Development of high strength ferritic steel for interconnect application in SOFCs

High-Cr ferritic model steels containing various additions of the refractory elements Nb and/or W were studied with respect to oxidation behaviour (hot) tensile properties, creep behaviour and high-temperature electrical conductivity of the surface oxide scales. Whereas W additions of around 2 wt.% had hardly any effect on the oxidation rates at 800 and 900 °C, Nb additions of 1% led to a substantially enhanced growth rate of the protective surface oxide scale. It was found that this adverse effect can be alleviated by suitable Si additions. This is related to the incorporation of Si and Nb into Laves phase precipitates which also contribute to increased creep and hot tensile strength. The dispersion of Laves phase precipitates was greatly refined by combined additions of Nb and W. The high-temperature electrical conductivity of the surface oxide scales was similar to that of the Nb/W-free alloys. Thus the combined additions of Nb, W and Si resulted in an alloy with oxidation resistance, ASR contribution and thermal expansion comparable to the commercial alloy Crofer 22 APU, but with creep strength far greater than that of Crofer 22 APU.     

Development of Ni based single crystal superalloys for power generation gas turbine blades

Abstract

In advanced industrial gas turbine systems, there has been a great demand for new single crystal (SC) superalloys with an excellent combination of high temperature creep strength, hot corrosion resistance and oxidation resistance. In the present study, twelve nickel based SC superalloys were designed with the aid of the d-electrons concept. Their chemical compositions were in the range 1·2–1·5%Ti, 3·8–6·5%Cr, 11%Co, 0–1·4%Mo, 0–3·0%Ru, 6·5–7·4%Ta, 5·0–6·4%W, 3·6–5·4%Re, 5·1–5·5%Al, 0·12–0·14%Hf and balanced Ni in wt-%. A series of experiments such as creep rupture tests, burner rig tests and cyclic oxidation tests were conducted with the heat treated SC specimens of these alloys. Almost all the designed alloys were found to be superior in creep rupture life to a second generation superalloy currently used in the world. In the hot corrosion resistance estimated from the burner rig tests, any designed alloys were similar or even superior to a second generation superalloy. The oxidation resistance was very different among the designed alloys, but some of them showed better resistance than the second generation superalloy. Thus the SC alloys containing about 4–5 wt-%Re had ～30°C or more higher temperature capability than the second generation superalloy, while exhibiting excellent hot corrosion resistance and good oxidation resistance.     

Development of EQ coating for new TBC system in Ni based superalloys

Abstract

Ni based single crystal superalloys containing high concentrations of refractory elements are prone to generation of a diffusion layer called secondary reaction zone (SRZ) beneath their bond coating during exposure at high temperatures. The SRZ causes a reduction of the load bearing cross-section and is detrimental to the creep properties of thin wall turbine airfoils. In the present study, a new bond coat system, 'EQ coating' which is stable and suppresses formation of the SRZ is proposed. The characteristic of EQ system is that the coating stays in equilibrium state and never reacts with the substrate. Diffusion couples of coating materials and substrate alloys were made and were heat treated at 1100°C for 300 and 1000 h. The concentration profiles of alloying elements in these diffusion couples were analysed by electron probe microanalyser to investigate the existence of the diffusion zone. Cyclic oxidation examinations were carried out at 1100°C in air and the oxidation properties of EQ coating materials were discussed.     

高温合金叶片修复技术的应用与发展

高温合金因具有良好的耐腐蚀性、抗氧化性、蠕变强度等性能,在航空及电力设备制造行业中有着重要的作用．常用于制造燃气轮机叶片等复杂构件。目前燃气轮机叶片多采用铸造成型,铸造过程中不可避免的会产生缺陷．且叶片在服役过程中因为高温高压蒸汽的腐蚀和冲刷．也会产生损伤。文章综述了镍基合金叶片修复技术的研究进展,重点介绍了国内外一些先进的叶片修复方法,深入分析了不同修复方法的特点,并对燃气轮机叶片修复技术的发展前景进行了展望。     

