Study of the reactive element effect in ODS iron-base alumina-formers

Iron aluminide (Fe{sub 3}Al) and FeCrAl compositions were dispersed with 15 different oxides in order to study the effect of oxygen- active dopants on high-temperature growth and adhesion of {alpha}- Al{sub 2}O{sub 3} scales. In these model-type, oxide dispersion strengthened (ODS) systems, the chemical effects of various cation dopants were compared to the baseline effect of an Al{sub 2}O{sub 3} oxide dispersion. By conducting isothermal and cyclic oxidation tests and by characterizing the oxidation product, effects on scale adhesion, growth rate and microstructure were evaluated. The dopants were categorized based on effectiveness in modifying the alumina scale. An Al{sub 2}O{sub 3} dispersion yielded some improvement in oxidation behavior apparently by strengthening the relatively weak substrate. However, the type of improvements in adhesion and change in growth mechanism associated with addition of reactive elements such as Y were not achieved. In general, due to the weaker substrate and the inherently faster interfacial void formation, the dispersions were less effective in ODS Fe{sub 3}Al than in ODS FeCrAl

Evaluation of Durable Metallic Supports for Catalytic Combustors

In 2000, a Cooperative Research and Development Agreement (CRADA) was undertaken between the Oak Ridge National Laboratory (ORNL) and Catalytica Energy Systems Incorporated (CESI) to determine the properties of current metallic catalyst supports and examine new candidate alloys for this application. A team was established at ORNL to examine oxidation-limited lifetime of these thin-walled metallic components using standard lifetime models and to measure the mechanical properties of the foils (40-200:m in thickness) which can differ substantially from bulk properties. Oxidation experiments were conducted on foil specimens at 700-1100 C in laboratory air and in air with 10 vol.% water vapor to better simulate the combustor environment. At the higher test temperatures, time to oxidation-induced (i.e. breakaway oxidation) failure was determined in 1h cycles in order to verify predictions from a standard reservoir-type oxidation lifetime model. Selected specimens were run for >10,000h in 100 or 500h cycles at lower test temperatures in order to determine the oxidation kinetics for the model. The creep properties of selected foils were measured for 4,000-8,000h at operation-relevant stresses and temperatures. None of the new candidate alloys significantly out-performed currently used alloys in laboratory testing, particularly in oxidation lifetime testing. Therefore, engine testing was not performed on any of the new candidate alloys. Both the oxidation- and creep-resistance of FeCrAl alloys was greater than expected and the results of the CRADA allowed CESI to extend life or increase operating temperatures for these lower cost substrate alloys in the next generation of catalyst modules. Three work areas were defined for the CRADA. The first area was investigating the oxidation behavior of current and candidate alloy foils. The goal was to obtain data such as the oxidation rate as a function of temperature and environment, the time to breakaway oxidation at high test temperatures and the residual Al alloy content at breakaway oxidation. These inputs are needed for a standard reservoir-type oxidation model. The second work area was to use the experimental data to model the oxidation-limited lifetime of foil materials as a function of temperature, foil thickness and composition. The third task was to measure mechanical properties, in particular creep rates and creep to failure life, of similar foils at application relevant temperatures and stress levels

The effect of various oxide dispersions on the oxidation resistance of Fe{sub 3}Al

Oxide-dispersed Fe-28at.%Al-2%Cr alloys were produced by a powder metallurgy technique followed by hot extrusion. Yttria and ceria were added to the base alloy to assess the effect of these dopants on the oxidation behavior. The amount of dopant was varied from 0.05-0.5 at.% Y in a series of Y{sub 2}O{sub 3}-dispersed alloys. isothermal and cyclic oxidation testing was conducted at temperatures from 800{degrees} to 1300{degrees}C. A CeO{sub 2} addition was detrimental to the oxidation behavior. The Y{sub 2}O{sub 3} improved the {alpha}-Al{sub 2}O{sub 3} scale adhesion relative to an undoped alloy, but was not as effective as similar additions to an oxide-dispersed FeCrAl alloy

Maker Club in Pre-School

The project allows pre-school children to develop the problematization of what they are learning and, in secondary school, students’ perspectives of cooperative in the development of scientific literacy. In this type of activity, children, with the help of high school students, deepen and consolidate behavioral values for life, thus enabling a positive change in their attitudes, in the way of believing, innovating, planning and persisting to conquer. Theactivities developed are accessible, both in approach and availability as well as in the cost of materials

The Influence of Specimen Thickness on the High Temperature Corrosion Behavior of CMSX-4 during Thermal-Cycling Exposure

