Search Results for mechanical performance

The next generation of materials and assemblies designed to address challenges in power generation, such as molten salt or supercritical carbon dioxide thermal transfer systems, corrosion, creep/fatigue, and higher temperature operation, will likely be highly optimized for their specific performance requirements. This optimization often involves strict control over microstructure, including homogeneity, grain size, texture, and grain boundary phases, as well as precise alloy chemistry and homogeneity. These stringent requirements aim to meet the new demands for bulk mechanical performance and durability. Some advanced materials, like oxide-dispersion strengthened or high-entropy alloys, necessitate specialized synthesis, fabrication, or welding/joining processes. Traditional methods that involve melting and solidifying can compromise the optimized microstructure of these materials, making non-melting synthesis and fabrication methods preferable to preserve their advanced characteristics. This paper presents examples where solid-phase, high-shear processing has produced materials and semi-finished products with superior performance compared to those made using conventional methods. While traditional processing often relies on thermodynamics-driven processes, such as creating precipitate phases through prolonged heat treatment, high-shear processing offers kinetics-driven, non-equilibrium alternatives that can yield high-performance microstructures. Additionally, examples are provided that demonstrate the potential for more cost-effective manufacturing routes due to fewer steps or lower energy requirements. This paper highlights advances in high-shear extrusion processing, including friction extrusion and shear-assisted processing and extrusion, as well as developments in solid-phase welding techniques like friction stir welding for next-generation power plant materials.

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.

A compositional modification has been proposed to validate an alloy design which potentially eliminates the requirement of post-weld heat treatment (PWHT) while preserving the advantage of mechanical properties in a reduced activation bainitic ferritic steel based on Fe-3Cr-3W-0.2V- 0.1Ta-Mn-Si-C, in weight percent, developed at Oak Ridge National Laboratory in 2007. The alloy design includes reducing the hardness in the as-welded condition for improving toughness, while increasing the hardenability for preserving the high-temperature mechanical performance such as creep-rupture resistance in the original steel. To achieve such a design, a composition range with a reduced C content combining with an increased Mn content has been proposed and investigated. Newly proposed “modified” steel successfully achieved an improved impact toughness in the as- welded condition, while the creep-rupture performance across the weldments without PWHT demonstrated ~50% improvement of the creep strength compared to that of the original steel weldment after PWHT. The obtained results strongly support the validity of the proposed alloy design.

A creep resistant martensitic steel, CPJ7, was developed with an operating temperature approaching 650°C. The design originated from computational modeling for phase stability and precipitate strengthening using fifteen constituent elements. Approximately twenty heats of CPJ7, each weighing ~7 kg, were vacuum induction melted. A computationally optimized heat treatment schedule was developed to homogenize the ingots prior to hot forging and rolling. Overall, wrought and cast versions of CPJ7 present superior creep properties when compared to wrought and cast versions of COST alloys for turbines and wrought and cast versions of P91/92 for boiler applications. For instance, the Larson Miller Parameter curve for CPJ7 at 650°C almost coincides with that of COST E at 620°C. The prolonged creep life was attributed to slowing down the process of the destabilization of the MX and M 23 C 6 precipitates at 650°C. The cast version of CPJ7 also revealed superior mechanical performance, well above commercially available cast 9% Cr martensitic steel or derivatives. The casting process employed slow cooling to simulate the conditions of a thick wall full-size steam turbine casing but utilized a separate homogenization step prior to final normalization and tempering. To advance the development of CPJ7 for commercial applications, a process was used to scale up the production of the alloy using vacuum induction melting (VIM) and electroslag remelting (ESR), and underlined the importance of melt processing control of minor and trace elements in these advanced alloys.

This study evaluates the elevated temperature mechanical performance of 316H stainless steel produced using directed energy deposition (DED) additive manufacturing (AM) from three separate collaborative research programs focused on understanding how AM variables affect creep performance. By combining these studies, a critical assessment of variables was possible including the DED AM method (laser powder and gas metal arc wire), laser power, sample orientation relative to build orientation, chemical composition, and post-processing heat treatment. Detailed microstructure characterization was used to supplement creep and chemistry results to provide insights into potential mechanistic differences in behavior. The study found that sample orientation was a critical variable in determining lower-bound creep behavior, but that in general the lowest creep strength orientation and the lowest creep ductility orientation were not the same. Heat treatment was also an important variable with as-printed materials showing for specific test conditions improved performance and that underlying substructures formed due to inhomogeneous chemical distributions were not completely removed when using standard wrought solution annealing heat-treatments. The chemistry of the final deposited parts differed from the starting stock and may be an important consideration for long-term performance which is not fully appreciated. Overall, the study found that while all the DED materials tested fell within an expected wrought scatter band of performance, the actual creep performance could vary by an order of magnitude due to the many factors described.

