Search Results for iron-chromium-aluminum alloys

The U.S. Department of Energy (DOE) and the Ohio Coal Development Office (OCDO) are sponsoring the “Boiler Materials for Ultrasupercritical Coal Power Plants” program. This program is aimed at identifying, evaluating, and qualifying the materials needed for the construction of critical components for coal-fired boilers capable of operating at much higher efficiencies than the current generation of supercritical plants. Operation at ultrasupercritical (USC) conditions (steam temperatures up to 760°C (1400°F)) will necessitate the use of new advanced ferritic materials, austenitic stainless steels and nickel-based alloys. As well as possessing the required mechanical properties and fireside corrosion resistance, these materials must also exhibit acceptable steamside oxidation resistance. As part of the DOE/OCDO program, steamside oxidation testing is being performed at the Babcock & Wilcox Research Center. More than thirty ferritic, austenitic and nickel-based materials have been exposed for up to 4,000 hours in flowing steam at temperatures between 650°C (1202°F) and 800°C (1472°F). In addition to wrought materials, steamside oxidation tests have been conducted on weld metals, coated materials and materials given special surface treatments. Exposed specimens were evaluated to determine oxidation kinetics and oxide morphology. High chromium ferritic, austenitic and nickel-based alloys displayed very good oxidation behavior over the entire temperature range due to the formation of a dense chromium oxide. With increasing steam temperature, low chromium ferritic materials experienced breakaway oxidation, and low chromium austenitic materials experienced significant oxide exfoliation. Special surface treatments that were applied to these materials appeared to have a beneficial effect on their oxidation behavior.

As part of the Boiler Materials for Ultrasupercritical Coal Power Plants program, sponsored by the United States (U.S.) Department of Energy (DOE) and the Ohio Coal Development Office (OCDO), the steamside oxidation and oxide exfoliation behavior of candidate alloys have been thoroughly evaluated in steam at temperatures between 620°C and 800°C (1148°F and 1472°F) for times up to 10,000 hours. The results from this test program indicate that the oxidation rates and oxide morphologies associated with steamside oxidation are a strong function of the crystallographic lattice structure and the chromium content of the material. Oxide exfoliation correlates to oxide thickness. The time required to reach the critical oxide thickness for exfoliation can be estimated based on oxidation kinetic relationships. For austenitic stainless steels, shot peening is effective in reducing steamside oxidation/exfoliation, but the efficacy of this technique is limited by the operating temperature. Nickel-based alloys exhibit very low oxidation/exfoliation rates, but have a propensity to form aluminum/titanium oxides along near surface grain boundaries.

Nickel-base alloys were exposed to flowing supercritical CO 2 (P = 20MPa) at temperatures of 700 to 1000°C for up to 1000 h. For comparison, 316L stainless steel was similarly exposed at 650°C. To simulate likely service conditions, tubular samples of each alloy were internally pressurised by flowing CO 2 , inducing hoop stresses up to 35 MPa in the tube walls. Materials tested were Haynes alloys 188, 230 and 282, plus HR120 and HR160. These alloys developed chromia scales and, to different extents, an internal oxidation zone. In addition, chromium-rich carbides precipitated within the alloys. Air aging experiments enabled a distinction between carburisation reactions and carbide precipitation as a result of alloy equilibration. The stainless steel was much less resistant to CO 2 attack, rapidly entering breakaway corrosion, developing an external iron-rich oxide scale and internal carburisation. Results are discussed with reference to alloy chromium diffusion and carbon permeation of oxide scales.

Coal burning power companies are currently considering FeAlCr weld overlay claddings for corrosion protection of waterwall boiler tubes located in their furnaces. Previous studies have shown that these FeAlCr coatings exhibit excellent high-temperature corrosion resistance in several types of low NOx environments. In the present study, the susceptibility of FeAlCr weld overlay claddings to hydrogen cracking was evaluated using a gas-tungsten arc welding (GTAW) process. Microsegregation of alloying elements was determined for the FeAlCr welds and compared to a currently used Ni-based superalloy. Long-term gaseous corrosion testing of select weld overlays was conducted along with the Ni-based superalloy in a gaseous oxidizing/sulfidizing corrosion environment at 500°C. The sample weight gains were used along with analysis of the corrosion scale morphologies to determine the corrosion resistance of the coatings. It was found that although there were slight differences in the corrosion behavior of the selected FeAlCr weld coatings, all FeAlCr based alloys exhibited superior corrosion resistance to the Ni-based superalloy during exposures up to 2000 hours.