A review of degradation in single crystal nickel based superalloys

Abstract

Single crystal nickel based superalloys are in use for the front rows of blading in advanced land based gas turbines. This is for their higher creep capabilities as well as outstanding low cycle fatigue properties as compared with conventionally cast nickel based superalloys. The continuous efforts to improve the high temperature creep properties of single crystal (SX) superalloys resulted in the development of the fourth generation of these alloys. Nevertheless, this improvement in the high temperature capability of these materials required the use of higher contents of Re and Ru which increased their densities and costs. The application of SX superalloys at higher temperatures results in microstructural changes called rafting and a topological inversion of their γ/γ′ phases. These two microstructural features reduce the tensile and low cycle fatigue properties of the SX superalloys. The extent of degradation in the microstructure and mechanical properties, in a certain SX superalloy, depends on the exposure temperature, duration and to a lesser extent on the applied stress. The consequences of these changes in microstructure and properties should be taken into consideration in the evaluation of the mechanical properties of SX superalloys.     

CF8C plus: a new high temperature austenitic casting alloy for advanced power systems

Abstract

A new cast austenitic stainless steel, CF8C plus, has been developed by Oak Ridge National Laboratory and Caterpillar for a wide range of transportation and energy applications. CF8C plus steel has improved high temperature tensile, creep, fatigue, and creep–fatigue properties compared with standard CF8C steel. Changes to the CF8C steel composition, including additions of Mn and N, result in changes to the solidification behaviour and final microstructure of the alloy, which directly relate to the improved mechanical properties. Additionally, CF8C plus is a relatively inexpensive steel which exhibits good castability. The mechanical properties of the alloy have generated significant interest for the production/design of cast components for diesel engine turbochargers and other exhaust components, natural gas reciprocating engines for distributed power, and turbine end covers and casings for land based turbines. In the present paper, the microstructural evolution of CF8C and CF8C plus are presented in more detail, and the mechanical properties of the alloys are compared with each other and other engineering alloys.     

Methanol electrooxidation at Pt–Ru–W sputter deposited on Au substrate

This work reports an improved electrocatalytic activity for methanol oxidation at Pt–Ru–W electrode sputter deposited on Au substrate. The performance of Pt–Ru–W was compared with that of Pt–W and of Pt–Ru alloy electrodes. All the alloys tested exhibited catalytic activity higher than Pt. Among the alloys tested, the Pt–Ru–W demonstrated a significant cathodic shift in the onset potential and a remarkable enhancement in the current density for methanol oxidation reaction (MOR). The onset potentials for the MOR matched well the anodic peak potentials recoded in the base electrolyte (H₂SO₄), i.e., 0.15 V versus Ag/AgCl for Pt–Ru–W and 0.35 V versus Ag/AgCl for Pt–W and Pt–Ru electrodes. From these findings, it was postulated that the background peak current generates oxide species necessary to complete the methanol oxidation to CO₂. Next, it was observed that the current density at Pt–Ru–W electrode decreased when the Au substrate was changed to Pt, C, or Si, although, the onset potential for MOR remained almost unaffected by the nature of the substrate. Afterwards, the effect of Au substrate on methanol oxidation at Au-based alloy electrodes was investigated.     

High-temperature oxidation performance of a new alumina-forming Ni-Fe-Cr-Al alloy in flowing air

A new type of alumina-forming Ni-Fe-Cr-Al-based alloy (HAYNES^® HR-224™ alloy) has been recently developed for aggressive oxidizing environments. This paper evaluates cyclic oxidation resistance of newly developed HR-224 alloy and its comparative performance to selected alumina- and chromia-forming heat-resistant alloys in flowing air at 982 °C and 871 °C. The HR-224 alloy outperformed several chromia-forming alloys owing to its ability to form and maintain a continuous, slow-growing, and adherent alumina scale. In fact, results indicated that HR-224 alloy exhibits comparable oxidation resistance to one of the most oxidation resistant alumina-forming Ni-Cr-Al-based 214^® alloy. Moreover, in this paper, growth kinetics of alumina scale in these two alumina-forming alloys is discussed in detail. Among chromia-formers, Ni-Cr based alloys with low Fe contents exhibited excellent oxidation resistance.     

Formation and effect of topologically close-packed phases in nickel-base superalloys

The formation of topologically close-packed (TCP) phases in nickel-base superalloys is an issue of increasing importance as alloys are designed with higher refractory element contents to meet the requirements of next generation turbine engines. This review considers the factors that affect an alloy’s susceptibility to TCP formation. In particular, the debate surrounding the effect of certain individual elements, such as Co and Re, in promoting or suppressing TCP formation is examined alongside the various mechanisms that have been proposed to account for this behaviour. In addition, the detrimental effects of these phases on the alloy’s mechanical properties are discussed, including crack initiation at precipitates, depletion of solid solution strengthening refractory elements and the effect on γ/γ′ rafting behaviour.