Nimonic 263 alloy was selected for gas turbine combustor transition piece due to its excellent high temperature mechanical performance. In present work, Nimonic 263 alloy plate with thickness of 5mm was welded using 263 filler metal by GTAW, then post weld heat treatment of 800℃/8h/air cool was carried out. The hardness and impact toughness of welded joints were measured, and the microstructure evolution after aging at 750℃ for 3000h was investigated by scanning electron microscopy(SEM). The results show that, during the aging process, the hardness of weld metal increases firstly and then decreases. The impact toughness decreases significantly at first and then increase. Furthermore, some fluctuations can be detected in hardness and impact toughness after long-term thermal exposure. The significant decrease in the impact toughness of the aged welded joints mainly results from the precipitation of η phase around grain boundary and intergranular MC phase. The hardness of weld metal increases due to the precipitation of more carbides and γ′ phase after 1000h aging, then decreases owing to the growth of γ′ phase after 3000h aging.

Metallographic tests, micro-hardness tests, mechanics performance tests and Energy Dispersion Spectrum (EDS) were conducted for a 2.25Cr-1Mo main steam pipe weldment served for more than 32 years. Microstructural evolution of the 2.25Cr-1Mo base metal and weld metal was investigated. Degradation in micro-hardness and tensile properties were also studied. In addition, the tensile properties of subzones in the ex-service weldment were characterized by using miniature specimens. The results show that obvious microstructural changes including carbide coarsening, increasing inter lamella spacing and grain boundary precipitates occurred after long-term service. Degradation in micro-hardness is not obvious. However, the effects of long term service on tensile deformation behavior, ultimate tensile strength and yield stress are remarkable. Based on the yield stress of micro-specimens, the order of different subzones is: WM>HAZ>BM, which is consistent with the order of different subzones based on micro-hardness. However, the ultimate tensile strength and fracture strain of HAZ are lower than BM.

Diffusion bonded compact heat exchangers have exceptionally high heat transfer efficiency and might significantly improve the performance and reduce the cost of supercritical carbon-dioxide Brayton cycle power plants using high temperature heat sources, like high temperature nuclear reactors and concentrating solar power plants. While these heat exchangers have an excellent service history for lower temperature applications, considerable uncertainty remains on the performance of diffusion bonded material operating in the creep regime. This paper describes a microstructural modeling framework to explore the plausible mechanisms that may explain the reduced creep ductility and strength of diffusion bonded material, compared to wrought material. The crystal plasticity finite element method (CPFEM) is used to study factors affecting bond strength in polycrystals mimicking diffusion bonded microstructures. Additionally, the phase field method is also employed to simulate the grain growth and recrystallization at the bond line to model the bonding process and CPFEM is used to predict the resulting material performance to connect processing parameters to the expected creep life and ductility of the material, and to study potential means to improve the structural reliability of the material and the resulting components by optimizing the material processing parameters.

The microstructural evolution of the Ni-based superalloy CMSX-4 including the change in gamma prime size and distribution and the degree of rafting has been examined in detail using field emission gun scanning electron microscopy (FEGSEM) and transmission electron microscopy (TEM) after high temperature degradation and rejuvenation heat treatments. The relationship between the microstructure, mechanical properties and the applied heat treatment procedures has been investigated. It is shown that there are significant differences in the rafting behaviour, the size of the ‘channels’ between the gamma prime particles, the degree of rafting and the size of the tertiary gamma prime particles in each of the different microstructural conditions studied. Chemical segregation investigations were carried out to establish the cause of reduced mechanical properties of the rejuvenated sample after high temperature degradation compared to an as-received sample after the same degradation procedure. The results indicate that although the microstructure of as-received and rejuvenated samples were similar, the chemical segregation was more pronounced in the rejuvenated samples, suggesting that chemical segregation from partitioning of the elements during rejuvenation was not completely eliminated. The aim of this research is to provide greater understanding of the suitability of rejuvenation heat treatments and their role in the extension of component life in power plant applications.

