Gas turbine efficiency is typically limited by the maximum allowable temperature for components at the inlet side and in the hot gas flow. Refractory alloys and SiC/SiC ceramic-matrix composites (CMCs) are promising candidates for advancing operating temperatures beyond those of Ni-based alloys (>1200 °C). Refractory alloys are more suitable than SiC/SiC CMCs for dynamic components, due to the latter's low toughness and ductility. However, it is well known that refractory alloys suffer from poor oxidation behavior under service lifetimes and conditions, leading to embrittlement concerns. The ARPA-E ULTIMATE program has set out to combine new alloys with advanced coatings to mitigate oxidation/embrittlement effects, while increasing the mechanical performance benefits of refractory materials. Low oxygen (inert gas) or vacuum systems are needed to assess high temperature mechanical performance of developed alloys. To investigate the environmental sensitivity of candidate alloys and develop high temperature testing capabilities, four argon tensile frames were upgraded as well as a single vacuum system at Oak Ridge National Laboratory. Digital image correlation was incorporated into the vacuum frame allowing for surface strain determination and refined insight into thermomechanical response. Creep testing was performed at 1300 °C on two alloys, C-103 and MHC in vacuum and high purity argon environments. The Mo-based alloy showed less sensitivity to oxygen, indicating that testing in well-controlled argon environments may be suitable. The C-103 alloy demonstrated a stronger sensitivity to oxygen in the well-controlled argon environment, illustrating the need for the developed vacuum testing capabilities. “Small” 25 mm and “large” 76 mm MHC specimens showed comparable results in terms of strain rate during creep testing and ultimate tensile strength during tensile testing, suggesting the viability of smaller geometries that use less material of advanced developmental alloys.

This study investigates the influence of build orientation on the high-temperature mechanical properties of IN738LC manufactured via metal laser powder bed fusion (PBF-LB/M). Since the PBF-LB/M layer-wise manufacturing process significantly affects grain morphology and orientation—ranging from equiaxed to textured grains—mechanical properties typically exhibit anisotropic behavior. Samples were manufactured in three build orientations (0°, 45°, and 90°) and subjected to hot tensile and creep testing at 850°C following DIN EN ISO 6892-2 and DIN EN ISO 204 standards. While tensile properties of the 45° orientation predictably fell between those of 0° and 90° orientations, creep behavior over 100-10,000 hours revealed unexpected results: the 45° orientation demonstrated significantly shorter rupture times and faster creep rates compared to other orientations. Microstructural analysis revealed distinct creep deformation mechanisms active within different build orientations, with the accelerated creep rate in 45° specimens attributed to multiple phenomena, particularly η-phase formation and twinning. These findings provide crucial insights into the orientation-dependent creep behavior of PBF-LB/M-manufactured IN738LC components.

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.

The efforts of the European Union and Germany in particular to realize the transformation towards a climate-neutral economy over the coming decades have the establishing of a hydrogen economy as a fundamental milestone. This includes production, import, storage, transportation and utilization of great amounts of gaseous hydrogen in existing and new infrastructure. Metallic materials, mainly steels, are the most widely used structural materials in the various components of this supply chain. Therefore, the accelerated use of hydrogen requires the qualification of materials (i.e., ensuring they are hydrogen-ready) to guarantee the sustainable and safe implementation of hydrogen technologies. However, there is currently no easily applicable and standardized method to efficiently determine the impact of gaseous hydrogen on metallic materials. The few existing standards describe procedures that are complex, expensive, and only available to a limited extent globally. This article outlines the key milestones towards standardizing an efficient testing method as part of the TransHyDE flagship project. This new approach enables testing of metallic materials in gaseous hydrogen using tubular specimens. It uses only a fraction of the hydrogen required by the traditional autoclave method, significantly reducing costs associated with technical safety measures. Among the topics to be discussed are the factors influencing the test procedure, including geometrical considerations, surface quality, gas purity and strain rate.

