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• Sliding movement of hot particles along surface can result in material degradation. • The material degradation is combination of high-temperature oxidation and erosion. • Wear rate in metallic specimens is influenced by thermally grown oxide morphology. • Specimens with high chromium content exhibited greater resistance to wear. The use of solid particles as a heat transfer medium is being explored for concentrated solar power plants (CSP) to increase their efficiency by achieving operating temperature >700 °C. During operation, these hot particles are expected to move along the various components within the collector system, resulting in material degradation from a combination of high-temperature oxidation and erosion. In the present study, the performance of candidate materials was evaluated through a series of abrasion erosion experiments at room temperature as well as at 800 °C. Wear in metallic and refractory type materials was investigated using CarboBead® HSP 40/70 particles inside a resistance heated kiln. Cross-sectional scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDS) analysis on the specimens tested at 800 °C determined that the specific wear rate in Inconel 740H and stainless steel 316 metallic specimens was influenced by the thermally grown oxide morphology. High chromium Inconel 740H specimens exhibited greater resistance to wear with a steady state specific wear rate of 1.92E-4 mm3N-1m−1 compared to 5.7E-3 mm3N-1m−1 for Stainless Steel 316. [ABSTRACT FROM AUTHOR]

• The operation of solar tower external receiver is dynamically simulated in Modelica. • The creep-fatigue lifetime of each receiver panel is estimated for three materials. • Temperature spikes due to clouds passages significantly affect the panels lifetime. • Creep-fatigue analysis based on clear-sky operation leads to 30% error on lifetime. This paper proposes a methodology for the dynamic thermal analysis of solar tower external receivers and for the assessment of their lifetime. A dynamic thermal model is implemented in Modelica to assess the temperature distributions of receiver tubes and heat transfer fluid (HTF), considering real weather data with a 10-minute time resolution and PI controllers based mass flow rate control. Three representative days are simulated, and the resulting tubes temperature distributions are used to assess the elastic stresses distributions during the investigated days. Subsequently, the creep-fatigue lifetime of each receiver panel is evaluated accounting for material plasticity and creep-induced stress relaxation. The developed methodology is applied to compare three materials for solar receivers: Inconel 740H, Haynes 230, and Incoloy 800H. Results show that fatigue can be negligible with respect to creep for all investigated materials and the shortest lifetime is obtained for 800H, followed by H230, and 740H. The approximation of a receiver operation for 365 clear-sky days per year leads to errors around 30 % on the lifetime. For 740H and 800H, the damage accumulation during cloudy days with high DNI peaks is greater than during clear-sky days due to the remarkable creep damage originated during temperature spikes which follow clouds passages. H230 instead, accumulates more creep damage during clear-sky operation. This emphasizes the potential of the developed methodology to compare different HTF mass flow rate control strategies as well as receiver geometries and materials, HTFs, and heliostats aiming strategies, accounting for the trade-off between energy yield and receiver lifetime. [ABSTRACT FROM AUTHOR]

Supercritical CO 2 central-receivers must withstand high temperatures and pressures combined with cyclic operation, which makes the solar receiver susceptible to creep-fatigue failure. In this work, a creep-fatigue analysis of a sCO 2 Inconel 740H tubular receiver of a 2 MW e solar tower plant has been accomplished to study the influence of the tube size on the receiver and solar field design. A 2D numerical model of the tubular receiver that accounts for the thermal conduction in both radial and circumferential directions was developed to determine the sCO 2 and wall temperature profile, which is crucial for the creep-fatigue calculations. The receiver flux distribution, which is an input to the model, was obtained with SolarPILOT, while a conventional recompression model was used to calculate the cycle efficiency and inlet temperature to the receiver. Comparison of the results of the 2D model with those of a 1D model showed that the 1D model overestimates the creep fatigue rupture time by two orders of magnitude. Furthermore, the efficiency and costs of the heliostat field and receiver were calculated for different receiver tube sizes. Smaller tubes allowed a higher maximum heat flux leading to smaller receiver and heliostat field designs, which resulted in higher overall efficiency of the power plant and lower material costs. For a design ensuring 25 year receiver lifetime the minimum sCO 2 solar receiver cost, 345 €/kW th , was obtained for the smallest pipe diameter. [Display omitted] • A creep-fatigue analysis of a sCO 2 central-receiver is presented. • Circumferential thermal conduction is not negligible due to the thick receiver tubes. • The cost and thermo-mechanical behavior of the power plant for different receiver tubes size are studied. • Smaller tubes result in better performance and lower costs of sCO 2 tower power plants. [ABSTRACT FROM AUTHOR]

