Abstract: The U.S. Department of Energy (DOE) Office of Fossil Energy is intensely promoting research and development of materials for advanced steam cycle systems and for oxy-fuel combustion systems. At Argonne National Laboratory, we have conducted studies to evaluate the corrosion performance of candidate structural alloys in coal-ash and in steam environments, in support of advanced steam cycle systems. The laboratory tests simulate the combustion atmosphere of advanced steam-cycle systems and three deposit chemistries that included ash constituents, alkali sulfates, and NaCl. Corrosion rate data will be presented for several Fe- and Ni-base alloys along with the mechanistic understanding of the corrosion processes. In the study on materials for oxy-fuel applications, we have evaluated the corrosion performance of the materials in CO2, steam, and in steam-CO2 mixtures. Materials selected for the study include intermediate-chromium ferritic steels, Fe-Cr-Ni heat-resistant alloys, and nickel-based superalloys. Information will be presented for materials exposed at temperatures between 650 and 950°C for times up to 10,000 h. In the ongoing experiments, we have incorporated low levels of sulfur and chlorine compounds (in addition to CO2 and steam) in the exposure environment to establish the role of second/third reactant on the scaling, internal penetration, and long term performance of the structural alloys.
Abstract: Alloy-designing of high-strength ferritic heat resistant steels has been investigated for application to highly efficient, low emission ultra-supercritical power plant with maximum steam temperature of 700 oC. Ferritic heat resistant steels can be applied to the next highest temperature components and are strongly desired to expand their temperature range up to 650 oC in order to minimize the requirement of expensive nickel base superalloys. A dispersion of nanometer size MX nitrides along boundaries and the addition of boron significantly improve long-term creep strength. Newly alloy-designed 9Cr-3W-3Co-0.2V-0.05Nb steel with 160 ppm boron and 85 ppm nitrogen exhibits excellent creep strength of base metal and no degradation in welded joints at 650 oC. The protective Cr2O3-rich scale forms on the surface of 9Cr steel by pre-oxidation treatment in Ar gas, which significantly improves the oxidation resistance in steam at 650 oC.
Abstract: The alloys required for fossil fuel power systems are transitioning from stainless steels that operate below 600oC to nickel-based alloys that can operate up to 760oC in advanced ultra-super critical steam turbines. This transition brings with it major metallurgical as well as economic challenges related to alloy design, melt processing, and fabrication of these large size components. The alloys, in general, must maintain creep resistance over 100,000 hours of service life while at the same time maintaining resistance to severe steam oxidation and general oxidation. A need exists for nickel-based alloys that are not only highly alloyed, but are also impervious to phase instabilities during initial processing and service. The potential exists for severe segregation when casting large ingots. This possibility must be dealt with during thermo-mechanical processing to obtain the wrought structures that can be inspected to design defect levels.
This paper will detail these challenges as they have been addressed in smaller aerospace turbines and discuss strategies to overcome these problems. New computational modeling tools will play a critical role in engineering solutions for alloy design, solidification, forging, and heat treatment. Since any solution also needs to be economically viable, the paper also discusses processing cost issues in terms of the process yields and operational strategies. The use of powder metallurgy will also be reviewed as a cost effective alternative to alloys that have traditionally been cast-wrought processed.
Abstract: Discovery of new high temperature alloys is a multidimensional problem which encompasses the intrinsic thermodynamic stability and their thermo-chemical and thermo-mechanical response to the combustion environment. Even when considering only the transition metals in combination with stable oxide formers, the number of ternary combinations exceeds 104. Hence, the traditional Edisonian process is not a practical approach to develop new alloy systems. Using formation enthalpy as a guide to compound stability, we propose a hierarchical scheme for identifying potential alloy systems which involves sifting through large regions of phase space with increasingly more accurate analysis. The coarsest sieve is a semi-empirical method based on the Miedema model extended to ternary systems. The next stage is ab initio simulations for a more accurate assessment and the basis for selecting system to investigate experimentally. We describe the implementation of this approach through the discovering of ternary additions that improve the oxidation stability -NiAl alloy.
Abstract: The high temperature creep and fatigue properties of two -TiAl base intermetallic alloys, for gas turbine components, have been investigated within the Integrated European project IMPRESS. The alloys contain 8% at. of Ta or Nb, respectively. The microstructure of both alloys was cross convoluted lamellar rather than the well known conventional lamellar, typical of the usual -TiAl. The microstructure of the Ta containing alloy was homogeneous in all the analyzed batches whilst that of the Nb alloy appeared significantly spread out from specimen to specimen. The creep properties of the alloys were investigated in the temperature range 700-850°C with applied stresses in order to have times to rupture up to about 3,000 h. The creep behaviour presented no steady state regimes, but only minima of the creep rates between significant decelerating and accelerating regimes. The minimum creep rates of the Ta alloy resulted to be significantly slower than the Niobium alloy at the same creep conditions. In low cycle fatigue at 650 and 700°C the Ta -TiAl showed longer lives than the Nb alloy, whilst the fatigue crack propagation rate in the same temperature range did not show any significant difference. Threshold values of stress intensity factor range were accurately measured at different R ratio. The microstructures of the two alloys were analysed by scanning microscopy in order to rationalise the different mechanical behaviour.
Abstract: For energy production and also for the glass industry, finding new refractory alloys which could permit to increase the process temperatures to 1200°C or more is a permanent challenge. Chromium base alloys can be good candidates, considering the melting point of Cr itself, and also its low corrosion rate in molten glass. Two families of alloys have been studied for this purpose, Cr-Mo-W and Cr-Ta-X alloys (X= Mo, Si..). A finer selection of compositions has been done, to optimize their chemical and mechanical properties. Kinetics of HT oxidation by air, of corrosion by molten glass and also creep properties of several alloys have been measured up to 1250°C. The results obtained with the best alloys (Cr-Ta base) give positive indications as regards the possibility of their industrial use.
Abstract: Two chromia forming materials have been exposed to different test gases at low pO2. All used test gases contained hydrogen species in the form of molecular hydrogen and water vapour. To insure the validity of the measured H-profiles H2 was replaced by with D2 and H2O by D2O. The Deuterium profiles in the oxides formed were established using SIMS and GDOES techniques. The profiles were quantified and correlated with the growth rate and the microstructures of the chromia scales formed on in low pO2 test gases. The presence of H2/H2O in the low pO2 test gas alters the properties of thermally growing chromia scales.