The powder metallurgy (P/M) process has been applied to a high strength turbine disk alloy. It is known that P/M alloys show characteristic microstructures such as prior powder boundaries (PPB) compared to microstructures of conventional cast and wrought (CW) alloys. High temperature tensile tests were conducted on CW and P/M processed alloy720Li in order to reveal the effect of temperature and strain rate on deformation behavior and to demonstrate the effect of microstructure derived from P/M process on deformability. The fracture mode of the P/M material changed from grain interior fracture to fracture around large PPB with an increment of strain rate. In addition, samples ruptured at higher temperature showed grain boundary fracture regardless of strain rate. On the other hand, the CW material showed good deformability with chisel point fracture in the entire temperature and strain rate condition range. In the P/M material, melting of grain boundaries occurred at super solvus temperature conditions. Large PPB acts as nucleation site of voids at higher strain rate conditions. Precipitation strengthening by γ’ phase degrades deformability at sub solvus temperature conditions. However, deformability near the solvus temperature and low strain rate condition in as HIPed P/M material increased with fine grain size distribution in spite of the presence of large grains resulting from PPB.

In order to establish a induction bending technique for Ni-based alloy HR6W large pipe, induction bending test was conducted on HR6W, which is a piping candidate material of 700°C class Advanced Ultra-Super Critical. In this study, a tensile bending test in which tensile strain was applied and a compression bending test in which compression strain was applied to the extrados side of the pipe bending part. As the results of these two types of induction bending tests, it was confirmed that a predetermined design shape could be satisfied in both bending tests. In addition, the wall thickness of the pipe was equal to or greater than that of the straight pipe section in compression bending. Therefore, if compression bending is used, it is considered unnecessary to consider the thinning amount of the bent portion in the design. Next, penetrant test(PT) on the outer surface of the bending pipes were also confirmed to be acceptable. Subsequently, metallographic samples were taken from the outer surface of the extrados side, neutral side and intrados side of the pipe bending portion. Metallographic observation confirmed that the microstructures were normal at all the three selected positions. After induction bending, the pipe was subjected to solution treatment. Thereafter, tensile tests and creep rupture tests were carried out on samples that were cut from the extrados side, neutral side and intrados side of the pipe bending portion. Tensile strength satisfied the minimum tensile strength indicated in the regulatory study for advanced thermal power plants report of Japan. Each creep rupture strength was the almost same regardless of the solution treatment conditions. From the above, it was possible to establish a induction bending technique for HR 6W large piping.

CrMoV cast steels are widely utilized for steam turbine and valve casings, and are subjected to operating and loading conditions which can promote damage mechanisms such as thermal fatigue, creep, erosion, etc. These components are subjected to variable, and sometimes severe conditions because of flexible operation. Therefore, there is a growing need for weld repair techniques including those which do not mandate post weld heat treatment (PWHT), e.g. so-called ‘temper bead’ weld repair. In this study, a simulated weld repair was performed using a temper bead technique. The maximum hardness in the heat affected zone (HAZ) CrMoV steel was ≤400HV. The integrity of the repair methodology was investigated using destructive testing, including hardness mapping, Charpy impact tests, tensile tests, low cycle fatigue and cross-weld creep, and the microstructure was assessed using light optical microscopy and scanning electron microscopy (SEM).

