Modern polycrystalline Ni-base superalloys for advanced gas turbine engines have been a key component that has contributed to technological advances in propulsion and power generation. As advanced turbine engine designs are beginning to necessitate the use of materials with temperature and strength capabilities beyond those exhibited by existing materials, new alloying concepts are required to replace conventional Ni-base superalloys with conventional γ-γ’ microstructures. The phase stability of various high Nb content Ni-base superalloys exhibiting γ-γ’-δ -η microstructures have been the subject of a number of recent investigations due to their promising physical and mechanical properties at elevated temperatures. Although high overall alloying levels of Nb, Ta and Ti are desirable for promoting high temperature strength in γ-γ’ Ni-base superalloys, excessive levels of these elements induce the formation of δ and η phases. The morphology, formation, and composition of precipitate phases in a number of experimental alloys spanning a broad range of compositions were explored to devise compositional relationships that can be used to predict the microstructural phase stability and facilitate the design of Ni-base superalloys.

The aspiration to deploy Nb-based alloys as viable upgrade for Ni-based superalloys is rooted in their potential for superior performance in high-temperature applications, such as rocket nozzles and next-generation turbines. However, realizing this goal requires overcoming formidable design hurdles, including achieving high specific strength, creep resistance, fatigue, and oxidation resistance at elevated temperatures, while preserving ductility at lower temperatures. Additionally, the requisite for alloy bond-coatings, to ensure compatibility with coating materials, further complicates the design process. QuesTek Innovations has its Integrated Computational Materials Engineering (ICME) technologies to design a superior performance high-temperature Nb-based superalloy based on solid solution and precipitation strengthening. Additionally, utilizing a statistical learning method from very limited available data, QuesTek engineers were able to establish physics-based material property models, enabling accurate predictions of equilibrium phase fraction, DBTT, and creep properties for multicomponent Nb alloys. With the proven Materials by Design methodology under the ICME framework, QuesTek successfully designed a novel Nb superalloy that met the stringent design requirements using its advanced ICMD materials modeling and design platform.

An international initiative is underway to develop the first standardized high-temperature solid particle erosion test method for steam turbine applications, addressing limitations of the current room-temperature ASTM G76 standard. Led by EPRI, this program involves laboratories from seven countries in a “Round Robin” testing program, aiming to establish consistent testing procedures for evaluating erosion resistance of materials used in Ultra Supercritical (USC) and advanced USC turbines. The proposed standard will use Type 410 stainless steel tested at 30 and 90-degree impingement angles with 50-micron alumina particles at 200 m/s, both at room temperature and 600°C, providing more relevant conditions for current and next-generation steam turbine applications.

Solid particle erosion (SPE) harms steam and gas turbines, reducing efficiency and raising costs. The push for ultra-supercritical turbines reignited interest in SPE’s impact on high-temperature alloys. While the gas turbine industry researches methods to improve erosion resistance, a similar need exists for steam turbines. Existing room-temperature SPE test standards are insufficient for evaluating turbine materials. To address this gap, an EPRI program is developing an elevated-temperature SPE standard. This collaborative effort, involving researchers from multiple countries, has yielded a draft standard submitted to ASTM for approval. This presentation will detail the program, test conditions, and the draft standard’s development.