CMSX-4 is a single-crystalline Ni-base superalloy designed to be used at very high temperatures and high mechanical loadings. Its excellent corrosion resistance is due to external alumina-scale formation, which however can become less protective under thermal-cycling conditions. The metallic substrate in combination with its superficial oxide scale has to be considered as a composite suffering high stresses. Factors like different coefficients of thermal expansion between oxide and substrate during temperature changes or growing stresses affect the integrity of the oxide scale. This must also be strongly influenced by the thickness of the oxide scale and the substrate as well as the ability to relief such stresses, e.g., by creep deformation. In order to quantify these effects, thin-walled specimens of different thickness (t = 100500 lm) were prepared. Discontinuous measurements of their mass changes were carried out under thermal-cycling conditions at a hot dwell temperature of 1100 C up to 300 thermal cycles. Thin-walled specimens revealed a much lower oxide-spallation rate compared to thick-walled specimens, while thinwalled specimens might show a premature depletion of scale-forming elements. In order to determine which of these competetive factor is more detrimental in terms of a component’s lifetime, the degradation by internal precipitation was studied using scanning electron microscopy (SEM) in combination with energy-dispersive X-ray spectroscopy (EDS). Additionally, a recently developed statistical spallation model was applied to experimental data [D. Poquillon and D. Monceau, Oxidation of Metals, 59, 409–431 (2003)]. The model describes the overall mass change by oxide scale spallation during thermal cycling exposure and is a useful simulation tool for oxide scale spallation processes accounting for variations in the specimen geometry. The evolution of the net-mass change vs. the number of thermal cycles seems to be strongly dependent on the sample thickness

Paper Number GT2004-54254 SCREENING AND EVALUATION OF MATERIALS FOR MICROTURBINE RECUPERATORS

ABSTRACT The effects of stress, temperature and exposure to microturbine exhaust gases on the mechanical properties and corrosion resistance of candidate materials for microturbine recuperators were investigated. Results are presented for 347 stainless steel metallic foils after 500-hr exposure to temperatures between 620°C and 760°C at a tensile stress of 50 MPa. It was found that the material experienced accelerated attack at the highest temperature and that the corrosion products consisted of mixed oxides of iron and chromium. It was also found that exposure at the highest temperatures resulted in significant decrease in both tensile strength and ductility. ORNL's microturbine recuperator test facility, where the exposures were carried out, is also described. INTRODUCTION The challenging performance targets for the next generation of microturbines include fuel-to-electricity efficiency of 40%, capital costs less than $500/kW, NOx emissions reduced to single parts per million, several years of operation between overhauls, life of 40,000 hours and fuel flexibility One of the critical components in low-compression ratio microturbines is the recuperator, which is responsible for a significant fraction of the overall efficiency of the microturbin

Substrate Effect on the High Temperature Oxidation Behavior of a Pt-modified Aluminide Coating. Part II: Long-term Cyclic-oxidation Tests at 1,050 C

This second part of a two-part study is devoted to the effect of the substrate on the long-term, cyclic-oxidation behavior at 1,050 C of RT22 industrial coating deposited on three Ni-base superalloys (CMSX-4, SCB, and IN792). Cyclicoxidation tests at 1,050 C were performed for up to 58 cycles of 300 h (i.e., 17,400 h of heating at 1,050 C). For such test conditions, interdiffusion between the coating and its substrate plays a larger role in the damage process of the system than during isothermal tests at 900, 1,050, and 1,150 C for 100 h and cyclicoxidation tests at 900 C which were reported in part I [N. Vialas and D. Monceau, Oxidation of Metals 66, 155 (2006)]. The results reported in the present paper show that interdiffusion has an important effect on long-term, cyclic-oxidation resistance, so that clear differences can be observed between different superalloys protected with the same aluminide coating. Net-mass-change (NMC) curves show the better cyclic-oxidation behavior of the RT22/IN792 system whereas uncoated CMSX-4 has the best cyclic-oxidation resistance among the three superalloys studied. The importance of the interactions between the superalloy substrate and its coating is then demonstrated. The effect of the substrate on cyclic-oxidation behavior is related to the extent of oxide scale spalling and to the evolution of microstructural features of the coatings tested. SEM examinations of coating surfaces and cross sections show that spalling on RT22/CMSX-4 and RT22/SCB was favored by the presence of deep voids localized at the coating/oxide interface. Some of these voids can act as nucleation sites for scale spallation. The formation of such interfacial voids was always observed when the b to c0 transformation leads to the formation of a two-phase b/c0 layer in contact with the alumina scale. On the contrary, no voids were observed in RT22/IN792, since this b to c0 transformation occurs gradually by an inward transformation of b leading to the formation of a continuous layer of c0 phase, parallel to the metal/scale interface