Nitridation is a high-temperature material degradation issue that can occur in air and in environments containing nitrogen, ammonia, etc., and in a variety of industrial processes. The nitridation behavior of several commercial nickel- and cobalt-based alloys is reviewed in this paper. The alloys include Haynes 230, Haynes 188, Haynes 625, Haynes 617, Haynes 214, Hastelloy X, and Haynes 233. The environments discussed are high-purity nitrogen gas between 871°C and 1250°C, 100% ammonia gas at 982°C and 1092°C, and a simulated combustion atmosphere at 982°C. The results showed that nitridation occurred in all the environments containing nitrogen. The nitridation attack was strongly influenced by the alloy compositions and the type of oxide formed (i.e., chromia or alumina), as some degree of oxidation was expected in the environments in which residual oxygen was present. Thermal cycling is briefly discussed because the integrity of protective oxides is also an important factor in resisting high-temperature oxidation and nitridation attack.

Oxide scale growth kinetics of 1.25Cr-0.5Mo and 2.25Cr-1Mo steels have been investigated and oxide scales have been characterized. In spite of the considerable difference in chromium contents of the two steels, their oxidation kinetics have been found to be similar. The similarity in oxidation behavior has been associated with formation of a protective innermost layer of silicon rich oxide in the case of 1.25Cr-0.5Mo steel and absence of such a layer in the case of 2.25Cr-1Mo steel, suggesting predominating role of Si over Cr.

Miniature specimen tests are necessary to assess the mechanical properties of new fuel cladding alloys for next-generation nuclear reactors. The small specimen allows for extensive testing programs from limited volumes of material. However, there is a lack of testing equipment to perform high-temperature mechanical tests on the miniature specimen. This work presents the development of a high-temperature creep test system for miniature specimens with in situ scanning electron microscope (SEM) testing capability for real-time characterization. Here, we discuss the challenges of the development of the system, such as gripping the samples, loading, heating, cooling mechanisms, and strain measurement. The equipment is used to investigate the creep behavior of FeCrAl alloy Kanthal APMT, and the results are compared with conventional creep test data from the same batch of this material.

In the “Boiler Materials for Ultrasupercritical Coal Power Plants” program, sponsored by the U.S. Department of Energy and the Ohio Coal Development Office, various materials are being assessed for their suitability in the high-temperature, high-pressure environment of advanced ultrasupercritical (A-USC) boilers. Beyond mechanical properties and fireside corrosion resistance, these materials must also exhibit adequate steamside oxidation and exfoliation resistance. A comprehensive database of steamside oxidation test results at temperatures relevant to A-USC conditions has been compiled over recent years. These tests have been conducted on ferritic and austenitic materials with chromium content ranging from 2 to 26%. The specimens were evaluated for oxidation kinetics and oxide morphology. The findings indicate that steamside oxidation behavior is significantly affected by temperature, the chromium content of the material, and the ability of chromium to diffuse through the material's crystallographic lattice structure. Additionally, surface treatments have been applied to enhance the steamside oxidation resistance of certain materials. While these treatments have shown potential, their effectiveness can be limited by the operational temperatures.

The INCONEL filler metals 72 and 72M have been utilized significantly for weld overlay protection of superheaters and reheaters, offering enhanced corrosion and erosion resistance in this service. Laboratory data conducted under simulated low-NOx combustion conditions, field exposure experience, and laboratory analysis (microstructure, chemical composition, overlay thickness measurements, micro-hardness) of field-exposed samples indicate that these overlay materials are also attractive options as protective overlays for water wall tubes in low-NOx boilers. Data and field observations will be compared for INCONEL filler metals 72, 72M, 625 and 622.

The steam oxidation behaviour of boiler tubes and steam piping components is a limiting factor for improving the efficiency of the current power plants. Spallation of the oxide scales formed during service can cause serious damage to the turbine blades. Vallourec has implemented an innovative solution based on an aluminum diffusion coating applied on the inner surface of the T/P92 steel. The functionality of this coating is to protect the tubular components against spallation and increase the actual operating temperature of the metallic components. In the present study, the newly developed VALIORTM T/P92 product was tested at the EDF La Maxe power plant (France) under 167b and 545°C (steam temperature). After 3500h operation, the tubes were removed and characterized by Light Optical Metallography (LOM), Scanning Electron Microscopy (SEM), with Energy Dispersive X-ray spectrometry (EDX) and X-Ray Diffraction (XRD). The results highlight the excellent oxidation resistance of VALIORTM T/P92 product by the formation of a protective aluminum oxide scale. In addition, no enhanced oxidation was observed on the areas close to the welds. These results are compared with the results obtained from laboratory steam oxidation testing performed on a 9%Cr T/P92 steel with and without VALIORTM coating exposed in Ar-50%H 2 O at 650°C.