This review was chosen as a runner up of the 2016 Materials Literature Review Prize of the Institute of Materials, Minerals and Mining, run by the Editorial Board of MST. Sponsorship of the prize by TWI Ltd is gratefully acknowledged.     

Microstructural evolution of cast TiAlNb intermetallics for third generation gas turbine components

Abstract

The microstructural evolution of Ti–45·9Al–8Nb and Ti–45·9Al–8Nb–0·2C (at-%) alloys fabricated by centrifugal investment casting is studied. It is found that both Ti–45·9Al–8Nb and Ti–45·9Al–8Nb–0·2C are mainly composed of γ phase and a small amount of α₂ phase, and the microstructure is mainly fully lamellar in C containing alloy but again the C free alloy is mainly nearly fully lamellar. Nb distributes homogeneously in the γ and α₂ phases but is depleted in the interdendritic regions. The addition of C reduces the lamellar spacing of Ti–45·9Al–8Nb alloy. The mean interlamellar spacings between α₂ and α₂ of Ti–45·9Al–8Nb and Ti–45·9Al–8Nb–0·2C alloys are about 950 and 700 nm, and those between γ and α₂ lamellae are about 320 and 220 nm respectively. Oil cooling from the α phase field after 1h treatment at 1360°C leads to massive transformation to γ_m for both Ti–45·9Al–8Nb and Ti–45·9Al–8Nb–0·2C alloys. Subsequent aging in the α+γ phase field results in α phase precipitates within the γ_m phase, which leads to convoluted microstructures. However, the C addition retards the precipitation of α₂ phase.     

Oxidation behaviour of cast Ni–Cr alloys in steam at 800°C

Abstract

To evaluate the steam oxidation resistance of cast Ni base alloy candidates for advanced steam turbine casings, laboratory experiments were conducted at 800°C. Alloys ranged from weaker, solid solution strengthened alloys 230 and 625 to stronger, precipitation strengthened alloys 105, 263 and 740, which are more difficult to fabricate and join. In general, these Ni–Cr based alloys exhibit low mass gains and form thin, protective Cr rich external oxides in 17 bar steam or 1 bar air. However, Al and Ti in these alloys internally oxidise in all cases. After 5000 h exposures, the average and maximum internal oxide penetration depths were measured, and the values were ranked based on the alloy Al+Ti contents. The middle range of Al+Ti compositions investigated, such as for alloys 617, 263, 282 and 740, showed the deepest penetrations. Further characterisation of the reaction products by electron microprobe showed a complex behaviour including significant Ti incorporation into the scale formed in both steam and air, and Ti rich oxide at both the gas and metal interfaces. Based on the Al and Ti contents, the internal oxidation observed in these alloys in steam was atypical.     

Insight into the effect of In addition on hydrogen generation behavior and hydrolysis mechanism of Al‐based ternary alloys in distilled water—A new active Al‐Ga‐In hydrolysis hydrogen generation alloy

(Al2Ga)‐xIn (x = 0, 2, 4, 6, 8 wt%) ternary aluminum (Al) alloys with different weight ratio of In for hydrolysis H₂ generation were prepared by melting‐casting technique. The phase compositions and microstructures of Al‐rich alloys were investigated by X‐ray diffraction (XRD) and high resolution scanning electron microscope (HR‐SEM) equipped with an energy dispersive spectrometer (EDS). The effect of In addition ratio on microstructures and H₂ generation performance were investigated, and the hydrolysis mechanism for Al‐Ga‐In ternary Al‐based alloys has been proposed. Al phase as matrix phase in the Al‐Ga‐In ternary alloy mainly determines the hydrolysis behavior, and the second phase In strongly promotes the hydrolysis process. The increase of In content can accelerate the H₂ generation rate as well as the final capacity and generation yield in neutral water. The generation yields for (Al2Ga)‐x In (x = 2, 4, 6, 8 wt%) alloys at 50°C are 0.56, 0.59, 0.62, and 0.66, respectively. The raising hydrolysis temperature can elevate the initial hydrolysis rate, final H₂ generation capacity, and yield. The H₂ generation capacities of (Al2Ga)‐8In alloy at 50°C, 60°C, and 70°C are 262, 290, and 779 mL·g^?1, respectively.     

