Cast nickel-based superalloys used as structural materials for gas turbine parts need to withstand high temperatures and dynamic mechanical loads. When in contact with ambient air, the formation of protective oxide scales causes a depletion of γ’-precipitates in the surface-near region and leaves a weakened microstructure. This environmentally based degradation of the material might be accelerated under cyclic thermal exposure. In this paper, the cyclic oxidation behavior of two cast nickel-based superalloys and one single crystalline variant are investigated: C1023, CM-247 LC and M-247 SX. Exposure tests were carried out under both isothermal and cyclic conditions in air at 850 °C, 950 °C and 1050 °C for times up to 120 h to investigate the impact of thermal cycling. The differences in oxidation mechanisms are analyzed phenomenologically via light and electron microscopy and brought in correlation with the oxidation kinetics, determined based on net mass change and depletion zone growth. An assessment of the impact of precipitation loss on local mechanical strength is attempted via nano-indentation method. The found relations can be transferred onto an acceleration of crack growth under creep-fatigue and thermo-mechanical fatigue 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.

Gas turbine blades made from nickel-based superalloys, valued for their high temperature stability and creep resistance, undergo various forms of microstructural degradation during extended service at elevated temperatures that can ultimately lead to blade failure. To extend blade and turbine rotor life, Sulzer has developed evaluation and rejuvenation processes that include microstructural assessment and stress rupture testing of specimens from service-exposed blades. While stress rupture testing presents certain limitations and challenges in evaluating material condition, Sulzer has successfully rejuvenated hundreds of gas turbine blade sets across multiple superalloy types, including GTD 111, IN 738 LC, and U 500, demonstrating the effectiveness of heat treatment rejuvenation in improving microstructure and mechanical properties of service-degraded components.

High gamma prime Ni-based superalloys comprising ≥3.5 % Al are difficult to weld due to high propensity of these materials to weld solidification, heat affected zone liquation, and stress-strain cracking. In this study the root cause analysis of cracking and overview on the developed weldable Ni-based superalloys for repair of turbine engine components manufactured from equiaxed (EA), directionally solidified (DS), and single crystal (SX) materials as well as for 3D AM is provided. It is shown that the problem with the solidification and HAZ liquation cracking of turbine engine components manufactured from EA and DS superalloys was successfully resolved by modification of welding materials with boron and silicon to provide a sufficient amount of eutectic at terminal solidification to promote self-healing of liquation cracks along the weld - base material interface. For crack repair of turbine engine components and 3D AM ductile LW4280, LW7901 and LCT materials were developed. It is shown that LW7901 and LCT welding materials comprising 30 - 32 wt.% Co produced sound welds by GTAW-MA on various SX and DS materials. Welds demonstrated high ductility, desirable combination of strength and oxidation properties for tip repair of turbine blades. Examples of tip repair of turbine blades are provided.

Creep-fatigue tests strain-controlled with different strain amplitudes and different hold times at 725 were done on nickel-based alloy 617 as a typical candidate material for turbine rotor of advanced ultra-supercritical power plant. Stress relaxes during the hold time when the strain remains at the tensile peak. The analysis of the stress relaxation during different strain hold times shows that the ratio of the relaxation stress and the maximum stresses has strong correlation with strain amplitude and hold time. The failure life also has a certain dependence on the relaxation stress ratio. The failure life decreases and the relaxation stress ratio increases as the strain amplitude increases. The failure life decreases and the relaxation stress ratio increases as the hold time increases. Therefore the stress relaxation ratio was used as an intermediate variable to obtain the corresponding relationship model by establishing the relationship between the relaxation stress ratio and the strain and the relationship between the relaxation stress ratio and the failure life. This model can be used to predict the creep-fatigue interaction life more simply and directly.

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.

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-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.

An approach to phase analysis called multiphase separation technology (MPST) has been developed to determine phase chemistries of precipitated particles with sizes visible under SEM/EPMA observations based on the data from the conventional EDS measurements on bulk steel/alloy material samples. Quite accurate results from its applications have successfully been demonstrated by comparisons of SEM/EPMA - EDS + MPST with some other currently available means, for instance, chemical extractions (CA), TEM-EDS, AP-FIM and Thermo-Calc. etc. Applied examples regarding the relations of change in phase parameters including type, composition, volume fraction, size and distribution of the precipitated particles with material qualities, creep rupture lives, property stabilities, property recovery and boiler tube failures for some advanced heat resistant steels (P92, Super304H, HR3C, TP347HFG (H)) are given through the use of the SEM/EPMA - EDS + MPST in this contribution. Examples on phase quantifications of some nickel base superalloys (Nimonic263, Inconel 740 and Rhenium-containing alloys) are also shown to reveal the feasibility of its use in determining phase chemistries of precipitated particles under different measurement conditions. Practical applications of this combined technology to the material quality control and assessments, processing parameter improvements, as well as fracture/failure analyses of high temperature components have shown that this technology is quite convenient and effective when used for microstructural analysis purposes during R&D, manufacturing and operating processes.

