Effect of carbon content on the oxidation behaviour of FeCrAlY alloys in the temperature range 1200–1300°C

Abstract

Two FeCrAlY alloys with different carbon contents (90 and 500 ppm respectively) were investigated in respect to their oxidation behaviour at 1200 and 1300°C in air. Oxidation tests, with exposure times ranging from a few hundred to several thousands of hours, revealed that the growth rate of the protective alumina scale was hardly affected by the alloy C-content. However, the time to occurrence of breakaway oxidation for the specimens (1 mm thickness) was substantially shorter for the high-than for the low-C alloy. This was primarily caused by poorer oxide scale adherence but additionally by a higher critical Al-content for occurrence of breakaway of the high-C alloy compared to the low-C alloy.

Extensive microstructural studies revealed formation of Cr-carbides at the grain boundaries in both alloys. The high-C alloy additionally showed carbide formation at the scale/metal interface, thus deteriorating scale adhesion. Furthermore, inter- and intra-granular carbide precipitation is considered to induce strengthening of the metal, thus hindering relaxation of the thermally-induced oxide stresses by substrate creep. In a series of experiments with variations in the cooling rates, it was verified that carbide formation very likely occurs during specimen cooling.     

Improvements in honeycomb abradable seals

Abstract

The concept of utilising honeycomb abradable seals to improve gas turbine engine performance has been under development for many years. Engine operating temperatures, in the region of the seals, have been restricted to below 950°C by the reliance on a chromia scale for degradation protection. The introduction of nickel brazed FeCrAlY based alloys within the honeycomb seal could facilitate a safe increase in operating temperatures to over 1100°C. This is aided by the formation of a more stable, α-alumina scale.

These Fe–20Cr–5Al–0.5Y foils, including the commercially produced variant designated MI2100, have been designed for a service lifetime of up to 24,000 hours. However, burner tests and isothermal oxidation tests in laboratory air at 1100 and 1200°C have shown them to fail after much shorter times. The major degradation of the foils occurs adjacent to the brazed region and limits the lifetime of the honeycomb seal.

Cross-sectional analysis in a scanning electron microscope of seals manufactured from MI2100 foils, after oxidation testing at 1200°C, has shown that voids form beneath the protecting α-alumina scale. In some cases, these voids are filled with silica, with some chromia present, and may be the origin of the subsequent degradation process. The results contrast with tests on free-standing thin foils (100 μm thickness) of MI2100 and other FeCrAlY alloys, where a continuous layer of chromia is formed below the alumina outer scale, once the aluminium content of the alloy drops below a critical composition. Although there is a small amount of silicon in MI2100, the main source of the high level of silicon found in the honeycombs is most likely to be the brazing alloy, since both nickel and silicon from the braze are very mobile in FeCrAlY alloys at high temperatures.

The formation and filling of voids with silica may be associated with the subsequent failure of the protective alumina scales on these brazed alloys, and this mechanism will be developed further in this paper.     

Failure of Alloy 625 tube stub ends – Effect of primary nitrides

This paper reports a failure analysis of Alloy 625 stub ends of ammonia cracker tubes that failed during operation after 47,000 h of service operation. The failure occurred due to the formation of M₂₃C₆ carbides at grain boundaries, which made them very weak and brittle. However, the formation of M₂₃C₆ carbides at grain boundaries was surprising since they formed at temperatures around 550 °C, which is much below their expected temperature of formation and occurred in a period less than half the designed life. Precipitation of the grain boundary carbides has been attributed to the presence of primary nitride particles in the present alloy instead of primary carbides which are usually observed in these alloys. Formation of nitrides consumed Ti that binds C in the form of primary MC carbides in these alloys. This left free carbon in the alloy matrix for easy formation of M₂₃C₆ carbides which otherwise form due to degeneration of primary MC carbides.     

Characterization of chromia scales formed in supercritical carbon dioxide

Abstract

Initial experimental work at 700°–800 °C is in progress to develop a lifetime model for supercritical CO₂ (sCO₂) compatibility for a 30-year lifetime of a >700 °C concentrated solar power system. Nickel-based alloys 282, 740H and 625 and Fe-based alloy 25 are being evaluated in 500-h cycles at 1 and 300 bar, and 10-h cycles in 1 bar industrial grade CO₂. The alloys showed similar low rates of oxidation in 1 and 300 bar CO₂ in 500-h cycles at 750 °C. However, in 10-h cycles, alloy 25 showed accelerated attack at 700° and 750 °C. Transmission electron microscopy scale cross-sections on alloy 25 after 1000 h at 700 °C in sCO₂ and in air only showed a small row of carbides beneath the scale in the former environment. Similar characterisation was performed on alloys 625 and 282 after sCO₂ exposure at 750 °C.     