The unexpectedly short Holocene Humid Period in Northern Arabia

The early to middle Holocene Humid Period led to a greening of today's arid Saharo-Arabian desert belt. While this phase is well defined in North Africa and the Southern Arabian Peninsula, robust evidence from Northern Arabia is lacking. Here we fill this gap with unprecedented annually to sub-decadally resolved proxy data from Tayma, the only known varved lake sediments in Northern Arabia. Based on stable isotopes, micro-facies analyses and varve and radiocarbon dating, we distinguish five phases of lake development and show that the wet phase in Northern Arabia from 8800-7900 years BP is considerably shorter than the commonly defined Holocene Humid Period (similar to 11,000-5500 years BP). Moreover, we find a two century-long peak humidity at times when a centennial-scale dry anomaly around 8200 years BP interrupted the Holocene Humid Period in adjacent regions. The short humid phase possibly favoured Neolithic migrations into Northern Arabia representing a strong human response to environmental changes

High Temperature Corrosion Behavior of Iron Aluminide Alloys and Coatings

A multi-year effort has been focused on optimizing the long-term oxidation performance of ingot-processed (IP) and oxide-dispersion strengthened (ODS) Fe{sub 3}Al and iron aluminide-based coatings. Based on results from several composition iterations, a Hf-doped alloy (Fe-28Al-2Cr-0.05at.%Hf) has been developed with significantly better high temperature oxidation resistance than other iron aluminides. The scale adhesion is not significantly better; however, the {alpha}-Al{sub 2}O{sub 3} scale grows at a slower rate, approximately a factor of 10 less than undoped iron aluminide. The benefit of Hf is greatest at 1100-1200 C. Long-term oxidation resistance of commercially fabricated ODS Fe{sub 3}Al has been determined and compared to commercially available ODS FeCrAl. Scale spallation rates for ODS Fe{sub 3}Al are higher than for ODS FeCrAl. To complement studies of iron-aluminide weld-overlay coatings, carbon steel was coated with Fe-Al-Cr by thermal spraying. These specimens were then exposed in air at 900 and 1000 C and in air-1%SO{sub 2} at 800 C. Most likely due to an inadequate aluminum concentration in the coatings, continuous protective Al{sub 2}O{sub 3} could not be maintained and, consequently, the corrosion performance was significantly worse than what is normally observed for Fe{sub 3}Al

Compressive Creep Performance and High Temperature Dimensional Stability of Conventional Silica Refractories

Furnace designers and refractory engineers recognize that optimized furnace superstructure design and refractory selection are needed as glass production furnaces are continually striving toward greater output and efficiencies. Harsher operating conditions test refractories to the limit, while changing production technology (such as the conversion to oxy-fuel from traditional air-fuel firing) can alter the way the materials perform. Refractories for both oxy- and air-fuel fired furnace superstructures are subjected to high temperatures during service that may cause them to excessively creep or subside if the refractory material is not creep resistant, or if it is subjected to high stress, or both. Furnace designers can ensure that superstructure structural integrity is maintained if the creep behavior of the refractory material is well understood and well represented by appropriate engineering creep models. Several issues limit the abilities of furnace designers to (1) choose the optimum refractory for their applications, (2) optimize the engineering design, or (3) predict the service mechanical integrity of their furnace superstructures. Published engineering creep data are essentially non-existent for almost all commercially available refractories used for glass furnace superstructures. The limited data that do exist are supplied by the various refractory suppliers. Unfortunately, these suppliers generally have different ways of conducting their mechanical testing and they also interpret and report their data differently; this makes it hard for furnace designers to draw fair comparisons between competing grades of candidate refractories. Furthermore, the refractory supplier's data are often not available in a form that can be readily used for furnace design and for the prediction and design of long-term structural integrity of furnace superstructures. With the aim of providing such comparable data, the US DOE's Office of Industrial Technology and its Advanced Industrial Materials program is sponsoring work to conduct creep testing and analysis on refractories of interest to the glass industry. An earlier stage of the project involved identifying which refractories to test and this is described elsewhere. Conventional silica was one such identified refractory category, and the present report describes the creep behavior of this class of refractories. To portray a more complete understanding of how these refractories perform at service temperatures, their fundamental corrosion resistances, dimensional stabilities, and microstructure were characterized as well