Components such as tubes, pipes and headers used in power generation plants are operated in a creep regime and have a finite life. During partial replacement, creep exhausted materials are often welded to virgin materials with superior properties. The aim of this study was to identify a suitable weld filler material to join creep aged X20CrMoV12-1 to a virgin P91 (X10CrMoVNbV9-1) steel. Two dissimilar joints were welded using the gas tungsten arc welding (GTAW) process for the root passes, and manual metal arc (MMA) welding for filling and capping. The X20 and the P91 fillers were selected for joining the pipes. The samples were further heat treated at 755°C to stress relief the samples. Microstructural evolution and mechanical properties of the weld metals were evaluated. The average hardness of X20 weld metal (264 HV10) was higher than the hardness measurement of P91 weld metal (206 HV10). The difference in hardness was attributed to the high carbon content in X20 material. The characterisation results revealed that the use of either X20 or P91 weld filler for a butt weld of creep aged X20 and virgin P91 pipes material does not have a distinct effect on the creep life and creep crack propagation mechanism. Both weld fillers (X20 and P91) are deemed to be suitable because limited interdiffusion (<10 μm) of chromium and carbon at the dissimilar weld interface was observed across the fusion line. The presence of a carbon ‘denuded’ zone was limited to <10 μm in width, based on the results from local measurements of the precipitate phase fractions using image analysis and from elemental analysis using EDS. However the nanoindentation hardness measurements across the fusion line could not detect any ‘soft’ zone at the dissimilar weld interface. The effect of the minute denuded zone was also not evident when the samples were subjected to nanoindentation hardness testing, tensile mechanical testing, Small Punch Creep Test (SPCT) and cross weld uniaxial creep testing.

By utilizing computational thermodynamics in a Design of Experiments approach, it was possible to design and manufacture nickel-base superalloys that are strengthened by the eta phase (Ni3Ti), and that contain no gamma prime (Ni3Al,Ti). The compositions are similar to NIMONIC 263, and should be cost-effective, and have more stable microstructures. By varying the aging temperature, the precipitates took on either cellular or Widmanstätten morphologies. The Widmanstätten-based microstructure is thermally stable at high temperatures, and was found to have superior ductility, so development efforts were focused on that microstructure. High temperature tensile test and creep test results indicated that the performance of the new alloys was competitive with NIMONIC 263. SEM and TEM microscopy were utilized to determine the deformation mechanisms during creep.

High temperature strength of a nickel-based superalloy, Alloy 740H, was investigated to evaluate its applicability to advanced ultrasupercritical (A-USC) power plants. A series of tensile, creep and fatigue tests were performed at 700°C, and the high temperature mechanical properties of Alloy 740H was compared with those of other candidate materials such as Alloy 617 and Alloy 263. Although the effect of the strain rate on the 0.2% proof stress was negligible, the ultimate tensile strength and the rupture elongation significantly decreased with decreasing strain rate, and the transgranular fracture at higher strain rate changed to intergranular fracture at lower strain rate. The time to creep rupture of Alloy 740H was longer than those of Alloy 617 and Alloy 263. The fatigue limit of Alloy 740H was about half of the ultimate tensile strength. Further, Alloy 740H showed greater fatigue strength than Alloy 617 and Alloy 263, especially at low strain range.

This study investigates the impact of adding small amounts of rare earth (RE) elements on the properties and microstructures of SA335P91 steel welds. The RE elements were incorporated into the weld metal using a coating process. The researchers then proposed an optimal RE formula aimed at achieving improved properties and microstructures. To evaluate the effectiveness of this approach, various tests were conducted on both welds with and without RE additions. These tests included tensile testing (both at room and high temperatures), impact testing, metallographic analysis to examine the microstructure, determination of phase transformation points, scanning electron microscopy, and X-ray diffraction. The results revealed that the addition of RE elements has the potential to enhance the properties and modify the microstructure of SA335P91 welds.