Microstructure evolution in bulk and surface states of chromium rich nickel based cast alloys reinforced by hafnium carbides after exposure to high temperature air

Abstract

Three alloys based on nickel, with a high level of chromium (25 wt-%) and containing varied quantities of carbon, 0·25 and 0·50 wt-%, and hafnium, 3·7 and 5·6 wt-%, fabricated by casting, were subjected to a 46 h long exposure at 1200°C in dry industrial air. The aim of the work was to investigate the thermal stability of their carbide interdendritic network and to discover their general behaviour in high temperature oxidation. The volume fraction of the hafnium carbides decreased slightly during high temperature exposure but their fragmentation was rather limited. In contrast, chromium carbides appeared in the two alloys, which initially contained exclusively HfC, and this may result in a decrease in their high temperature properties. The evolution of the carbides appeared to be responsible for a moderate lowering of room temperature hardness. The behaviour of the three alloys during high temperature oxidation was very good, despite the unusually high content of hafnium. All were chromia-forming, although oxidation of Hf led to HfO₂ islands in the external scale and in the subsurface region. In summary, the behaviour of these three alloys showed that the HfC containing Ni–25Cr family is potentially interesting for applications at very high temperatures.     

Bimodal grain size distribution: an effective approach for improving the mechanical and corrosion properties of Fe–Cr–Ni alloys

The effect of nanocrystalline grain size and bimodal distribution of nano- and microcrystalline grain sizes on the oxidation resistance and mechanical properties of Fe-based alloys has been investigated. Nanocrystalline and bimodal Fe–10Cr–5Ni–2Zr alloy pellets, prepared by mechanical alloying route, have been compared with conventional microcrystalline stainless steel alloys having 10 and 20 wt% Cr. Zr addition has been shown to improve the grain size stability at high temperatures. A significant improvement in the ductility of bimodal alloys with respect to nanocrystalline alloys was seen presumably due to the presence of the microcrystalline grains in the matrix. The high temperature oxidation of nanocrystalline and bimodal alloys at 550 °C shows superior oxidation resistance over microcrystalline alloy of similar composition (Fe–10Cr–5Ni) and comparable to that of microcrystalline alloy having twice as much Cr (Fe–20Cr–5Ni). Secondary Ion Mass Spectroscopy depth profiling confirms the hypothesis that nanostructure facilitates the enrichment of Cr at the oxide metal interface resulting in the formation of a passive oxide layer.     

Effect of overheating temperature on the microstructure and creep behavior of HP40Nb alloy

HP40Nb alloy has been widely used as a high temperature material in petrochemical plants. However, overheating or local temperature excursion occurs occasionally in service and leads to serious damage on the material. Effect of temperature on the microstructure and creep performance of the HP40Nb alloy is investigated in the present work. Several specimens are cut from serviced components of the alloy and heat-treated at different temperatures from 900 °C to 1250 °C for its possible working conditions, in which the temperature of 1200 or 1250 °C is used to simulate the overheating condition of HP40Nb tubes. The microstructure of specimens is examined by scanning electron microscope (SEM) and transmission electron microscope (TEM). The creep behavior is evaluated through using impression creep tests with a flat-ended cylindrical indenter. The content of inter- and intra-dendritic carbides in the specimens is represented by the surface fraction of each phase, which has been estimated by image processing method. The results show that the total of the surface fraction of inter- and intra-dendritic carbides in the HP40Nb alloy does not significantly change at the temperature lower than 1100 °C. However, the surface fraction of inter-dendritic carbides reaches the maximum at 1100 °C. A maximal steady state impression rate is also observed at 1100 °C. The results suggest that the content of inter-dendritic carbides is the main influencing factor on the creep performance of HP40Nb alloys comparing with that of the intra-dendritic carbides.     

Effects of carbon and chromium on the solidification structure and properties of ferrite-austenite duplex heat-resistant alloy

In order to design a new kind of low-cost high-temperature ferrite-austenite duplex alloy, the effects of carbon and chromium on the alloy solidification structure and properties have been investigated with orthogonal experiments. The addition of carbon promotes strongly the formation of austenite and that of carbides in the alloy solidification structure and refines the alloy grains. With the increase of carbon content, the alloy high temperature strength and oxidation resistance at 1250°C improves at first, but then begins to deteriorate greatly when the carbon content exceeds 0.15%. The addition of chromium facilitates the formation of ferrite in the alloy solidification structure. As the chromium content increases, the alloy rupture strength at 1250°C initially is enhanced, but then reduces rapidly, while the alloy oxidation resistance improves continuously.     