The power industry has been faced with continued challenges around decarbonization and additive manufacturing (AM) has recently seen increased use over the last decade. The use of AM has led to significant design changes in components to improve the overall efficiency of gas turbines and more recently, hot-section components have been fabricated using AM nickel-base superalloys, which have shown substantial benefits. This paper will discuss and summarize extensive studies led by EPRI in a novel AM nickel-base superalloy (ABD·900-AM). A comprehensive high temperature creep testing study including >67,000 hours of creep data concluded that ABD-900AM shows improved properties compared to similar ~35% volume fraction gamma prime strengthened nickel-base superalloys fabricated using additive methods. Several different creep mechanisms were identified and various factors influencing high temperature behavior, such as grain size, orientation, processing method, heat treatment, carbide structure, chemistry and porosity were explored. Additional studies on the printability, recyclability of powder, wide range of process parameters and several other factors have also been studied and results are summarized. A summary on the alloy -by-design approach and accelerated material acceptance of ABD-900AM through extensive testing and characterization is further discussed. Numerous field studies and examples of field use cases in ABD-900AM are also evaluated to showcase industry adoption of ABD-900AM.

In this study, fatigue crack propagation behavior at lower temperature in single crystal nickel-base superalloys was investigated experimentally and analytically. Four types of compact specimens with different combinations of crystal orientations in loading and crack propagation directions were prepared, and fatigue crack propagation tests were conducted at room temperature and 450°C. It was revealed in the experiments that the crack propagated in the shearing mode at room temperature, while the cracking mode transitioned from the opening to shearing mode at 450°C. Both the crack propagation rate and the transition behavior were strongly influenced by the crystallographic orientations. To interpret these experimental results, crystal plasticity finite element analysis was carried out, taking account some critical factors such as elastic anisotropy, crystal orientations, 3-D geometry of the crack plane and the activities of all 12 slip systems in the FCC crystal. A damage parameter based on the slip plane activities derived from the crystal plasticity analysis could successfully rationalize the effect of primary and secondary orientations on the crystallographic cracking, including the crack propagation paths and crack propagation rates under room temperature. The proposed damage parameter could also explain the transition from the opening to crystallographic cracking observed in the experiment under 450°C.

The aim of this work was to reveal the effects of trace elements on the creep properties of nickel-iron base superalloys, which are the candidate material for the large components of the advanced-ultrasupercritical (A-USC) power generation plants. High temperature tensile and creep properties of forged samples with seven different compositions were examined. No significant differences were observed in the creep rate versus time curves of the samples, of which contents of magnesium, zirconium, manganese and sulfur were varied. In contrast, the curves of phosphorus-added samples showed very small minimum creep rates compared to the other samples. The creep rupture lives of phosphorus-added samples were obviously longer than those of the other samples. Microstructure observation in the vicinity of grain boundaries of phosphorus-added samples after aging heat treatment revealed that there were fine precipitates consisting of phosphorus and niobium at the grain boundaries. The significant suppression of the creep deformation of phosphorus-added sample may be attributed to the grain boundary strengthening caused by the fine grain boundary precipitates.

As conventional coal-fired power plants seek to reduce greenhouse gas emissions by increasing efficiency, the temperature limitations of traditional ferritic/martensitic steels used in high-temperature components present a significant challenge. With Advanced Ultra Supercritical (A-USC) power plants proposing steam temperatures of 760°C, attention has turned to nickel-based superalloys as potential replacements, since ferritic/martensitic steels cannot withstand such extreme conditions. However, the current absence of cast nickel-based superalloys combining high strength, creep-resistance, and weldability has led to the development of cast analogs of wrought nickel-based superalloys, including H263, H282, and N105. This paper examines the alloy design criteria, processing experiences, as-processed and heat-treated microstructures, and selected mechanical properties of these materials while also discussing their potential for full-scale development.

The goal of improving the efficiency of pulverized coal power plants has been pursued for decades. The need for greater efficiency and reduced environmental impact is pushing utilities to ultra supercritical conditions (USC), i.e. steam conditions of 760°C and 35 MPa. The long-term creep strength and environmental resistance requirements imposed by these conditions are clearly beyond the capacity of the currently used ferritic steels and other related alloys. Consequently, new materials based on austenitic stainless steels and nickel-base superalloys are being evaluated as candidate materials for these applications. In the present work, the nickel-base superalloys CCA617, Haynes 230 and Inconel 740, and an austenitic stainless steel Super З04H, were evaluated. The materials were aged for different lengths of time at temperatures relevant to USC applications and the corresponding microstructural changes were characterized by x-ray diffraction, optical, scanning and transmission electron microscopy, with particular attention being given to the structure, morphology and compositions of phases (including γ, γ’, carbides, ordered phases, etc.) and the nature, density and distribution of dislocations and other defects. The results are presented and discussed in light of accompanying changes in microhardness.