Prior to utilizing new advanced materials in coal power plants, a large number of experimental testing is required. Test procedures are needed in specialized high temperature laboratories with state of the art facilities and precise, accurate analytical equipment capable of performing tests at a variety of temperatures and environments. In this study, the results of a unique technique involving salt spray testing at high temperatures are presented. The Haynes 282 gamma – prime (γ’) strengthened alloy fabricated by means of three different manufacturing processes: HAYNES 282 WROUGHT alloy, Haynes 282-SINT alloy, and finally Haynes 282-CAST alloy have been tested. The materials have been exposed to a salt spray corrosion atmosphere using 1% NaCl - 1% Na 2 SO 4 . Post exposure investigations have included SEM, EDS and XRD examinations. The test using salt spray of 1% NaCl - 1% Na 2 SO 4 water solution at 550 °C for 500 hours indicted no influence on the corrosion products formation, where Cr 2 O 3 has been developed in all three alloys, whereas NiO has been found only in Haynes 282-CAST material. On the other hand, it has been found that the fabrication process of HAYNES 282 alloy strongly influences the corrosion products formation under the high temperature exposures.

New Fe-base ferritic alloys based on Fe-30Cr-3Al-Nb-Si (wt.%) were proposed with alloy design concepts and strategies targeted at improved performance of tensile and creep-rupture properties, environmental compatibilities, and weldability, compared to Grade 91/92 type ferritic-martensitic steels. The alloys were designed to incorporate corrosion and oxidation resistance from high Cr and Al additions and precipitate strengthening via second-phase intermetallic precipitates (Fe2Nb Laves phase), with guidance from computational thermodynamics. The effects of alloying additions, such as Nb, Zr, Mo, W, and Ti, on the properties were investigated. The alloys with more than 1 wt.% Nb addition showed improved tensile properties compared to Gr 91/92 steels in a temperature range from 600-800°C, and excellent steam oxidation at 800°C as well. Creep-rupture properties of the 2Nb-containing alloys at 700°C were comparable to Gr 92 steel. The alloy with a combined addition of Al and Nb exhibited improved ash-corrosion resistance at 700°C. Additions of W and Mo were found to refine the Laves phase particles, although they also promoted the coarsening of the particle size during aging. The Ti addition was found to reduce the precipitate denuded zone along the grain boundary and the precipitate coarsening kinetics.

A combined pilot-scale combustion test and long-term laboratory study investigated the impact of oxy-firing on corrosion in coal-fired boilers. Four coals were burned under both air and oxy-firing conditions with identical heat input, with oxy-firing using flue gas recirculation unlike air-firing. Despite higher SO 2 and HCl concentrations in oxy-firing, laboratory tests showed no increase in corrosion rates compared to air-firing. This is attributed to several factors: (1) Reduced diffusion: High CO 2 in oxy-firing densified the gas phase, leading to slower diffusion of corrosive species within the deposit. (2) Lower initial sulfate: Oxy-fired deposits initially contained less sulfate, a key hot corrosion culprit, due to the presence of carbonate. (3) Reduced basicity: CO 2 and HCl reduced the basicity of sulfate melts, leading to decreased dissolution of metal oxides and mitigating hot corrosion. (4) Limited carbonate/chloride formation: The formation of less corrosive carbonate and chloride solutes was restricted by low O 2 and SO 3 near the metal surface. These findings suggest that oxy-firing may not pose a greater corrosion risk than air-firing for boiler materials.

Laboratory-scale tests are frequently used to generate understanding of high-temperature oxidation phenomena, to characterise and rank the performance of existing, future materials and coatings. Tests within the laboratory have the advantage of being well controlled, monitored and offer the opportunity of simplification which enables the study of individual parameters through isolating them from other factors, such as temperature transients. The influence of pressure on the oxidation of power plant materials has always been considered to be less significant than the effects of temperature and Cr content, but still remains a subject of differing opinions. Experimental efforts, reported in the literature, to measure the influence of steam pressure on the rate of oxidation have not produced very consistent or conclusive results. To examine this further a series of high pressure steam oxidation exposures have been conducted in a high pressure flowing steam loop, exposing a range of materials to flowing steam at 650 and 700 °C and pressure of 25, 50 and 60 bar. Data is presented for ferritic-martensitic alloys showing the effect of increasing pressure on the mass change and oxide thickness of these alloys in the flowing steam loop. In addition the effect observed on the diffusion of aluminium from an aluminised coating in these alloys is also presented and the differences in the extent of diffusion discussed.