Creep-resistant austenitic stainless steels are known to be the potential candidate materials for use as super- and re-heater tubes in ultra-super critical (USC) power plants. Among them, ASTM A213/A213M S30432, a novel 18-8 stainless steel (18Cr- 9Ni-3Cu-Nb-N), has attracted considerable attention from electric industry due to its combined lower cost and more excellent performance in contrast to traditional TP347H steel. More than 10 years of service in Japan laid a solid foundation for the steel being selectable USC boiler materials. Steels of S30432 have been recently developed in China during the past few years. This paper presents the evaluation results of S30432 tubes manufactured by four steel plants in China as well as Sumitomo super304H tubes for comparison. A detailed microstructural analysis of the tubes has been performed by using optical and electron microscope, and mechanical properties of the tubes have been evaluated using hardness testing as well as tensile testing up to 700°C. It was found that the impurity elements, nonmetallic inclusions and grain size of the S30432 tubes were well controlled. TEM observation revealed the microstructural changes for a selected batch of S30432 specimens in condition of hot rolled material, as-extruded tube, solution treated tube and 650°C/1000h aged tube. Most attention was paid to the morphology and distribution of precipitates in the microstructure which should be responsible for the enhanced performance of the steel. Although the hardness of all the evaluated tubes was measured to be similar, they showed more or less differences in tensile properties between each other. Creep rupture testing is still in progress, and the steel might exhibit excellent long-term creep rupture strength at 650°C as was predicted from the currently available testing results.

The effects of pre-strain on creep properties of Alloy 740 have been investigated. Tensile strain was 7.5% and introduced by room temperature tensile test. Creep tests were conducted under 750 degree C, 275-350MPa. Creep rupture life of pre-strained sample decreased by half compared with as-heat treated sample. Creep behaviors of both samples were almost similar in primary creep stage, but onset of creep rate acceleration of pre-strained sample was faster than those of as-heat treated sample. As a result, minimum creep rate of pre-strained sample were two times larger than that of as-heat treated sample. From the observation of ruptured specimen, pre-strained sample had much more sub cracks than as-heat treated sample. On the other hand, microstructure of both samples was also different. There were MC precipitates on grain boundary in both ruptured specimens, but both size and number of MC precipitates were larger in pre-strained sample although creep life of pre-strained sample was shorter than that of as-heat treated sample. In this paper, the difference of creep behavior will be discussed in terms of both the microstructural change and mechanical damage.

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.

Dissimilar metal welds (DMWs) between ferritic and austenitic materials at elevated temperatures have long posed challenges for boiler manufacturers and operators due to their potential for premature failure. As the industry moves towards higher pressures and temperatures to enhance boiler efficiencies, there is a need for superior weld metals and joint designs that optimize the economy of modern boilers and reduce reliance on austenitic materials for steam headers and piping. EPRI has developed a new filler metal, EPRI P87, to enhance the performance of DMWs. Previous work has detailed the development of EPRI P87 for shielded metal arc welding electrodes, gas-tungsten arc welding fine-wire, and its application in an ultra-supercritical steam boiler by B&W. This study examines the weldability of EPRI P87 consumables through various test methods, including Varestraint testing (both trans and spot), long-term creep testing (approximately 10,000-hour running tests), procedure qualification records for tube-to-tube weldments between traditional/advanced austenitic steels and creep-strength enhanced ferritic steels, and elevated temperature tensile testing. Macroscopic examinations from procedure qualification records using light microscopy confirmed the weldability and absence of cracking across all material combinations. The findings demonstrate that EPRI P87 is a weldable alloy with several advantages for DMW applications and highlight that specific weld joint configurations may necessitate the use of high-temperature tensile data for procedure qualifications.

To develop 10-ton class forgings with adequate long-term strength and without segregation defects for A-USC steam turbine rotors, researchers modified the chemical composition of Alloy 706 to improve its microstructure stability and segregation properties. The modified Alloy, named FENIX-700, is a γ' phase strengthened alloy without a γ" phase, and its microstructure stability is superior to Alloy 706 at 700°C, as demonstrated by short-term aging tests and phase stability calculations using the CALPHAD method. A trial disk 1-ton class forging of FENIX-700 was manufactured from a double-melted ingot, with tensile and creep strength of the forging equivalent to that of 10-kg class forgings, indicating a successful trial. Long-duration creep tests were performed using 10-kg class forgings, revealing an approximate 105-hour creep strength at 700°C higher than 100 MPa. Manufacturability tests showed that FENIX-700 performs better than Alloy 706, as evidenced by segregation tests using a horizontal directional solidification furnace and hot workability tests. Microstructure observation and tensile tests on 10,000-hour aged specimens (at temperatures of 650, 700, and 750°C) revealed degradation of tensile strength and yield stress due to coarsening of the γ' phase, but also showed enhanced ductility through aging. The microstructure stability of FENIX-700 at 700°C was confirmed as excellent through microstructure observation of the 10,000-hour aged sample and supporting thermodynamic considerations.