Ultrasupercritical (USC) steam boiler and heat recovery steam generator (HRSG) technology is constantly evolving to improve efficiency and reduce emissions. Currently, temperatures are pushing beyond the capabilities of even the most advanced ferritic steels with some applications requiring nickel-based superalloys. Cost-effective design of these systems requires the application of a variety of alloys representing a range of cost/property trade-offs. CF8C-Plus is a cast austenitic stainless steel recently developed for application in high temperatures similar to those in power plants (600 - 900 °C) with creep strength comparable to several superalloys. This makes it an attractive alternative for those expensive alloys. EPRI, with assistance from PCC subsidiaries Special Metals and Wyman Gordon Pipes and Fittings, has produced and characterized two pipe extrusions nominally 5.25 inch OD x 0.5 inch wall thickness and 6 inch OD x 0.75 inch wall (13.3 x 1.3 cm and 15.2 x 1.9 cm), each about 1000 lbs, to continue to assess the feasibility of using a wrought version of the alloy in power piping and tubing applications. The mechanical properties from these extrusions show performance in the same population as earlier forging trials demonstrating capability exceeding several austenitic stainless steels common to the industry. Creep-rupture performance in these extrusions continues to be competitive with nickel-based superalloys.

In advanced ultra-supercritical (A-USC) power plants, which operate at steam temperatures of 700 °C or higher, there is a need to replace 9 to 12Cr martensitic steels with high-strength nickel-base superalloys or austenitic steels for components exposed to the highest temperatures. However, due to the high cost of nickel-base superalloys, it is desirable to use 9 to 12% Cr martensitic steels for components exposed to slightly lower temperatures, ideally expanding their use up to 650 °C. Key challenges in developing ferritic steels for 650 °C USC boilers include enhancing oxidation resistance and long-term creep rupture strength, particularly in welded joints where resistance to Type IV cracking is critical for constructing thick-section boiler components. The current research aims to investigate the creep deformation behavior and microstructure evolution during creep for base metals and heat-affected-zone (HAZ) simulated specimens of tempered martensitic 9Cr steels, including 9Cr-boron steel and conventional steels like grade 91 and 92. The study discusses the creep strengthening mechanisms and factors influencing creep life. It proposes an alloy design strategy that combines boron strengthening and MX nitride strengthening, avoiding the formation of boron nitrides during normalizing heat treatment, to improve the creep strength of both base metal and welded joints.

The advancement of additive manufacturing (AM) technology has heightened interest in producing components from nickel-based superalloys for high-temperature applications; however, developing high gamma prime (γ’) strengthened alloys suitable for AM at temperatures of 1000°C or higher poses significant challenges due to their “non-weldable” nature. Traditional compositions intended for casting or wrought processes are often unsuitable for AM due to their rapid heating and cooling cycles, leading to performance compromises. This study introduces ABD-1000AM, a novel high gamma prime Ni-based superalloy designed using the Alloys-by-Design computational approach to excel in AM applications at elevated temperatures. Tailored for AM, particularly powder bed fusion, ABD-1000AM demonstrates exceptional processing capability and high-temperature mechanical and environmental performance at 1000°C. The study discusses the alloy design approach, highlighting the optimization of key performance parameters, composition, and process-microstructure-performance relationships to achieve ABD-1000AM’s unique combination of processability and creep resistance. Insights from ABD-1000AM’s development inform future directions for superalloy development in complex AM components.

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.

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.

A study is being conducted on turbine materials for use in ultra-supercritical (USC) steam power plants, with the objective of ensuring no material-related impediments regarding maximum temperature capabilities and the ability to manufacture turbine components. A review of the state-of-the-art and material needs for bolting and casing applications in USC steam turbines was performed to define and prioritize requirements for the next-generation USC turbines. For bolting, several potentially viable nickel-base superalloys were identified for service at 760°C, with the major issues being final material selection and characterization. Factors limiting inner casing material capabilities include casting size/shape, ability to inspect for discontinuities, stress rupture strength, and weldability for fabrication and repairs. Given the need for precipitation-strengthened nickel-base alloys for the inner casing at 760°C, the material needs are two-fold: selection/fabrication-related and characterization. The paper provides background on turbine components and reviews the findings for bolting and casing materials.