Microstructural analysis of creep exposed IN617 alloy

Abstract

Ni base alloys such as IN617 are one of the preferred choices for steam turbine components used by fossil fuelled power generation plants. IN617 is a solid solution strengthened Ni based superalloy containing ～23%Cr, 12%Co and 9%Mo with low content of precipitation strengthening elements Al, Ti and Nb. In the 'as received' (solution annealed condition), the microstructure consists of primary carbides (M₂₃C₆) and occasional TiN particles dispersed in a single phase austenitic matrix. Owing to high temperature exposure and the creep deformation processes that occur in service, evolution of the microstructure occurs. This results in secondary precipitation and precipitate coarsening, both on grain boundaries and intragranularly in areas of high dislocation density. The influence of creep deformation on the solution treated IN617 alloy at an operating condition of 650 ～C/574 h, with emphasis on the morphology and distribution of carbide/nitride precipitation is discussed. The applied stress was at an intermediate level.     

High-temperature microstructural evolution and quantification for alloys IN740 and IN740H: comparative study

In ultra-supercritical power plants, Ni-base alloys are candidate materials for long-term, high-temperature applications, operating at temperatures and pressures as high as 750°C and 35?MPa. Alloy IN740 and its modification, alloy IN740H, are considered for such applications. Their microstructural evolution, at 750°C for times ranging between 3000 and 5000 hours, has been investigated by means of scanning electron microscopy, electron back-scattered diffraction, energy dispersive X-ray analysis and phase quantification. All phases were identified and quantified allowing comparison between the two microstructures, their evolution and stability. Particular attention was paid to γ′, η and G phases. The results are used within a broader investigation aimed at improving and further developing a predictive creep model based on continuous damage mechanics.     

Influence of strength differential effect on material effort of a turbine guide vane based on thermoelastoplastic analysis

The strength differential (SD) effect has been observed in many iron-based metals such as 4310, 4330, maraging steel, and HY80 steels as well as titanium, aluminium 2024-T351, magnesium, and nickel-based super alloys such as aged Inconel 718. Moreover, the SD effect increases with temperature. The Huber–Mises–Hencky (HMH) J₂ yield condition is insu?cient to simulate the response of metals that exhibit the SD effect. Our work demonstrates the importance of taking into account the SD effect during strength analysis of turbine components. Two yield conditions are considered: the HMH condition and the SD-dependent Burzynski condition. The equivalent stresses produced by these conditions in the elastic state are compared. Plastic zone areas and effective strain values predicted by the two conditions are compared. Our investigation was performed based on thermal-fluid-structure interaction (FSI) analysis of a turbine guide vane made of a nickel-based super alloy that exhibits the SD effect. Conjugate heat transfer analysis was performed, and then elastoplastic stress analysis was performed with boundary conditions obtained from the computational fluid dynamics (CFD) analysis. The paraboloid Burzynski yield condition was implemented in an FE code. Implementation was based on the Euler backward method with consistency tangent moduli evaluated in the explicit form.     

Preparation of creep-resistant Ni–5 wt.% Al anodes for molten carbonate fuel cells

Ni–5 wt.% Al anodes for molten carbonate fuel cells (MCFCs) are fabricated using relatively cheap elemental powders instead of expensive alloy powders. The tape-cast green sheets are sintered in various atmospheres: reduction, full oxidation–reduction, and partial oxidation–reduction atmospheres. The anode sintered in a reduction atmosphere shows a morphology of a network structure of an NiAl solid solution with its surface covered with thin Al₂O₃ films, and has relatively low creep resistance. On the other hand, the anode sintered in a full oxidation–reduction atmosphere or the one sintered in a partial oxidation–reduction atmosphere has a morphology of small Al₂O₃ particles dispersed in a network structure. In the former, however, a large number of micropores are created during sintering. The latter does not have the micropore problem and generally exhibits high creep resistance. The highest creep resistance is shown by the anode sintered in a partial oxidation–reduction atmosphere with an oxidation time of 2.5 h.     