Effects of carbon and chromium on the solidification structure and properties of ferrite–austenite duplex heat-resistant alloy

In order to design a new kind of low-cost high-temperature ferrite–austenite duplex alloy, the effects of carbon and chromium on the alloy solidification structure and properties have been investigated with orthogonal experiments. The addition of carbon promotes strongly the formation of austenite and that of carbides in the alloy solidification structure and refines the alloy grains. With the increase of carbon content, the alloy high temperature strength and oxidation resistance at 1250°C improves at first, but then begins to deteriorate greatly when the carbon content exceeds 0.15%. The addition of chromium facilitates the formation of ferrite in the alloy solidification structure. As the chromium content increases, the alloy rupture strength at 1250°C initially is enhanced, but then reduces rapidly, while the alloy oxidation resistance improves continuously.     

Mathematical Model for high temperature properties of cast nickel superalloys with high content of gamma prime phase

This investigation relates to structural cast superalloys with improved oxidation resistance in the temperature range of 1100–1200°C. It was tried to clarify the role of heavy metal elements on mechanical properties of an alloy in the system Ni-17at% Al-3at% Cr-Mo-Ta-W. Solidus temperature, yield strength at 20°C, ultimate tensile strength at 1200°C of polycrystal alloys and creep-rupture life at 1000°C and 1200°C of single crystals <001> were investigated by means of experimental design and regression analysis techniques, It was determined that solidus temperature do not depend from W content, yield strength at 20°C and ultimate tensile strength at 1200°C strongly depend on Ta and W contents. The properties sharply increase with simultaneous adding of Ta+W. At 1200°C Mo content is a critical factor. The influence of alloying with Ta and W on the creep resistance of single crystals was examined. With increase of test temperature dependence of creep resistance from refractory additives appears to increase, but interval of an optimum alloying narrows.     

FEG-SEM examination of alumina scales formed on FeCrAl alloys

Abstract

Field emission gun scanning electron microscopy (FEGSEM) with electron backscattered diffraction (EBSD) has been used to investigate the microstructure and the oxidation behaviour of ultra-high purity Fe–20Cr–5Al model alloys and a commercial Fe–20Cr–5Al alloy. The model alloys contain controlled additions of phosphorus and carbon impurities and increased levels of more beneficial elements including yttrium, hafnium and titanium. The samples studied were oxidised at 800°C and 1200°C in humidified air for up to 3100 h, and 900°C and 1000°C for 1 h in laboratory air. At the higher temperature, well-adhered, compact and highly protective α-alumina scales formed, whereas at the lower temperature the scales formed were a less protective type of metastable alumina.

Preliminary examination showed that the texture of the formed alumina scale was unaffected by the texture of the underlying substrate and the substrate compositions. At the higher temperature, the study revealed that the alumina scale comprised two distinct regions; the outer region at the scale/gas interface contained small, equiaxed (0.5–1 micron) grains and the inner region at the scale/metal interface contained, columnar grains that are 2–3 times larger than the equiaxed ones. However, at the lower temperature these two distinct regions were not apparent. Instead, grains of predominantly metastable alumina were observed. The links between texture morphology and oxide growth mechanisms will be discussed in this paper.     

Microstructure and oxidation behaviour at 1100°C of HfC containing Cr rich iron based cast alloys

Three alloys containing 0·25–0·50 wt-%C, 26–28 wt-%Cr and 4–6 wt-%Hf were elaborated by foundry. They contained a dendritic matrix and HfC carbides, which are expected to strengthen the alloy at high temperatures. They were exposed in air at 1100°C during 46 h. The aged microstructures displayed coarsened chromium carbides but no significant changes to the fraction or morphology of the HfC carbides. The surface characterisation of the oxidised samples showed that the alloys behaved well despite some localised instances of fast oxidation. The hardness was modified by the microstructure stabilisation achieved during the high temperature exposure. A preliminary test showed that the reinforcement by HfC may indeed lead to interesting creep resistance at 1100°C as had been hoped.     