In the late 1980s, the domestic utility industry experienced failures in dissimilar metal welds (DMWs) between low-alloy ferritic tubing and austenitic tubing in superheaters and reheaters. Extensive research by EPRI identified that nickel-based filler metals significantly improved service life compared to 309 SS filler metals. Additionally, optimized joint geometries and increased weld metal reinforcement were found to further enhance durability. A new nickel-based filler metal was developed with thermal expansion properties similar to the low-alloy base metal, along with a low chromium content designed to minimize the carbon-denuded zone. However, this filler metal was never commercialized due to its tendency to microfissure, which resulted in a shorter-than-expected service life. This paper explores further investigations into the microfissuring of this filler metal and examines long-term testing to assess its suitability for high-temperature applications.

Diffusion aluminide coatings have been evaluated as a strategy for improving the oxidation resistance of austenitic and ferritic-martensitic (FM) steels, particularly in the presence of steam or water vapor. The objective was to evaluate the strengths and weaknesses of these coatings and quantify their performance and lifetime. Long-term diffusion and oxidation experiments were conducted to study the behavior of various model diffusion coatings and produce a better data set for lifetime predictions. The key findings are that (1) thin coatings (<50μm) with relatively low Al contents appear to be more effective because they avoid high thermal expansion intermetallic phases and have less strain energy to nucleate a crack; and (2) the low Al reservoir in a thin coating and the loss of Al due to interdiffusion are not problematic because the low service temperatures of FM steels (<600°C) and, for austenitic steels at higher temperatures, the phase boundary between the ferritic coating-austenitic substrate inhibits Al interdiffusion. Unresolved issues center on the effect of the coating on the mechanical properties of the substrate including the reaction of N in the alloy with Al.

Inconel Filler Metal 72 (FM 72) and Incoclad 671/800H co-extruded tubing have been successfully used for over 20 years to protect boiler tubing from high-temperature degradation. A newer alloy, FM 72M, offers superior weldability and the lowest corrosion rate in simulated low NOx environments. Both FM 72 and 72M show promise in addressing challenges like circumferential cracking and corrosion fatigue in waterwall tubing overlays. Additionally, 72M’s superior wear resistance makes it ideal for replacing erosion shields in superheater and reheater tubing. Beyond improved protection, these alloys exhibit increased hardness and thermal conductivity over time, leading to reduced temperature difference across the tube wall and consequently, enhanced boiler efficiency and lower maintenance costs. This paper discusses the historical selection of optimal alloys for waterwall and upper boiler tubing overlays, analyzes past failure mechanisms, and highlights the key properties of successful choices like FM 72 and 72M.

Various carbon and nitrogen free martensitic alloys were produced for the application which required long time creep properties at high temperatures. But they were easy transformed to austenite phase before the creep tests because of low Ac1 temperature. In this paper, a new attempt has been demonstrated using carbon and nitrogen free austenitic alloys strengthened by intermetallic compounds. We choose Fe-12Ni-9Co-10W-9Cr-0.005B based alloy. Furthermore, we discussed about creep characteristics among the wide range of the testing conditions more over 700°C and steam oxidation resistance to confirm the possibility of the alloys for the future USC power plants under the severe environments.

Abradability, erosion and steam oxidation tests were conducted on commercial and experimental abradable coatings in order to evaluate their suitability for applications in steam turbines. Steam oxidation tests were carried out on free-standing top coat samples as well as coating systems consisting of a bond and an abradable top coat. Mapping of the abradability performance under widely varied seal strip incursion conditions was carried out for a candidate abradable coating that showed good steam oxidation performance in combination with good erosion resistance. The abradability tests were carried out on a specially designed test rig at elevated temperatures. The steam oxidation analysis combined with the abradability mapping results provide a potentially improved seal coating system that can be integrated into existing steam turbine designs for various seal locations. Such design integration is easily achieved and can be applied to steam turbine components that are sprayed in dedicated coating shops or even at the site of final turbine assembly.

Large heat-to-heat variation of creep rupture strength in weldments of mod.9Cr-1Mo steels was observed in the creep rupture tests conducted for two different heats at 600°C and 650°C. One heat showed consistently lower time-to-rupture than the other for 130-60MPa at 600°C. Detailed microstructural investigations revealed that the number density of precipitates in the weaker heat was remarkably lower than that associated with the stronger heat through most of the creep region. Accordingly, heat-to-heat variation of creep rupture strength was attributed to the difference in the precipitate strengthening effects throughout creep. Equilibrium calculation predicted that the smaller phase fraction of M 23 C 6 and VN precipitates due to the lower content of chromium and lower ratio of nitrogen/aluminum in the weaker heat. However, given that long-term creep rupture strength at 650°C converged for the two heats, the microstructure including precipitates may settle into a similar level for subsequent longer hours even at 600°C.