Solid particle erosion (SPE) and liquid droplet erosion (LDE) cause severe damage to turbine components and lead to premature failures, business loss and rapier costs to power plant owners and operators. Under a program funded by the Electric Power Research Institute (EPRI), nano­coatings are under development for application in steam and gas turbines to mitigate the adverse effects of PE and LPE on rotating blades and stationary vanes. Based on a thorough study of the available information, most promising coatings such as nano-structured titanium silicon carbo-nitride (TiSiCN), titanium nitride (TiN) and multilayered nano coatings were selected. TurboMet International (TurboMet) teamed with Southwest Research Institute (SwRI) with state-of-the-art nano-technology coating facilities with plasma enhanced magnetron sputtering (PEMS) method to apply these coatings on various substrates. Ti-6V-4Al, 12Cr, 17-4PH, and Custom 450 stainless steel substrates were selected based on the current alloys used in gas turbine compressors and steam turbine blades and vanes. Coatings with up to 30 micron thickness have been deposited on small test coupons. These are extremely hard coatings with good adhesion strength and optimum toughness. Tests conducted on coated coupons by solid particle erosion (SPE) and liquid droplet erosion (LDE) testing indicate that these coatings have excellent erosion resistance. The erosion resistance under both SPE and LDE test conditions showed the nano-structured coatings have high erosion resistance compared to other commercially produced erosion resistance coatings. Tension and high-cycle fatigue test results revealed that the hard nano-coatings do not have any adverse effects on these properties but may provide positive contribution.

While the water vapor content of the combustion gas in natural gas-fired land based turbines is ~10%, it can be 20-85% with coal-derived (syngas or H 2 ) fuels or innovative turbine concepts for more efficient carbon capture. Additional concepts envisage working fluids with high CO 2 contents to facilitate carbon capture and sequestration. To investigate the effects of changes in the gas composition on thermal barrier coating (TBC) lifetime, furnace cycling tests (1h cycles) were performed in air with 10, 50 and 90 vol.% water vapor and in CO 2 -10%H 2 O and compared to prior results in dry air or O 2 . Two types of TBCs were investigated: (1) diffusion bond coatings (Pt diffusion or simple or Pt-modified aluminide) with commercially vapor-deposited yttria-stabilized zirconia (YSZ) top coatings on second-generation superalloy N5 and N515 substrates and (2) high velocity oxygen fuel (HVOF) sprayed MCrAlYHfSi bond coatings with air-plasma sprayed YSZ top coatings on superalloy X4 or 1483 substrates. In both cases, a 20-50% decrease in coating lifetime was observed with the addition of water vapor for all but the Pt diffusion coatings which were unaffected by the environment. However, the higher water vapor contents in air did not further decrease the coating lifetime. Initial results for similar diffusion bond coatings in CO 2 -10%H 2 O do not show a significant decrease in lifetime due to the addition of CO 2 . Characterization of the failed coating microstructures showed only minor effects of water vapor and CO 2 additions that do not appear to account for the observed changes in lifetime. The current 50°-100°C de-rating of syngas-fired turbines is unlikely to be related to the presence of higher water vapor in the exhaust.

Ni-base superalloys used for hot section hardware of gas turbine systems experience thermomechanical fatigue (TMF), creep, and environmental degradation. The blades and vanes of industrial gas turbines (IGTs) are made from superalloys that are either directionally-solidified (DS) or cast as single crystals (SX). Consequently, designing and evaluating these alloys is complex since life depends on the crystallographic orientation in addition to the complexities related to the thermomechanical cycling and the extent of hold times at elevated temperature. Comparisons between the more complex TMF tests and simpler isothermal low cycle fatigue (LCF) tests with hold times as cyclic test methods for qualifying alternative repair, rejuvenation, and heat-treatment procedures are discussed. Using the extensive set of DS and SX data gathered from the open literature, a probabilistic physics-guided neural network is developed and trained to estimate life considering the influence of crystallographic orientation, temperature, and several other cycling and loading parameters.

In both direct- and indirect-fired supercritical CO 2 (sCO 2 ) cycles, there is considerable interest in increasing the size and efficiency of such systems, perhaps by increasing the peak temperature to >700°C. However, relatively little experimental data are available under these conditions with pressures of 200-300 bar. Furthermore, impurities such as O 2 and H 2 O in the CO 2 may greatly alter the compatibility of structural alloys in these environments. While an experimental rig is being constructed that can measure and control the impurity levels in sCO 2 at 200-300 bar, initial 1 bar experiments at 700°-800°C for 500 h have been conducted in high-purity and industrial grade CO 2 , CO 2 +0.15O 2 and CO 2 +10%H 2 O and compared to exposures in dry air and 200 bar sCO 2 . These results, focusing on Fe- and Ni-base structural alloys and commercial chromia- and alumina-forming alloys, indicate that modifications in the environment did not strongly affect the reaction products at 700°-800°C.

