Significant development is being carried out worldwide for establishing advanced ultra supercritical power plant technology which aims enhancement of plant efficiency and reduction of emissions, through increased inlet steam temperature of 750°C and pressure of 350 bar. Nickel base superalloy, 50Ni-24Cr-20Co-0.6Mo-1Al-1.6Ti-2Nb alloy, is being considered as a promising material for superheater tubes and turbine rotors operating at ultra supercritical steam conditions. Thermal fluctuations impose low cycle fatigue loading in creep regime of this material and there is limited published fatigue and creep-fatigue characteristics data available. The scope of the present study includes behavior of the alloy under cyclic loading at operating temperature. Strain controlled low cycle fatigue tests, carried out within the strain range of 0.2%-1%, indicate substantial hardening at all temperatures. It becomes more evident with increasing strain amplitude which is attributed to the cumulative effects of increased dislocation density and immobilization of dislocation by γ′ precipitates. Deformation mechanism which influences fatigue life at 750°C as a function of strain rate is identified. Hold times up to 500 seconds are introduced at 750°C to evaluate the effect of creep fatigue interaction on fatigue crack growth, considered as one of the primary damage mode. The macroscopic performance is correlated with microscopic deformation characteristics.

Nickel-based Alloy 617B (DIN 2.4673) and Alloy C-263 (DIN 2.4650) with high creep strength and good fabricability are promising material candidates for the design of next generation coal-fired “Advanced Ultra-Super-Critical A-USC” power plants with advanced steam properties and thus higher requirements on the material properties. Microstructural studies of the precipitation hardened alloy C-263 were performed with Electron Microscopy (TEM) with respect to their strengthening precipitates like carbides and intermetallic gamma prime. Specimens were subjected to different ageing treatments at elevated temperatures for different times. The microstructural results of the investigated nickel alloy C-263 are presented and discussed with respect to their correlation with required properties for A-USC, e.g. the mechanical properties, the creep resistance and the high temperature stability and compared to Alloy 617B. The manufacturing procedure for the prematernal and forgings as well as for thin walled tube components for A-USC power plants is presented.

Haynes 282 is a great candidate to meet advanced ultra-super-critical (A-USC) steam conditions in modern coal-fired power plants. The standard 2-step aging treatment has been designed for optimizing microstructure therefore providing excellent mechanical properties. We studied an alternative, more economical, 1-step aging treatment and compared microstructure, tensile properties at 750˚C and deformation behavior. Moreover, three cooling rates from the solution temperature were studied to simulate large-scale components conditions. We found that as much as about 20% of fine spherical intragranular γ' particles were successfully precipitated in all cases. Their average size increased as the cooling rate decreased. All four heat-treated alloys exhibited good mechanical properties at 750˚C with a yield strength well over 620MPa. As expected, the yield strength increased and the ductility decreased as the average γ' size decreased. The alloys exhibited a mixed mode of deformation, though the dominant deformation mechanism depended on the different γ' characteristics. The major operative deformation mechanism could be well predicted by strength increment calculations based on the precipitation strengthening model. Our results suggest that wrought Haynes 282 produced by a more economical 1-step aging treatment may be a reliable candidate for high temperature applications under A-USC conditions.

Carbon Capture and Storage (CCS) has become promising technology to reduce CO 2 emissions. However, as a consequence of CCS installation, the electrical efficiency of coal fired power plant will drop down. This phenomenon requires increase in base efficiency of contemporary power plants. Efficiency of recent generation of power plants is limited mainly by maximum live steam temperature of 620°C. This limitation is driven by maximal allowed working temperatures of modern 9–12% Cr martensitic steels. Live steam temperatures of 750°C are needed to compensate the efficiency loss caused by CCS and achieve a net efficiency of 45%. Increase in the steam temperature up to 750°C requires application of new advanced materials. Precipitation hardened nickel-based superalloys with high creep-rupture strength at elevated temperatures are promising candidates for new generation of steam turbines operating at temperatures up to 750°C. Capability to manufacture full-scale forged rotors and cast turbine casings from nickel-based alloys with sufficient creep-rupture strength at 750°C/105 hours is investigated. Welding of nickel-based alloys in homogeneous or heterogeneous combination with 10% Cr martensitic steel applicable for IP turbine rotors is shown in this paper. Structure and mechanical properties of prepared homogeneous and heterogeneous weld joints are presented.

