Year 2012, Volume 32

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 oxyfuel 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 nickelbased 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.

Abstract

Discovery of new high temperature alloys is a multidimensional problem which encompasses the intrinsic thermodynamic stability and their thermo-chemical and thermomechanical 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 β-NiAl alloy.

Abstract

The high temperature creep and fatigue properties of two γ -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 γ-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 γ -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

In some cases chromium, necessary for the resistance to hot corrosion, unfortunately tends to lower the solidus temperature of alloys destined to high temperature applications. However a decrease of its content in the bulk can be allowed by a Cr-enrichment of the alloys sub-surfaces. To examine this possibility in the case of refractory TaC-strengthened iron-based alloys, three {Fe(bal.) – 10Cr – 0.2 or 0.4C – 3 or 6Ta} – alloys were elaborated by foundry in inert atmosphere and subjected to Cr deposit and inwards diffusion by a pack-cementation process. Bulk and subsurface microstructures examinations and concentrations profiles were performed after each step of elaboration of the future coated alloys as well as after a 50 hours / 1000°C oxidation test in air. A 500μm-deep carbide-free sub-surface with a chromium content twice the bulk one was obtained for the 0.2C-3Ta and 0.4C-6Ta alloys (less deep for the 0.4C-3Ta alloy) and allowed the alloys resisting high temperature oxidation.

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.

Abstract

Thermal barrier coatings (TBCs) are an enabling materials technology to improve the efficiency and durability of gas turbines and thus through such efficiency improvements offer reduce fuel usage and an associated reduction in CO₂ emission. This commercial drive is pushing both aero- and industrial turbines to be lifetime dependent on TBC performance – the TBC must be “prime reliant”.
However, the prediction of the durability of the TBC system has proved difficult, with lifetimes varying from sample to sample and component to component. One factor controlling this is the inability to measure accurately the bondcoat/ceramic interface temperature when buried under a TBC. In operating engines this is further exacerbated by the fact that such TBC systems operate in strong temperature gradients due to the need to cool aerofoil components.
This research examines the design and manufacture of self diagnostic thermal barrier coatings capable of accurately measuring the interface temperature under the TBC, whilst providing the requisite thermal protection. Data on the temperature sensing capability of various rare earth doped EB-PVD thermal barrier coatings will be reported. It will be shown that systems exist capable of measuring temperatures in excess of 1300oC. Details of the measurement method, the compositions and the thermal stability of such systems will be discussed in this paper. The ability to produce a sensing TBC capable of measuring interface temperature, surface temperature and heat flux will further be discussed permitting the design of thermal barrier protected components capable of insitu performance monitoring.

Abstract

In studies of erosion-corrosion at materials at elevated temperatures, there have been many attempts to model the process. Such models have comprised quasi-static and simulation models. However in many environments, erosion-corrosion occurs in aqueous conditions and can be considerable affected by flowing gases in dry conditions. In such cases, any accurate models of erosioncorrosion must include an algorithm due to flow in addition to the mechanics and chemistry of the tribo-corrosion interactions. In this paper, a CFD model is generated of the tribo-chemical interaction at elevated temperatures. The initial work has concentrated on modeling temperature effects in wet conditions; however, the potential application of the model to dry conditions is also outlined. The results are discussed in relation to existing erosion-corrosion models in the literature at elevated temperatures.

Abstract

In the present work, the melting behaviour of ashes obtained from the combustion of coals from different seams were investigated by a laboratory-scale equipment. The ash melting behaviour was studied by heating the specimens in a tubular furnace under a controlled gas atmosphere, while continuous monitoring the shape transformation by a digital camera. The ash fusibility temperatures (AFT) were determined by using an application which allows the in-line identification of the AFTrelated shapes specified in the ASTM D 1857-04.
The effect of the furnace gas-atmosphere on the determination of the ash fusion temperatures were studied by performing the measurements under dry-air, SO₂-rich-air and humidified air environments. It was found that under dry conditions, AFT determination is significantly affected by slag foaming, leading to an overestimation of the melting temperatures. Low water vapour concentrations does not appreciably change the results obtained under dry air, while SO₂-rich atmosphere seems to reduce foaming. As a result, since no volume expansion was observed, samples heated under SO₂-air flow, apparently melt at lower temperatures with respect to dry air conditions. The mineral phases transformation of the ashes during the heating process was studied by X-Ray diffraction and it was found that the low temperature transformations are essentially related to Iron and Alkalis reaction with aluminosilicates to form a glassy phase. On the other hand, high temperatures transformations are related to quartz and mullite solubilization which is closely dependent to the amount of Calcium, and therefore of anorthite phase, in the ash sample.

Abstract

In order to improve the oxygen permeation rates of oxygen transport membranes (OTM), asymmetric membranes with a thin dense layer must be fabricated on a porous support. In this paper, we focus on preparing and characterizing structures with functional layers consisting of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF) and La_0.68Sr_0.3Fe_0.8Co_0.2O_3-δ (LSFC). The supports for these structures were prepared by tape casting and warm pressing, and samples were analysed in terms of nitrogen permeability and porosity. The coating of the functional layers was performed via screen printing. For BSCF, multiple layers were necessary to obtain a gas-tight structure. Oxygen permeation measurements of the asymmetric structures showed an increase in the oxygen flux in comparison to bulk membranes with a thickness of L =1 mm. However, this enhancement is subproportional to the reduced thickness, indicating rate limiting effects, i.e. surface exchange kinetics in the case of LSFC and concentration polarization in the porous support in the case of BSCF.