Hydrogen as a clean fuel is increasingly being used to propel gas turbines and to power combustion engines. Metallic materials including Ni-based alloys are commonly used in conventional gas turbines and combustion engines. However, hydrogen may cause embrittlement in these materials, depending on their chemical composition. In this work, the hydrogen embrittlement behavior of Ni-based alloys containing up to 50 wt.% Fe has been investigated using slow strain rate tensile testing, under cathodic hydrogen charging at room temperature. It was found that the larger the Ni equivalent concentration in an alloy, the more severe the hydrogen embrittlement. It was also found that solid solution alloys have less severe hydrogen embrittlement than precipitation alloys of the same Ni equivalent concentration. In solid solution alloys, hydrogen embrittlement led to cleavage type fracture, which is in line with literature where hydrogen enhanced planar deformation. In precipitation alloys, hydrogen embrittlement resulted in a typical intergranular fracture mode.

Large scale components of the conventional 600°C class steam turbine were made of the ferritic steel, but the steam turbine plants with main steam temperatures of 700°C or above (A-USC) using the Ni-base superalloys are now being developed in order to further improve the thermal efficiency. The weight of the turbine rotor for the A-USC exceeds 10ton. A lot of high strength superalloys for aircraft engines or industrial gas turbines have been developed up to now. But it is difficult to manufacture the large-scale parts for the steam turbine plants using these conventional high strength superalloys because of their poor manufacturability. To improve high temperature strength without losing manufacturability of the large scale components for the A-USC steam turbine plants, we developed Ni-base superalloy USC800(Ni-23Co-18Cr-8W-4Al-0.1C [mass %]) which has temperature capability of 800°C with high manufacturability achieved by controlling microstructure stability and segregation property. The 700°C class A-USC materials are the mainstream of current development, and trial production of 10 ton-class forged parts has been reported. However, there have been no reports on the development and trial manufacturing of the A-USC materials with temperature capability of 800°C. In this report, results of trial manufacturing and its microstructure of the developed superalloy which has both temperature capability 800°C and good manufacturability are presented. The trial manufacturing of the large forging, boiler tubes and turbine blades using developed material were successfully achieved. According to short term creep tests of the large forging and the tube approximate 100,000h creep strength of developed material was estimated to be 270MPa at 700 °C and 100MPa at 800°C.

Improving power plant efficiency through supercritical steam pressures and very high steam temperatures up to 700°C and beyond is an effective approach to reducing fuel consumption and CO2 emissions. However, these extreme steam temperatures necessitate the use of nickel-base alloys in the high-pressure/intermediate-pressure turbine sections requiring very large component sections that cannot be met by steels. Saarschmiede, involved in manufacturing large components for the power generation industry and research programs on advanced 9-12% chromium steels, has extensive experience producing nickel and cobalt-base alloy forgings for applications like aircraft engines, aerospace, land-based gas turbines, and offshore. This paper reports on the manufacturing and testing of large-section forgings made from candidate nickel-base alloys like 617 and 625 for high-pressure/intermediate-pressure turbine components in power stations operating at 700°C and higher steam temperatures.