This overview paper summarizes part of structure stability study results in China on advanced heat-resistant steels, nickel-iron and nickel base superalloys such as 12Cr2MoWVTiB(GY102) ferritic steel, Super 304H austenitic steel, GH2984, Nimonic 80A and INCONEL 740 superalloys for fossil power plant application. China had established first USC power plant with steam parameters of 650°C and 25 MPa in the year of 2006. Austenitic heat-resistant steel Super 304H is mainly used as boiler superheater and reheater material. Ni-Cr-Fe base superalloy GH2984 was used as tube material for marine power application. Ni-Cr-Co type INCONEL 740 has been studied in a joint project with Special Metals Corp., USA for European USC model power plant with the steam temperature of 700°C. Nimonic 80A has been used as several stage USC steam turbine bucket material at 600°C in China. Structure stability study of Nimonic 80A shows its possibility of 700°C application for USC steam turbine buckets.

Creep-fatigue lives of nickel-based Alloy 617 and Alloy 740H were investigated to evaluate their applicability to advanced ultrasupercritical (A-USC) power plants. Strain controlled push-pull creep-fatigue tests were performed using solid bar specimen under triangular and trapezoidal waveforms at 700°C. The number of cycles to failure was experimentally obtained for both alloys and the applicability of three representative life prediction methods was studied.

Advanced UltraSupercritical (A-USC) Steam fossil power plants will operate at steam temperatures up to 760°C, which will require the use of Ni-based superalloys for steam boiler/superheater and turbine systems. In 2008, the Oak Ridge National Laboratory (ORNL) and the National Engineering Technology Laboratory/Albany (NETL/Albany) collaborated to make and test castings of Ni-based superalloys, which were previously only commercially available in wrought form. These cast Ni-based based alloys are envisioned for the steam turbine casing, but they may also be applicable to other large components that connect the steam supply to the steam turbine. ORNL and NETL/Albany have produced small vacuum castings of HR 282, Nimonic 105, Inconel 740, and alloy 263, which are precipitation-hardened Ni-based superalloys, as well as solid-solution superalloys such as alloys 625, 617 and 230. The initial alloy screening included tensile and creep-testing at 800°C to determine which alloys are best suited for the steam turbine casing application at 760°C. HR 282 has the best combination of high-temperature strength and ductility, making it a good candidate for the cast-casing application. Cast and wrought versions of HR 282 have similar creep-rupture strength, based on the limited data available to-date. Detailed comparisons to the other alloys and microstructures are included in this paper.

A new nickel-base superalloy GH750 has been developed as boiler tube of advanced ultrasupercritical (A-USC) power plants at temperatures about and above 750°C in China. This paper researched the weld solidification of GH750 filler metal, microstructure development and property of GH750 welded joint by gas tungsten arc weld. Liquid fraction and liquid composition variation under non-equilibrium state were calculated by thermo-dynamic calculation. The weld microstructure and the composition in the dendrite core and interdendritic region were analyzed by SEM(EDX) in detail. The investigated results show that there is an obvious segregation of precipitation-strengthening elements during the weld solidification. Titanium and Niobium are the major segregation elements and segregates in the interdendritic region. It was found that the changing tendency of the elements’ segregation distribution during the solidification of GH750 deposit metal is agree with the thermodynamic calculation results. Till to 3,000hrs’ long exposure at 750°C and 800°C, in comparison with the region of dendrite core of solidification microstructure, not only the coarsening and the accumulation of γʹ particles are remarkable in the interdendritic region, but also the small quantity of the blocky and needle like η phases from. The preliminary experimental results indicate that the weakening effect of creep-rupture property of the welded joint is not serious compared with GH750 itself.

Finite element (FE) modeling has been applied to a stress relaxation cracking (SRC) test in order to evaluate the effects of changing sample geometry and material type. This SRC test uses compressive pre-straining to create a tensile residual stress in modified compact-tension specimens and has been used to test 316H stainless steel. The FE model is first used to verify that sample integrity will not be compromised by modifying the geometry. The FE model is then applied to candidate Advanced Ultra Supercritical nickel-base alloys 617, 740H, and 800. It is determined that this stress relaxation test will be appropriate for these alloys.

Advanced power generation systems, including advanced ultrasupercritical (A-USC) steam and supercritical carbon dioxide (sCO 2 ) plants operating above 700°C, are crucial for reducing carbon dioxide emissions through improved efficiency. While nickel superalloys meet these extreme operating conditions, their high cost and poor weldability present significant challenges. This study employs integrated computational materials engineering (ICME) strategies, combining computational thermodynamics and kinetics with multi-objective Bayesian optimization (MOBO), to develop improved nickel superalloy compositions. The novel approach focuses on utilizing Ni 3 Ti (η) phase strengthening instead of conventional Ni 3 (Ti,Al) (γ’) strengthening to enhance weldability and reduce costs while maintaining high-temperature creep strength. Three optimized compositions were produced and experimentally evaluated through casting, forging, and rolling processes, with their microstructures and mechanical properties compared to industry standards Nimonic 263, Waspaloy, and 740H. Weldability assessment included solidification cracking and stress relaxation cracking tests, while hot hardness measurements provided strength screening. The study evaluates both the effectiveness of the ICME design methodology and the practical potential of these cost-effective η-phase strengthened alloys as replacements for traditional nickel superalloys in advanced energy applications.