Microstructural evolutions of IN738LC during multiple reheat treatment and aging

Abstract

IN738LC is a polycrystalline superalloy which is still widely used for gas turbine blading in many industrial applications. It has been the centre of many research programmes during the last four decades in areas such as alloy design, processing, various degradation regimes and even after heat treatment during repair. The subject of this study has been the microstructural evolution during multiple reheat treatments combined with long term aging. Material has been exposed at two different aging temperatures (875 and 950°C) for a period of 4000 h. This has been followed by a reheat treatment and then further aging, up to a total of 16?000 h. The microstructural evolution of the alloy, γ′ coarsening and carbide transformations, have been investigated in detail after the heat treatments and further aging periods. The results highlighted that although the γ′ structure has largely been recovered by the heat treatments, that of the carbides did not follow a similar trend. This partial recovery of the alloy microstructure and its potential impact on the long term integrity of gas turbine blading which has been heat treated periodically will be discussed in this paper.     

Development of low thermal expansion nickel base superalloy for steam turbine applications

Abstract

The target operating temperature of ultrasupercritical power plants is increasing and is planned to reach 700°C. Austenitic superalloys are promising materials for these applications to replace ferritic heat resistant steels, because of their high strength at 650–700°C. In general, austenitic nickel base superalloys show higher creep rupture strength than ferritic heat resistant steels; however, they have higher coefficients of thermal expansion, lower creep rupture ductilities, and higher costs. The effect of the Mo and Co content, amount of γ' phase, and Al/Ti ratio in the γ' phase on the thermal expansion behaviour of a Mo containing superalloy has been investigated by use of the conventional Mo containing Alloy 252 as a reference. Tensile and creep rupture properties were also measured. Following a modified heat treatment, the Co free superalloy developed on the basis of these tests showed higher creep rupture ductility than Alloy 252, while retaining comparable low thermal expansion and high creep rupture strength. Creep rupture properties at 700°C for up to 20 000 h were satisfactory, suggesting that the alloy is suitable for long term applications. Initial assessments of the weldability and mechanical properties of weld joints at 750°C are encouraging for boiler tube applications.     

Effect of Mg3MnNi2 on the electrochemical characteristics of Mg2Ni electrode alloy

Mg₂Ni–x mol% Mg₃MnNi₂ (x = 0, 15, 30, 60, 100), the novel composite alloys employed for hydrogen storage electrode, have been successfully synthesized by a method combining electric resistance melting with isothermal evaporation casting process (IECP). X-ray diffraction (XRD) analysis results show that the composite alloys are composed of Mg₂Ni phases and the new Mg₃MnNi₂ phases. It is found on the electrochemical studies that maximum discharge capacities of the composite alloys increase with the increasing content of the Mg₃MnNi₂ phase. The discharge capacity of the electrode alloy is effectively improved from 17 mAh g⁻¹ of the Mg₂Ni alloy to 166 mAh g⁻¹ of the Mg₃MnNi₂ alloy. Among these alloys, the Mg₃MnNi₂ phase possesses a positive effect on the retardation of cycling capacity degradation rate of the electrode materials. Cyclic voltammetry (CV) results confirm that the increasing content of the Mg₃MnNi₂ phase effectively improves the reaction activity of the electrode alloys. Surface analyses indicate that the Mg₃MnNi₂ phase can enhance the anti-corrosive performance of the particle surface of these composite alloys.     

The roles of cellular and dendritic microstructural morphologies on the corrosion resistance of Pb–Sb alloys for lead acid battery grids

During the past 20 years, lead acid batteries manufacturers have modified grid manufacturing processes and the chemical composition of the used alloys in order to decrease battery grid weight as well as to reduce the production costs, and to increase the battery life-time cycle and the corrosion resistance. The aim of this study was to evaluate the effects of cellular and dendritic microstructures of two different Pb–Sb alloys on the resultant corrosion behavior. A water-cooled unidirectional solidification system was used to obtain cellular and dendritic structures. Macrostructural and microstructural aspects along the casting have been characterized by optical microscopy and SEM techniques. Electrochemical impedance spectroscopy and potentiodynamic polarization curves were used to analyze the corrosion resistance of samples in a 0.5 M H₂SO₄ solution at 25 °C. For cellular microstructures the corrosion rate decreases with increasing cell spacing. In contrast, finer dendritic spacings exhibit better corrosion resistance than coarser ones. The microstructural pre-programming may be used as an alternative way to produce Pb alloy components in conventional casting, rolled-expanded, and continuous drum casting with better corrosion resistance.