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An Inconel 740H nickel-based alloy was fabricated via wire arc additive manufacturing. The as-welded and heat-treated samples were analyzed to investigate their phase composition, microstructure, crystal structure, and mechanical properties. After heat treatment, the sample exhibited a columnar crystal zone microstructure consisting of a γ matrix + precipitated phase, the remelting zone metallographic structure was a γ matrix + precipitated phase, and the HAZ metallographic structure was a γ matrix + precipitated phase. Transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) were used to show that the welded sample exhibited many dislocations, a few inclusions, and carbides, nitrides, and γ' precipitates in its crystal structure. In contrast, the crystal structure of the heat-treated sample exhibited a lower number of dislocations and significantly higher carbide and γ' precipitate content. Moreover, the mechanical performance of these samples was excellent. This heat-treatment process improved the sample strength by about 200 MPa, leading to better high-temperature mechanical properties. This work is anticipated to offer theoretical and experimental support for using additive manufacturing methods in the manufacturing of nickel-based superalloy components. [ABSTRACT FROM AUTHOR]

An Inconel 740H nickel-based alloy was fabricated via wire arc additive manufacturing. The as-welded and heat-treated samples were analyzed to investigate their phase composition, microstructure, crystal structure, and mechanical properties. After heat treatment, the sample exhibited a columnar crystal zone microstructure consisting of a γ matrix + precipitated phase, the remelting zone metallographic structure was a γ matrix + precipitated phase, and the HAZ metallographic structure was a γ matrix + precipitated phase. Transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) were used to show that the welded sample exhibited many dislocations, a few inclusions, and carbides, nitrides, and γ’ precipitates in its crystal structure. In contrast, the crystal structure of the heat-treated sample exhibited a lower number of dislocations and significantly higher carbide and γ’ precipitate content. Moreover, the mechanical performance of these samples was excellent. This heat-treatment process improved the sample strength by about 200 MPa, leading to better high-temperature mechanical properties. This work is anticipated to offer theoretical and experimental support for using additive manufacturing methods in the manufacturing of nickel-based superalloy components.

Advanced ultra-supercritical power plants, in which steam temperatures exceed 700 °C, demand the development of protective coatings, particularly for valves governing the steam flow where sealing surfaces are subjected to high static tribological solicitations. In this context, we systematically investigated the friction (static and dynamic) and wear of two cobalt-based coatings and one nickel-based substrate. Results indicate that oxidation kinetics exhibits a double-edged sword effect in terms of tribological properties at elevated temperatures. Fast oxidation kinetics promotes the sintering of oxides, generating a smoother oxide tribofilm and lowering the dynamic coefficient of friction (COF). However, this can also significantly increase the static COF over loading time due to welding/sintering of oxidized asperities, leading to increased torque requirements for valve operation. [ABSTRACT FROM AUTHOR]

Fireside corrosion behavior of Inconel 740H superalloy was studied at 750 °C in simulated coal ash/flue gas environments by means of XRD, SEM and EDS. The results indicated that the corrosion behavior was strongly related to the SO2 levels and was significantly affected by NaCl additions. In presence of the atmospheres with 0.1 % SO2, the alloy exhibited the highest corrosion resistance due to formation of a stable and dense Cr2O3 film. In presence of the atmosphere with 1.5 % SO2, however, a non-coherent and porous Cr2O3 film was formed. The thickness of film and internal sulfides were substantially increased. The NaCl additions significantly accelerated the corrosion process. A non-protective outer oxide film was formed, composed by multiple layers with serious inner sulfide and spallation. The depths of internal oxidizing and sulfuration zones were significantly increased. The mechanism of ash corrosion formation was also discussed.

This paper presents the results of investigations on microstructure changes after the long-term impact of temperature. The microstructure investigations were carried out by light microscopy and scanning electron microscopy. The qualitative and quantitative identification of the existing precipitates was carried out using X-ray phase composition analysis. The effect of elevated temperature on precipitation processes of test material were described. The obtained results of investigations form part of the material characteristics of new-generation alloys, which can be indirectly associated with the stability of functional properties under the simultaneous effect of high temperature and stress.