INCONEL alloy 740H has been specified for tube and pipe for the boiler and heat exchanger sections of AUSC and sCO 2 pilot plants currently designed or under construction. These systems also require fittings and complex formed components such as flanges, saddles, elbows, tees, wyes, reducers, valve parts, return bends, thin-wall cylinders and tube sheets. The initial evaluation of alloy 740H properties, leading to ASME Code Case 2702, was done on relatively small cross-section tube and plate. The production of fittings involves the use of a wide variety of hot or cold forming operations. These components may have complex geometric shapes and varying wall thickness. The utility industry supply chain for fittings is largely unfamiliar with the processing of age-hardened nickel-base alloys. Special Metals has begun to address this capability gap by conducting a series of trials in collaboration with selected fittings manufacturers. This paper describes recent experiences in first article manufacture of several components. The resulting microstructure and properties are compared to the published data for tubular products. It is concluded that it will be possible to manufacture most fittings with properties meeting ASME Code minima using commercial manufacturing equipment and methods providing process procedures appropriate for this class of alloy are followed. INCONEL and 740H are registered trademarks of Special Metals Corporation.

700°C advanced ultra-supercritical system and supercritical CO 2 turbine system are developed for high efficiency turbine systems for next generation. This study covered the feasibility of creep life assessment of γ’-Ni 3 (Al,Ti) precipitation strengthened Ni-based superalloy rotor material, TOS1X-2, a modified alloy of UNS N06617 for these systems, based on hardness measurement method. It was found that the hardness of TOS1X-2 was governed by the change in precipitation strengthening and strain hardening during creep. The clear relationship between hardness increase in crept portion and macroscopic creep strain was observed, suggesting that it might be possible to estimate the creep strain or initiation of acceleration from hardness measurement. Microstructure inhomogeneity and microstructure evolutions during creep especially focused on dispersion of creep strain were characterized by EBSD quantitative analysis. It was found that creep strain was accumulated along the grain boundary, while it was relatively absent in coarse grains with low Schmid factor of {111} <110> slip system in fcc structure. The upper limit of hardness scatter band is thought to be important, since it represents the local and critical creep damage of the alloy.

In wrought nickel-base alloys used at elevated temperatures for extended periods of time, it is commonly observed that unwanted phases may nucleate and grow. One such phase is the eta phase, based on Ni 3 Ti, which is a plate-shaped precipitate that nucleates at the grain boundaries and grows at the expense of the strengthening gamma prime phase. In order to study the effects of eta phase on creep performance, Alloy 263 was modified to contain 3 different microstructures: standard (contains gamma prime); aged (contains gamma prime and eta); and modified (contains only eta and no gamma prime). These microstructures were then creep tested in the range of 973-1123 K (700-850°C). An extensive test matrix revealed that the eta-only modified alloy had creep rupture strengths within 10% of the standard alloy even though this alloy had no strengthening gamma prime precipitates. It also exhibited superior creep ductility. A preliminary test matrix on the aged material containing eta and gamma prime prior to the creep tests revealed that the performance of this microstructure was generally between that of the standard alloy (best) and the eta-only alloy (worst). The aged material exhibited far superior creep ductility. These results suggest that the presence of the eta phase may not be deleterious to creep ductility, and in fact, may enhance it.

A new nickel-based superalloy, designated as GH750, was developed to meet the requirements of high temperature creep strength and corrosion resistance for superheater/reheater tube application of A-USC power plants at temperatures above 750°C. This paper introduces the design of chemical composition, the process performance of tube fabrication, microstructure and the properties of alloy GH750, including thermodynamic calculation, room temperature and high temperature tensile properties, stress rupture strength and thermal stability. The manufacturing performance of alloy GH750 is excellent and it is easy to forge, hot extrusion and cold rolling. The results of the property evaluation show that alloy GH750 exhibits high tensile strength and tensile ductility at room and high temperatures. The 760°C/100,000h creep rupture strength of this alloy is larger than 100MPa clearly. Microstructure observation indicates that the precipitates of GH750 consist of the precipitation strengthening phase γ’, carbides MC and M 23 C 6 and no harmful and brittle TCP phases were found in the specimens of GH750 after long term exposure at 700~850°C. It can be expected for this new nickel-based superalloy GH750 to be used as the candidate boiler tube materials of A-USC power plants in the future.

