as polycrystalline nickel superalloys, and state of the art 9% chrome creep resistant ferritic steels, austenitic steels, and ferro-nickel alloys. These materials are more expensive than the structural alloys used in most existing FE power plants. Design of components that use advanced and state of the art creep resistant structural alloys will utilize finite element modeling (FEM) tools to calculate the long term mechanical behavior and the optimum designs of these components.

A key requirement for more accurate life assessment calculations of FE power plant component in existing plants, and for optimal mechanical design of new components is a physics based mathematical description of the creep behavior (strain evolution versus time as a function of applied stress, temperature and microstructure; and where the model describes extreme conditions, e.g., zero creep strain at zero applied stress, time to creep rupture approaches zero as applied stress approaches ultimate hot tensile stress, creep strain rate approaches zero as operating temperature is reduced) of structural alloys and their weldments. Such models need to be applicable to operating times of at least 300,000 hours, describe creep behavior over the expected range of temperature and applied stresses, account for multi-axial conditions on creep, and be numerically stable when used in FEM simulations of three dimensional (3D) components. For some creep resistant ferritic and stainless steel alloys (e.g., P91 steel and 316 stainless steel) large lab scale uniaxial creep test data exist run to at least 100,000 hours. Some long term plant data is also available.

Applications are sought for physics based models that accurately describe the creep behavior of state of the art and advanced structural alloys or their weldments under uniaxial stress conditions, and to greater than 300,000 hours. Applications should provide the lab and plant data sets that will be used to develop the creep model, a method to analyze variability in the experimental data, a statistical based measure of goodness of fit of the model(s) to the data set, and a procedure to account for multi-axial stress conditions. Applications can focus on either the base alloy only or weldments of an alloy. An application that focuses on weldments will need to consider the base alloy because the creep behavior of the base alloy will affect the creep behavior of the weldment.

The creep model(s) and numerical algorithm for calculating creep strain vs time developed should be verified and validated using the method described in ASME PTC 60/V&V (Guide for Verification & Validation of in Computational Solids Mechanics). Numerical stability of the computational model should be demonstrated in at least one typical commercially available Finite Element Modeling (FEM) software package that is used for modeling time dependent stresses and strains of solids. Examples of commercially available FEM software are ANSYS and ABAQUS. The programming language of the numerical algorithm of the creep model should be compatible for direct use of the numerical algorithm in commercially available FEM software packages.

Questions – Contact: Rick Dunst at richard.dunst@netl.doe.gov

Reference

1. Weitzel, P., and Tanzosh, J., 2011, Development of Advanced Ultra Supercritical Coal Fired Steam Generators for Operation above 700C, Rao, K.R., ed., Energy & Power Generation Handbook, ASME, New York, p. 17.1-17.26. http://ebooks.asmedigitalcollection.asme.org/content.aspx?bookid=312&sectionid=38782261

2. Viswanathan, R., et al, 2013, U.S Program on Materials Technology for Ultra-Supercritical Coal Power Plants, Asthana, R., ed., Journal of Materials Engineering & Performance, Vol. 22 (10), Sprinder US, New York, p. 2904. http://link.springer.com/article/10.1007/s11665-013-0717-6

3. Abson, D.J., and Roswell, J.S., 2013, International Materials Review, Vol 58(8), ASM International, p. 437. http://www.maneyonline.com/toc/imr/58/8

4. Yao, H-T., et al, 2007, A Review of Creep Analysis and Design Under Multi-axial Stress States, Hassan, Y.A., Nuclear Engineering & Design, Vol 237, p. 1969. http://www.sciencedirect.com/science/article/pii/S0029549307001331