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.

A significant research and development effort is underway to support the qualification of Alloy 709 as a Class A construction material in the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, Section III, Division 5, High Temperature Reactors. This initiative includes a comprehensive Alloy 709 code qualification plan aimed at generating extensive material testing data crucial for compiling the code case data package. The data package is essential in establishing material-specific design parameters for Alloy 709 to be used as Section III, Division 5 Class A construction material for fast reactors, molten salt reactors and gas-cooled reactors. An ASME Section III, Division 5 material code case requires the evaluation of mechanical properties from a minimum of three commercial heats, covering anticipated compositional ranges. A key part of the data package involves fatigue and creep-fatigue testing at elevated temperatures, needed for developing the fatigue design curves and the damage envelope of the creep-fatigue interaction diagram (D-diagram). This paper summarizes the strain-controlled fatigue testing on three commercial heats of Alloy 709 at 760 and 816°C with strain ranges between 0.25% and 3%. The fatigue failure data are used to generate a preliminary fatigue design curve. Additionally, the creep-fatigue testing results at 816°C with tensile hold times of 10, 30, and 60 minutes are presented in support of developing the D-diagram for Alloy 709.

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.

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.

More efficient, sustainable, flexible and cost-effective energy technologies are strongly needed to fulfil the new challenges of the German “Energiewende”. For a reduction of consumed primary resources higher efficiency steam cycles with increased operating parameters, pressure and temperature, are mandatory. Hence, advanced materials are needed. The present study focuses on stainless, high strength, ferritic (non-martensitic) steel grades, regarding thermal treatment effects on particle evolution. The heat treatment includes variations, e.g. a two phase pre heat treatment. Effects of the treatment were analysed and connected to creep performance. Experiments at differently heat treated materials show promising improvement of creep performance. These results can be linked to the stability and evolution of strengthening Laves phase particles.

An innovative additively manufactured gradient composite transition joint (AM-GCTJ) has been designed to join dissimilar metals, to address the pressing issue of premature failure observed in conventional dissimilar metal welds (DMWs) when subjected to increased cyclic operating conditions of fossil fuel power plants. The transition design, guided by computational modeling, developed a gradient composite material distribution, facilitating a smooth transition in material volume fraction and physical properties between different alloys. This innovative design seeks to alleviate structural challenges arising from distinct material properties, including high thermal stress and potential cracking issues resulting from the thermal expansion mismatch typically observed in conventional DMWs. In this study, we investigated the creep properties of transition joints comprising Grade 91 steel and 304 stainless steel through a combination of simulations and creep testing experiments. The implementation of a gradient composite design in the plate transition joint resulted in a significant enhancement of creep resistance when compared to the baseline conventional DMW. For instance, the creep rupture life of the transition joint was improved by > 400% in a wide range of temperature and stress testing conditions. Meanwhile, the failure location shifted to the base material of Grade 91 steel. Such enhancement can be primarily attributed to the strong mechanical constraint facilitated by the gradient composite design, which effectively reduced the stresses on the less creep-resistant alloy in the transition zone. Beyond examining plate joints, it is crucial to assess the deformation response of tubular transition joints under pressure loading and transient temperature conditions to substantiate and demonstrate the effectiveness of the design. The simulation results affirm that the tubular transition joint demonstrates superior resistance compared to its counterpart DMW when subjected to multiaxial stresses in tubular structures. In addition, optimization of the transition joint’s geometry dimensions has been conducted to diminish the accumulated deformation and enhance the service life. Lastly, the scalability and potential of the innovative transition joints for large-diameter pipe applications are addressed.