MoSiBTiC alloy is a promising material for advanced aerospace applications and next generation high pressure turbine blades in jet engines and gas turbines. It mainly consists of Mo solid solution, TiC and Mo 5 SiB 2 phases and has creep strength much stronger than Ni-base superalloys and better than SiC/SiC ceramic matrix composites. Furthermore, the fracture toughness of the alloy is much better (>15 MPa(m) 1/2 ) than Mo-Si-B ternary alloys (<10 MPa(m) 1/2 ) even if the volume fraction of Mo solid solution is less than 50 %. The improvement of fracture toughness would be caused not only by the continuity of Mo solid solution in solidification microstructure but also by TiC phase affecting as a fracture-resistant phase. In order to understand the microstructure evolution during solidification and the effect of TiC phase on the fracture toughness of the MoSiBTiC alloy, Mo-Ti-C ternary model alloys are dealt with in this study. Then, (1) liquidus surface projection and (2) isothermal section and the elastic moduli of TiC phase in equilibrium with Mo solid solution were focused on. The obtained liquidus surface projection suggests that the ternary transition peritectic reaction (L+ Mo 2 C->Mo+TiC) takes place in Mo-rich region. At 1800 °C, TiC phase in equilibrium with Mo phase contains at least 20.2 at% Mo and the Mo/TiC/Mo 2 C three phase region should exist around Mo-15Ti-10C.

Titanium is extensively utilized in the aerospace industry due to its low density and excellent mechanical and chemical properties. Given that components in this sector are exposed to temperatures up to 873 K, representing 45% of the metal's melting point, understanding the mechanical properties in this temperature range is crucial for ensuring flight safety. This study focuses on examining the creep behavior of pure titanium to gain insights into its fundamental mechanical response. Creep was observed to occur at stresses exceeding micro-yielding levels around 297 K, primarily attributed to overcoming the pinning effect caused by interstitial atoms. Interestingly, at intermediate temperatures, an inverted primary creep phenomenon was noted, with an activation energy of approximately 240 kJ/mol within this range. This value, significantly larger than those associated with lattice or dislocation-core diffusions, suggests the potential movement of dislocations with interstitial atoms, similar to the diffusion of oxygen or nitrogen within titanium. Moreover, fracture strain exceeded 80% at temperatures surpassing 673 K, possibly resulting from grain boundary diffusion mechanisms akin to superplasticity. The activation energy for this mechanism, at 97 kJ/mol, is adequate for activating grain boundary deformation at intermediate temperatures.

Following the successful completion of a 14-year effort to develop and test materials which would allow advanced ultra-supercritical (A-USC) coal-fired power plants to be operated at steam temperatures up to 760°C, a United States-based consortium has started on a project to build an A-USC component test facility, (A-USC ComTest). Among the goals of the facility are to validate that components made from the advanced alloys can perform under A-USC conditions, to accelerate the development of a U.S.-based supply chain for the full complement of A-USC components, and to decrease the uncertainty for cost estimates of future commercial-scale A-USC power plants. The A-USC ComTest facility will include a gas fired superheater, thick-walled cycling header, steam piping, steam turbine (11 MW nominal size) and valves. Current plans call for the components to be subjected to A-USC operating conditions for at least 8,000 hours by September 2020. The U.S. consortium, principally funded by the U.S. Department of Energy and the Ohio Coal Development Office with co-funding from Babcock & Wilcox, General Electric and the Electric Power Research Institute, is currently working on the Front-End Engineering Design phase of the A-USC ComTest project. This paper will outline the motivation for the project, explain the project’s structure and schedule, and provide details on the design of the facility.

Alloy 718 is one of the most widely used for aircraft engine and gas turbine components requiring oxidation and corrosion resistance as well as strength at elevated temperatures. Alloy 718 has been produced in both wrought and cast forms, but metal injection molding and metal-based additive manufacturing (AM) technologies have the potential to create a three-dimensional component. Their mechanical properties are highly dependent on the types of powder processing, but the relationship between microstructures and properties has not been clarified. In this study, the mechanical properties of Alloy 718 manufactured by AM are compared to cast and wrought properties. The electron beam melting processed specimens with strong anisotropy showed higher yield strength, which can be explained by critical resolved shear stress. In addition, the creep deformation showed a complicated behavior which was different from that of wrought alloy. Such abnormal behavior was characterized by γ-channel dislocation activity.