Microstructure,precipitates and mechanical properties of powder bed fused inconel 718 before and after heat treatment

IN718 alloy was fabricated by laser powder bed fusion (PBF) for examination of microstructure, precipitates and mechanical properties in the as-built state and after different heat treatments. The as-built alloy had a characteristic fine cellular-dendritic microstructure with Nb, Mo and Ti segregated along the interdendritic region and cellular boundary. The as-built alloys were then subjected to solution heat treatment (SHT) at 980 °C or 1065 °C for 1 h. SHT at 980 °C led to the formation of δ-phase in the interdendritic region or cellular boundary. The segregation was completely removed by the SHT at 1065 °C, but recrystallization was observed, and the carbides decorated along the grain boundaries. The as-built alloy and alloys with SHT at 980 °C and 1065 °C were two-step aged, which consisted of annealing at 720 °C for 8 h followed by annealing at 620 °C for 8 h. Transmission electron microscopy revealed the precipitation of γ' and γ” in all alloys after two-step aging, but the amount and uniformity of distribution varied. The Vickers hardness of the PBF IN718 alloy increased from 296 HV to 467 HV after direct aging. The hardness decreased to 267 HV and 235 HV after SHT at 980 °C and 1065 °C, respectively, but increased to 458 HV and 477 HV followed by aging. The evolution of Young’s modulus after heat treatment exhibited similar trend to that of hardness. The highest hardness was observed for IN718 after SHT at 1065 °C and two-step aging due to precipitation with greater amount and uniform distribution.     

Transient oxidation of alumina forming Ti–Al–Ag-based alloys and coatings studied by SEM,AFM, XPS and LRS

Abstract

γ-TiAl based intermetallics possess poor oxidation properties at temperatures above approximately 700°C. Previous studies showed that protective alumina scale formation on γ-TiAl can be obtained by small additions (around 2 at.%) of Ag. Recently, this type of materials has therefore been proposed as oxidation resistant coatings for high strength TiAl alloys. In the present study, a number of cast Ti–Al–Ag alloys and magnetron sputtered Ti–Al–Ag coatings were investigated in relation to transient oxide formation in air at 800°C. After various oxidation times the oxide composition, microstructure and morphology were studied by combining a number of analysis techniques, such as SEM, ESCA, AFM and LIOS-RS. The γ-TiAl–Ag alloys and coatings appear to form an α-Al₂O₃ oxide scale from the beginning of the oxidation process, in spite of the relatively low oxidation temperature of 800°C. The formation of metastable alumina oxides seems to be related to the presence of Ag-rich precipitates in the alloy matrix.     

Microstructure evolution and stress rupture properties of K4750 alloys with various B contents during long-term aging

The relationship among B content,microstructure evolution and stress rupture properties of K4750 alloy during long-term aging were investigated.After aging at 800 ℃ for 1000 h,the decomposition degree of MC carbides of K4750 alloys with 0B,0.007 wt.％ B and 0.010 wt.％ B were basically identical,which indicated that B has no inhibition on MC carbide decomposition during long-term aging.The MC carbide decomposition was accompanied by the formation of M23C6 carbides and a small number of η phases,which was controlled by the outward diffusion of C and Ti combined with the inward diffusion of Ni and Cr from the γ matrix.In addition,M23C6 carbides in boron-free alloy were in continuous chain and needle-like η phases were precipitated near them,while M23C6 carbides in boron-containing alloys remained in granular distribution and no η phases precipitation around them.Adding B could delay the agglomeration and coarsening of M23C6 carbides during long-term aging,which was because the segregation of B at grain boundary retarded the diffusion of alloy elements,thus weakened the local fluctuation of chemical composition near grain boundary.The stress rupture samples of K4750 alloys with various B contents after aging at 800 ℃ for 1000 h were tested at 750 ℃/380 MPa.The results indicated that the stress rupture properties of boron-containing alloys were significantly better than that of boron-free alloy,which could be attributed to the increase of grain boundary cohesion strength and the optimization of M23C6 carbide distribution due to the addition of B.     

Segregation of reactive elements at oxide grain boundaries in FeCrAlRE alloys

Abstract

Extensive previous research has established that the oxidation of FeCrAl alloys at temperatures ≥1000°C results in the formation of α-Al₂O₃ oxide scales, and that minor alloy constituents (particularly Reactive Elements (RE) such as Y, Hf, Zr, etc.) can change the oxide growth mechanism. A knowledge of the segregation behaviour of these REs is thus central to our understanding of the oxidation behaviour of these, technologically important, range of alloys.

The new SuperSTEM microscope at the Daresbury Laboratory offers considerable potential for a detailed study of the segregation to oxide grain boundaries at the atomic level. The microscope has an aberration corrector fitted to the objective lens, allowing the formation of sub-Angstrom probe for simultaneous ultra-high resolution high angle annular dark field (HAADF) imaging and atomic-column electron energy loss spectroscopy (EELS). This paper reports on an initial study of oxide grain boundary segregation in commercial and model FeCrAlRE alloys containing controlled additions of the reactive elements, yttrium, zirconium and hafnium oxidised at 1250°C, in air, for 50 hours. Both yttrium and hafnium are shown to segregate to the grain boundaries while hafnium rich particules form in the outer region of the scale.     