Unstressed exposure tests of Inconel 740H alloy tube were carried out at 750℃ for 500-3000h. The microstructure evolution and microhardness were studied by means of thermodynamic simulation, OM, FEG-SEM and microhardness testing. The results show that the tube is qualified if both chemical composition and tensile properties of the as-received alloy meet the corresponding requirements of ASME. After long term exposure, the main precipitates are γ' and M23C6, and no η and σ phase. With the prolonging of exposure time, the coarsening of γ' becomes faster and the law of relationship between the radius of γ' and time accords with LSW Ostwald ripening law; meanwhile, the change in size of M23C6 is not so obvious. During the whole process, microhardness increases firstly and then decreases, but the fluctuation is slight. The changes of microstructure and hardness indicate that, after long time exposure, the domestic Inconel 740H has good stability and can be used for further carrying out the investigation on the mechanical property of creep-rupture.

Inconel 740H alloy is a candidate material for 700°C advanced ultra-supercritical (A-USC) coal-fired power plants application as superheater/reheater tube. In this work, oxidation behavior of Inconel 740H alloy was studied in static air at 750°C and 850°C, and in dynamic pure steam at 750°C, respectively. The alloy was oxidized approximately following a parabolic law in three test environment. In the static air, the oxidation rate at 850°C was about 50 times of that at the 750°C. More NiCrMn spinal and TiO2 were detected after oxidation at 850°C. Cr2O3, however, was the main oxidation product at 750°C. In the pure steam, Cr2O3 was still the main oxidation product. The oxidation rate was about 2.6 times of that in static air, but the surface roughness was much smaller and edges of oxide particles were more blurred. There was no evidence of cracks or spallation in three test environments.

Abstract The supercritical carbon dioxide Brayton cycle is of significant interest due to the potential for high efficiency and low capital cost compared to Rankine power cycles. Maximum temperature and pressure conditions produce higher efficiencies, leading to the desire for materials with high strength at elevated temperatures, such as INCONEL 740H. This alloy has been found to have good corrosion properties, but the mechanical and weld performance in carbon dioxide has not been studied. This study has attempted to address this issue by testing the mechanical properties in both base material and transverse gas tungsten arc welded samples before and after exposure to supercritical carbon dioxide at 750 °C, 20 MPa for 1000 h. After exposure to CO 2 , base material showed an increase in strength with a moderate drop in ductility, while welded samples exhibited a decrease in both yield strength and elongation. This matched well with samples which were thermally aged in a neutral environment. Further analysis indicated the CO 2 environment had little effect beyond a depth of 5 µm of the surface, and no effect beyond 10–15 µm. The evolution of mechanical properties of 740 H in this environment, therefore, is dominated by the effect of thermal aging, with only trace impact by the CO 2 environment. The thermal aging effect was shown to be mechanically detrimental within the fusion zone of the weld due to micro-segregation during the welding process. [ABSTRACT FROM AUTHOR]

This paper presents the results of investigations on microstructure changes after the long-term impact of temperature. The microstructure investigations were carried out by light microscopy and scanning electron microscopy. The qualitative and quantitative identification of the existing precipitates was carried out using X-ray phase composition analysis. The effect of elevated temperature on precipitation processes of test material were described. The obtained results of investigations form part of the material characteristics of new-generation alloys, which can be indirectly associated with the stability of functional properties under the simultaneous effect of high temperature and stress. [ABSTRACT FROM AUTHOR]

INCONEL® alloy 740H® is an age-hardenable nickel-based alloy approved for pressure vessels and piping within ASME Section I, VIII, and B31.1. Currently, the code applies a weld strength reduction factor (WSRF) of 0.7 to the allowable stresses for longitudinal seam welds in the time dependent creep regime. In this work, a full-scale seam weld was successfully produced using typical industrial practices. The component was solution heat-treated and aged after fabrication to improve the performance of the weldment. Tensile, bend, impact, and low-cycle fatigue tests showed the component met all the specification requirements and elevated temperature properties were within base metal expectations with failures predominately in the weld metal. Long-term creep tests, including large samples more representative of the entire weldment, were fabricated and tested to times in excess of 10,000 h at multiple temperatures. Analysis of the creep data supports a WSRF of 0.9 for the solution annealed + aged weldments in contrast to the WSRF of 0.7 currently applied to welded + aged weldments. Metallurgical analysis shows that the solution annealing causes recrystallization of the weld metal and reduces the chemical compositional and microstructural gradients in the weldments resulting in acceptable short-term performance. In long-term creep, evidence for coarsened zones in the weldments due to discontinuous coarsening reactions were identified as the mechanism leading to accelerated creep damage formation in the weld metal and sample failure. • Solution annealing dramatically improves the high-temperature performance of alloy 740H® matching filler metal weldments. • Tensile, impact, bend, and low cycle fatigue testing met material minimum behavior. • A weld strength reduction factor of 0.9 is proposed based on long-term creep testing at times >10,000 h. • Coarsened zones form at high-temperature due to subtle compositional gradients in weld metal which lead to creep failure. [ABSTRACT FROM AUTHOR]