In dissimilar welds between martensitic stainless steel F6NM and nitrogen-strengthened austenitic stainless steel FXM-19, type 209 austenitic welding consumables are used to align with the mechanical properties and chemical composition of FXM-19, with F6NM welds requiring post-weld heat treatment (PWHT) to restore ductility and toughness, raising concerns about sigma embrittlement in ER209 butter welds. This study investigated the mechanical properties and microstructure of F6NM+FXM-19 dissimilar welds, finding no detrimental sigma phase formation in the butter (PWHT) and groove weld metal (as welded) across various welding processes, indicating no sigma phase transformation due to PWHT. Submerged arc welding (SAW) and gas tungsten arc welding (GTAW) demonstrated good mechanical properties, while Gas Metal Arc Welding with 100% Ar gas shield (GMAW 100% Ar) could not be properly evaluated due to weld defects. SAW and GTAW were deemed suitable for this dissimilar weld joint, with several welding processes providing acceptable results using ER209 filler material for fabricating pressure vessels requiring F6NM to XM-19 joints.

Additive manufacturing (AM) is a process where, as the name suggests, material is added during production, in contrast to techniques such as machining, where material is removed. With metals, AM processes involve localised melting of a powder or wire in specific locations to produce a part, layer by layer. AM techniques have recently been applied to the repair of gas turbine blades. These components are often produced from nickel-based superalloys, a group of materials which possess excellent mechanical properties at high temperatures. However, although the microstructural and mechanical property evolution during the high temperature exposure of conventionally produced superalloy materials is reasonably well understood, the effects of prolonged high temperature exposure on AM material are less well known. This research is concerned with the microstructures of components produced using AM techniques and an examination of the effect of subsequent high temperature exposures. In particular, the paper will focus on the differences between cast and SLM IN939 as a function of heat treatment and subsequent ageing, including differences in grain structure and precipitate size, distribution and morphology, quantified using advanced electron microscopy techniques.

This study addresses the welding challenges encountered when joining Haynes 282, a heat-resistant superalloy, to 3.5NiCrMoV high-strength low alloy steel (HSLA) for advanced power plant applications, particularly in thick-section components like rotors. The project demonstrated successful thick-section dissimilar metal welding up to 76 mm (3 in.) using two techniques: keyhole tungsten inert gas welding and conventional gas tungsten arc welding with Haynes 282 filler metal. Various groove weld geometries were evaluated, supported by computational weld modeling to predict and minimize weld distortion. The results validate these welding approaches for critical power plant components requiring both high-temperature performance and cost-effectiveness.

Diffusion aluminide coatings have been evaluated as a strategy for improving the oxidation resistance of austenitic and ferritic-martensitic (FM) steels, particularly in the presence of steam or water vapor. The objective was to evaluate the strengths and weaknesses of these coatings and quantify their performance and lifetime. Long-term diffusion and oxidation experiments were conducted to study the behavior of various model diffusion coatings and produce a better data set for lifetime predictions. The key findings are that (1) thin coatings (<50μm) with relatively low Al contents appear to be more effective because they avoid high thermal expansion intermetallic phases and have less strain energy to nucleate a crack; and (2) the low Al reservoir in a thin coating and the loss of Al due to interdiffusion are not problematic because the low service temperatures of FM steels (<600°C) and, for austenitic steels at higher temperatures, the phase boundary between the ferritic coating-austenitic substrate inhibits Al interdiffusion. Unresolved issues center on the effect of the coating on the mechanical properties of the substrate including the reaction of N in the alloy with Al.

Delivered condition of Inconel740H specified in ASME Code Case 2702 is solution heat treated and aged condition, fabricating performances are also based on the condition, and a re-annealing and aging shall be performed if cold forming to strains is over 5%. These almost harsh requirements bring great inconvenience for its engineering practice and utilization. The comparative bending tests on 740H tubes in solution heat treated + aged condition and solution heat treated condition were performed, and the rules’ reasonability of the specification on delivered condition was discussed and analyzed from point view of deformability and weldability in the paper. The bending test results showed that tube bent was difficult because of its high strength and limited deforming capacity in solution heat treated + aged condition. Therefore, the material supplied in the solution condition may be better from fabricating points. Whether re-solution for the bent tube is performed after bending depends on its bending radius, followed by welding and post weld heat treatment of component (this treatment can also be the aging treatment for annealed sector at the same time), this treating manner can meet regulatory requirements. For solution tubes, however, there are some inconveniences to its engineering application because fewer research studies were carried out on its properties up to now, and no regulations on them were given for the material in the specification. Suggestions are: 1) deeply investigating the properties of tubes in solution condition, including mechanical and fabricating performances, 2) adding the mechanical properties, maximum allowable cold forming to stain without performing re-solution and weld strength reduction factor of solution material to the code case.