As-cast microstructures and behavior at high temperature of chromium-rich cobalt-based alloys containing hafnium carbides

Hafnium is often used to improve the high temperature oxidation resistance of superalloys but not to form carbides for strengthen them against creep. In this work hafnium was added in cobalt-based alloys for verifying that HfC can be obtained in cobalt-based alloys and for characterizing their behavior at a very temperature. Three Co–25Cr–0.25 and 0.50C alloys containing 3.7 and 7.4 Hf to promote HfC carbides, and four Co–25Cr– 0 to 1C alloys for comparison (all contents in wt.%), were cast and exposed at 1200 °C for 50 h in synthetic air. The HfC carbides formed instead chromium carbides during solidification, in eutectic with matrix and as dispersed compact particles. During the stage at 1200 °C the HfC carbides did not significantly evolve, even near the oxidation front despite oxidation early become very fast and generalized. At the same time the chromium carbides present in the Co–Cr–C alloys totally disappeared in the same conditions. Such HfC-alloys potentially bring efficient and sustainable mechanical strengthening at high temperature, but their hot oxidation resistance must be significantly improved.     

Rejuvenation of the microstructure and mechanical properties of a service-exposed IN939 superalloy by heat treatments

Effect of heat treatment on the recovery of microstructure and mechanical properties of a service-exposed IN939 superalloy was studied. Four-stage heat treatment was performed on the service-exposed alloy. The microstructures of the service-exposed and rejuvenated alloys were examined by optical and scanning electron microscopes. The hardness, tension, and stress-rupture tests were carried out to characterise the mechanical properties. The results showed that the heat treatment could rejuvenate the microstructure of the alloy that was deteriorated during the service at high temperatures. Decomposed MC carbides were transformed to new fine carbides, continuous M23C6 carbides were dissolved and new discontinuous carbides along the grain boundaries were regenerated and, finally, the fine γ? particles were reformed. The microstructural recovery resulted in an increase in the hardness and ultimate tensile strength of the alloy. The results also showed that the creep behavior of the alloy at a testing temperature of 850°C could be improved by the heat treatment.     

Microstructure and oxidation behaviors of near-α Ti-6.5Al-4Sn-4Zr-0.5Mo-based alloys with Ir addition

The microstructure and oxidation behaviors of near α-Ti-based alloys with small amount of iridium (Ir) additions were investigated. The microstructure of both Ir-free and Ir-containing alloys was observed to consist of α + β Widmanstätten colonies. The β lamellae gradually became continuous with increasing Ir additions since Ir acted as a β-stabilizer in the alloys. Isothermal oxidation test indicated that Ir addition reduced the oxidation resistance at 650 °C; while at 750 °C, the adherence of thermally grown oxides was enhanced, and a thin Al₂O₃-enriched layer on the oxide scale was promoted in the Ir-containing alloy, which suggests that Ir addition was effective in improving oxidation resistance of near-α-based alloys at 750 °C.     

Oxidation behaviour and microstructural stability of alloy 625 during long-term exposure in steam

Nickel-based alloys are being considered as construction materials for various components in high-efficiency steam turbines with envisaged operating temperatures around 700 °C. In the present study, the steam oxidation behaviour of the nickel-based alloy 625 in the temperature range of 700–800 °C was investigated whereby exposures up to 10000 h were carried out. Gravimetric data in combination with results from a variety of post exposure analysis techniques showed in all cases the formation of protective oxide scales mainly consisting of chromia with minor amounts of outer Cr/Mn spinel and internal silica. The phases found in the bulk alloy after long-term exposure were mainly needle-shaped δ-Ni₃(Nb,Mo) phase, μ-phase and Si-rich η-M₆C carbide. Microstructural features and phase formation were found to be related to minor variations in the alloy composition, especially iron and silicon content. The oxidation-induced chromium depletion caused a number of microstructural changes in the subsurface depletion layer. Most important was an enrichment of the intermetallic δ-phase at the scale–alloy interface. DICTRA modelling revealed this effect to be caused by uphill diffusion as a result of a negative niobium activity gradient in the subscale chromium depletion zone. Although the available kinetic and thermodynamic data allowed qualitative explanation of the δ-phase enrichment, the databases do not correctly describe the high molybdenum solubility in the δ-phase.