A constitutive equation, with parameters derived from the interpolation of primary and steady state stages of constant load creep curves, has been utilized to estimate the stress relaxation behavior of the martensitic steel X20Cr13, alloy used in many high temperature applications, including heavy duty gas turbines. Creep and stress relaxation tests have been performed at 350°C, close to the negligible creep temperature of the studied alloy for stresses of interest for engineering applications. The creep tests were carried out at stresses below and above the yield stress, whereas, for the relaxation stress tests, the imposed strain was in the range 0.2% to 1.2% with the purpose to have, at the beginning of the tests, the same initial stresses of the performed creep tests. After a stress relaxation period, lasting between 10 to 1000 hours, each specimen was generally reloaded at the initial stress and a new relaxation test, on the same specimen, was carried out. This “reloading procedure”, simulating the re-tightening of bolts, has been repeated several times. The proposed equation has shown to well predict the experimental creep and stress relaxation behavior of the steel under investigation.

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.

Gas turbine blades are operated in a high temperature and a high pressure. In order to cope with that harsh condition, the blades are made of Nickel based superalloys which show excellent performance in such environment. Manufacturers of the blades usually provide the standards for the blade inspection and replacement. According to their guide, the blades are replaced after 3 times of operations and 2 times of refurbishments. Howsoever, purchase the new blades is always costly and burdensome to the power plant owners hence, the assessment of the blade lifespan and the rejuvenation of the degraded blades are indeed crucial to them. In this study, the optimal rejuvenation conditions for gas turbine blades were derived and verified. In addition to that, the creep durability was evaluated based on the actual blade inspection interval. LCF tests have been carried out on the rejuvenated blade and the result was compared with the fatigue life of the new blades. In order to secure the safety of the rejuvenated blade during operation, a heat flow analysis was performed to simulate the operating conditions of the gas turbine during operation, and the main stress and strain areas were investigated through the analysis results. And then LCF and creep considering the actual operating conditions were evaluated. The calculated life of fatigue and creep life is compared to the hot gas path inspection interval. For the rejuvenated blades, the creep life and the LCF interval were reviewed based on the temperature, stress, and strain acquired by computational analysis. The creep life was calculated as 59,363 hours by LMP curve, and the LCF was calculated as 2,560 cycles by the Manson Coffin graph.

Modern gas turbines are operated with fuels that are very clean and within the allowances permitted by fuel specifications. However, the fuels that are being considered contain vanadium, sulfur, sodium and calcium species that could significantly contribute to the degradation of components in hot gas flow path. The main potential risk of material degradation from these fuels is “hot corrosion” due to the contaminants listed above combined with alkali metal salts from ambient air. Depending on the temperature regime hot corrosion can damage both TBC coatings and bond coat/substrate materials. Deposit-induced or hot corrosion has been defined as “accelerated oxidation of materials at elevated temperatures induced by a thin film of fused salt deposit”. For the initiation of hot corrosion, deposition of the corrosive species, e.g. vanadates or sulfates, is necessary. In addition to the thermodynamic stability, the condensation of the corrosive species on the blade/vane material is necessary to first initiate and then propagate hot corrosion. Operating temperatures and pressures both influence the hot corrosion damage. The temperature ranges over which the hot corrosion occurs depend strongly on following three factors: deposit chemistry, gas constituents and metal alloy (or bond coating/thermal barrier coating) composition. This paper reports the activities involved in establishing modeling and simulation followed by testing/characterization methodologies in relevant environments to understand the degradation mechanisms essential to assess the localized risk for fuel flexible operation. An assessment of component operating conditions and gas compositions throughout the hot gas paths of the gas turbines, along with statistical materials performance evaluations of metal losses for particular materials and exposure conditions, are being combined to develop and validate life prediction methods to assess component integrity and deposition/oxidation/corrosion kinetics.

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.