The long-term creep strength of creep strength-enhanced ferritic steels has been overestimated due to changes in the stress dependence of creep rupture life at lower stress levels. To address this, creep rupture strength has been reassessed using a region-splitting analysis method, leading to reductions in the allowable tensile stress of these steels as per Japan’s METI Thermal Power Standard Code in December 2005 and July 2007. This method evaluates creep rupture strength separately in high and low stress regimes, divided at 50% of the 0.2% offset yield stress, which corresponds approximately to the 0% offset yield stress in ASME Grade 122-type steels. In the high-stress regime, the minimum creep rate follows the stress dependence of flow stress in tensile tests, with the stress exponent (n) decreasing from 20 at 550°C to 10 at 700°C. In contrast, the low-stress regime exhibits an n value of 4 to 6 for tempered martensitic single-phase steels, while dual-phase steels containing delta ferrite show an even lower n value of 2 to 4. The significant stress dependence of creep rupture life and minimum creep rate in the high-stress regime is attributed to plastic deformation at stresses exceeding the proportional limit. Meanwhile, creep deformation in the low-stress regime is governed by diffusion-controlled mechanisms and dislocation climb as the rate-controlling process.

Material properties and damage mechanisms exhibit significant variation across weldments. Micro tensile (MT) testing of specimens machined from specific narrow weldment zones is one method to characterize local property variation. Although limited, the literature data on micro-tensile specimen testing reports on low-temperature behavior. However, cross-weld local material data at high service temperatures have not been reported yet. In the present study, MT tests are conducted across similar P22 and P91 steel welds at 550°C and 600°C, respectively. To study deformation mechanisms and the role of surface condition on properties, specimens with different surface conditions (machined, polished, and electropolished) are tested. Two different loading rates of 0.2 mm/min and 0.5 mm/min are used to study the effect of loading rate on deformation and mechanical properties. Variations in weldment material properties are presented as a function of specimen surface conditions and loading speeds. Deformation behavior is studied on the side surfaces of tested micro-tensile specimens using SEM. Deformation is correlated to the microstructural constituent observed on side surfaces.

Investigations on welded joints made from a modified parent material and welding consumables are described. Tubes and pipes with typical dimensions have been welded using different welding processes and consumables (GTAW, SAW, SMAW, modified filler metals). The influence of melting loss and chemical composition of the consumables on the weld performance was studied. Short-term tensile and long-term creep tests on cross weld specimens were carried out in order to evaluate strength. The results obtained so far show that the properties of the welded joints are rather optimistic, it could be assumed that the modified Alloy 617 and the welding consumables used will meet the requirements for use in a plant operated at ultra critical steam conditions with live steam temperatures up to 720°C and pressure up to 300 bar. This allows for first practical applications in test loops of plants. These applications including the Welding Procedure Qualifications are described.

This study examines the deformation behavior of P92 steel (ferritic, 9% Cr) at high temperatures (600°C to 900°C) using isothermal hot tensile tests. Particular focus is placed on the stress-strain behavior around its alpha-gamma transition temperature (825°C). Additionally, fire accident simulation heating tests were conducted to assess the integrity of P92 beyond 650°C (relevant for short-term creep) and compare it to stainless steel 1.4404 (potential building material). Finally, microstructural analysis was performed on tested samples, revealing that the martensitic structure with characteristic laths was retained at temperatures up to 750°C.