Inconel 740H is one of the most promising candidate Ni-base superalloys for the main steam pipe of 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants. After processing and welding in manufacturing plant in solution-annealed state, large components was commonly suggested to have an extra aging treatment at 800 °C for 16 h, in order to obtain homogeneous γ′ precipitates. In this present work, creep tests and microstructure analyses were conducted on Inconel 740H pipe specimens under two different heat treatments to verify the necessity of aging process. Here we show that aging treatment has limited effect on the creep rupture life of Inconel 740H pipe. Both in grain interiors and along grain boundaries, crept specimens under two different heat treatments have the same precipitates. But the shape and distribution of γ′ in solution annealed sample is not as regular as the aged ones. Our results provide the underlying insight that aging treatment is not so necessary for the straight pipes if the on-site condition was hard to control. But for both groups of specimens, a small amount of h particles and some banded like M 23 C 6 were emerged during creep, which would be harmful to mechanical properties for the long run.

In this work, the effects of phosphorus addition on the creep properties and microstructural changes of wrought γ’-strengthened Ni-based superalloys (Haynes 282) were investigated, focusing on the effects of carbides precipitation. In an alloy with a phosphorus content of 8 ppm, precipitation of M 23 C 6 carbides was observed in both grain boundaries and the grain interior prior to the creep tests. Grain boundary coverage by carbide increased with phosphorus content up to approximately 30 ppm. On the other hand, the amount of M 23 C 6 in the grain interior decreased with phosphorus content. The results of the creep tests revealed the relationship between the time to rupture and the grain boundary coverage by carbides. The microstructure of the crept specimens showed the existence of misorientation at the vicinity of grain boundaries without carbides, as demonstrated via electron backscattered diffraction (EBSD) analysis. These results suggest that the observed improvement in the time to rupture is due to a grain-boundary precipitation strengthening mechanism caused by grain boundary carbides and that phosphorus content affects the precipitation behavior of M 23 C 6 carbides in the grain interior and grain boundaries. These behaviors were different between alloys with the single addition of phosphorus and alloys with the multiple addition of phosphorus and niobium.

To address current energy and environmental demands, the development and implementation of more efficient power plants is crucial. This efficiency improvement is primarily achieved by increasing steam temperatures and pressures, necessitating the introduction of new materials capable of withstanding these extreme conditions. Nickel-based alloys emerge as prime candidates for high-temperature and high-pressure applications, offering significant creep strength and the ability to operate at metal temperatures above 750°C. This research focuses specifically on steam header and pipework systems, which are critical components carrying steam from boilers to turbines under severe operating conditions. The study emphasizes the importance of selecting suitable materials for these components and developing methodologies to predict their safe operating lifetimes, thereby ensuring the reliable and efficient operation of next-generation power plants.

This overview paper contains a part of structure stability study on advanced austenitic heat-resisting steels (TP347H, Super304H and HR3C) and Ni-base superalloys (Nimonic80A, Waspaloy and Inconel740/740H) for 600-700°C A-USC fossil power plant application from a long-term joint project among companies, research institutes and university in China. The long time structure stability of these advanced austenitic steel TP347H, Super304H, HR3C in the temperature range of 650-700 °C and Ni-base superalloys Nimonic80A, Waspaloy and Inconel740/740H in the temperature range of 600-800°C till 10,000h have been detailed studied in this paper.

The necessity to reduce carbon dioxide emissions of new fossil plant, while increasing net efficiency has lead to the development of not only new steels for potential plant operation of 650°C, but also cast nickel alloys for potential plant operation of up to 700°C and maybe 750°C. This paper discusses the production of prototype MarBN steel castings for potential plant operation up to 650°C, and gamma prime strengthened nickel alloys for advanced super critical plant (A-USC) operation up to 750°C. MarBN steel is a modified 9% Cr steel with chemical concentration of Cobalt and tungsten higher than that of CB2 (GX-13CrMoCoVNbNB9) typically, 2% to 3 Co, 3%W, with controlled B and N additions. The paper will discuss the work undertaken on prototype MarBN steel castings produced in UK funded research projects, and summarise the results achieved. Additionally, within European projects a castable nickel based super alloy has successfully been developed. This innovative alloy is suitable for 700°C+ operation and offers a solution to many of the issues associated with casting precipitation hardened nickel alloys.

The Alloys-by-Design approach, involving large-scale CALPHAD calculations to search a compositional range, has been used to isolate a suitable nickel-based superalloy for additive manufacturing (AM) by optimizing the trade-off between processability and increasing strength. This has been done in response to the limited focus on development of new superalloys designed to overcome the limitations of the AM process, specifically the high defect density of parts made from high-performance alloys. Selected compositions have been made using gas atomization, and laser powder-bed fusion AM trials were performed. The resulting properties were evaluated in the as-processed, heat treated and thermally exposed conditions. The assessment, combined with characterization techniques including scanning electron microscopy and atom probe tomography, rationalizes a temperature capability up to and above 850 °C, and demonstrate the opportunity to develop alloys with properties beyond the current state of the art.