The oxidation behavior and mechanism of as-cast Inconel 740H superalloy, newly developed for advanced ultra-supercritical power plants superheater and reheater pipes, were investigated in the temperature range of 1050-1170 °C in air. Isothermal oxidation testing, X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy were used in this study. The results showed that the oxidation of Inconel 740H was a diffusion-controlled process and controlled by the outward diffusion of metallic elements and inward penetration of oxygen. Owing to the complicated chemical composition of the alloy, different oxides tended to form in the oxidation process. Cross-sectional characterization investigation indicated that the oxide-scale layer consisted of three parts when the temperature was high or the oxidation time was long: an internal oxidation zone composed of AlO and TiO, a continuous and dense middle layer of CrO containing precipitates of TiO and an outmost layer of NiCrO spinel. In the formation process of NiCrO, NiO was found in the early transient stage. With further oxidation, TiO finally could not be detected in the internal oxidation zone as a result of outwards diffusion of Ti and its limited concentration in the matrix. [ABSTRACT FROM AUTHOR]

The microstructure evolution of Inconel 740H during aging at 750°C for up to 3000h was investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), three-dimensional atom probe (3DAP) analysis and micro-phase analysis. The mechanical properties of samples after aging were also studied. The grain size increased substantially during the whole aging period. Two different types of grain boundary carbides were observed; block-shaped and needle-shaped. Both were identified to be M23C6 by selected area diffraction measurements. The grain boundary carbides did not coarsen significantly during aging, with the weight fraction increasing only from 0.20% to 0.28%. In contrast, a much higher coarsening rate of γ′ precipitates was observed, as evidenced from both TEM observations and SAXS analysis. 3DAP was used to study the elemental partitioning behavior between γ′ precipitates and γ matrix as well as the evolution in width of the γ/γ′ interface. A large increase in the width of γ/γ′ interfaces was seen between 1000h and 3000h aging. In addition, for the sample aged at 750°C for 3000h, Cr enrichment on the γ matrix side of the γ/γ′ interface was found. Tensile tests at 750°C of the aged samples showed a gradual decrease in elevated-temperature yield strength after 500h, when this alloy was over-aged. The critical precipitate size for the transition from precipitate cutting by weakly coupled dislocations to strongly coupled dislocations for Inconel 740H was calculated to be approximately 50nm, which agrees well with the experiment measurements of elevated-temperature yield strength. The room temperature impact toughness of all samples decreased during aging as the grain size kept growing. [ABSTRACT FROM AUTHOR]

Abstract: Creep–rupture experiments were conducted on multiple heats of the nickel-based superalloy Inconel® 740 at temperatures between 923 and 1123K (650 and 850°C). The interactions between chemistry, microstructure, and creep performance were evaluated by analysis of creep data, optical microscopy, electron microscopy, and computational thermodynamics. The data show that grain size has a modest effect on the creep–rupture strength. Computational thermodynamics verified experimental observations of the formation of eta phase as a function of temperature and alloy chemistry, but the kinetics for the precipitation of eta phase did not agree with the experimental findings. Despite the formation of eta phase and the concomitant reduction in volume fraction of gamma prime, the creep resistance of the alloy is insensitive, within the range of chemistries tested, to the volume fraction of gamma prime. The creep ductility was found to increase with test temperature. Precipitation of a large volume fraction of eta phase (greater than 7%) appears to reduce the creep–rupture ductility, but smaller amounts do not produce adverse effects. [Copyright &y& Elsevier]

Inconel 740H is a nickel-based alloy for pressure components of ultra-supercritical boilers. Its chemical composition and strengthened matrix, as well as corrosion resistance, provide the highest creep resistance among the materials recommended for use in high-performance pressure components of power units. This paper investigates the changes in the microstructure and mechanical properties after ageing at 700 and 750 °C for 1000, 10,000, 20,000, and 30,000 h. Observation of the microstructure was performed using scanning and transmission electron microscopy. The identification of existing precipitates was conducted by X-ray phase analysis. The effects of time and ageing at elevated temperatures on the mechanical properties and precipitation process in the test alloy are discussed. The presented results are part of the material characteristics of the new-generation alloys to be used in the design of pressure equipment for steam boilers, as well as in diagnostic work during operation. [ABSTRACT FROM AUTHOR]