Low cost stainless steels (SS) with high performance, especially with good mechanical properties and oxidation resistance at high temperature, are continuous pursuit for manufacture industry. To achieve good combination of mechanical properties, oxidation resistance and cost, nanoprecipitate strengthened alumina forming austenitic (NSAFA) SS have been developed by removing expensive elements such as Nb and Ta, lowering Ni concentration in Fe–26Ni–15Cr-1.5Mo-(2.8–3.5)Al-(1–2.35)Ti (wt.%) alloys. Microstructure investigation demonstrates that NSAFA SS are strengthened by L12 structured (Ni, Fe)3(Al, Ti) γ′ nanoprecipitates with area fraction around 25%–40%. Oxidation experiments (750 °C/480 h) in air demonstrates that all the NSAFA steels have alumina forming ability. In addition, it is found that Al and Ti are all necessary for γ′ nanoprecipitates, but additional Al and Ti facilitate formation of the Ni2AlTi phase. To ensure high volume fraction of γ′ nanoprecipitates and reduce Ni2AlTi phase, 2.8–3.0 wt% Al and 1.8–2.35 wt% Ti are preferred. Benefit from the high volume fraction of γ′ nanoprecipitates, yield strength of NSAFA steels is much higher than that of Fe-base superalloy (e.g., A286) and can compare to some Ni-base superalloys (e.g., Hayners 282 & Inconel 740H).

The increasing performance requirements of power plant designs, such as advanced-ultra supercritical (A-USC), require the use of Ni-based superalloys to replace high-strength, ferritic-martensitic steels for components subjected to temperatures above 898 K (625 °C) and for austenitic stainless steels components at temperatures above 973 K (700 °C). To date, commercial Ni-based superalloy INCONEL 740H has been shown to be appropriate for use in A-USC power plants as boiler components in a wrought product. However, large complex components in boilers as well as other casings in the turbine and valve chest require castings of a thick-wall nature. Using the alloy in its cast form would be significantly valuable in terms of range of component size, geometry and complexity. Previous investigations revealed short creep lives from cast INCONEL alloy 740H. In this investigation, an alternative casting route that utilized a melt procedure resulting in a fine-grain casting, and in conjunction with a computationally optimized homogenization heat treatment, not only controlled the grain size and grain boundary structure but minimized chemistry variability and segregation. A primarily equiaxed and homogenous grain size distribution was obtained from this approach with better repartition of M23C6 carbides along the grain boundaries. Furthermore, better than 38 pct increase was obtained for this material in comparison to the creep life obtained from the best performing conventionally cast material. More importantly, the fine-grain homogenized (FGH) casting route resulted in the Larson–Miller plot for this material that coincided with that of wrought alloy 740. At low creep stresses (with a test still in progress), the FGH casting is resulting in higher values of the LMP than the wrought alloy. [ABSTRACT FROM AUTHOR]

Combinations of pure metals –Alloy 316H, Haynes 230, Inconel 625, Inconel 740H and Incoloy 800H– are analysed as functionally graded materials (FGM) to test their potential suitability for the solar central receiver tubes manufacturing, aiming to reduce their elastic stresses under operation to extend their lifetime, and to increase the solar power tower thermal efficiency. The proposed FGMs rely on a circumferential variation of their properties, with one material at the tube front side and the other at the rear one. The analytical investigation of the complete receiver shows lower elastic stresses at the tubes middle length when two materials with very disparate properties are combined since the temperature profile presents a peak at the front and gradually decreases towards the rear side. The mechanical properties are homogenized if the front material has a greater Young's modulus at a certain temperature and lower thermal expansion coefficient than the rear-side one. Yet, a compromise must be reached since the thermal gradients at the tube ends are almost negligible, and so greater elastic stresses are observed there when two very different materials are combined. Various FGMs tested manage to reduce the overall elastic stresses, with the Inconel 625-Incoloy 800H FGM showing the most promising results and a potential lifetime extension. Moreover, such stress reduction shows room for receiver thermal efficiency and heliostat field optical efficiency improvement: around 0.13% each in the case analysed since the proposed FGM can withstand the concentrated heat flux from more equatorial aiming strategies than the pure metals. • Analysis of FGM combining metals for SPT receiver tubes manufacturing. • Circumferential gradation of the material pair properties, from front to back. • Critical stress improved: thermal profile evens disparate material pairs properties. • Inc625-Inc800H FGM shows stress descends that can improve the tubes durability. • The FGM can endure more concentrated heat fluxes, increasing receiver efficiency. [ABSTRACT FROM AUTHOR]

Solidification cracking is a major obstacle when joining dissimilar alloys using additive manufacturing. In this work, location-specific solidification cracking susceptibility has been investigated using an integrated computational materials engineering (ICME) approach for a graded alloy formed by mixing P91 steel and Inconel 740H superalloy. An alloy mixture of 26 wt.% P91 and 74 wt.% Inconel 740H, with high configurational and total entropy, was fabricated using wire arc additive manufacturing. Microstructure characterization revealed intergranular solidification cracks in the FCC matrix, which increased in length along with the enrichment of Nb (~27 to 56 wt.%) and Cu (~87 wt.%) in the middle and top regions. DICTRA simulations to model location-specific solidification cracking susceptibility showed that the top region with the highest cooling rate (270 K/s) has the highest solidification cracking susceptibility in comparison with the middle and bottom regions. This is in good agreement with the experimentally observed varying crack length. From Scheil simulations, it was deduced that enrichment of Nb and Cu affected the solidification range as high as ~77%, in comparison with the matrix composition. The overall solidification cracking susceptibility and freezing range was highest for the 26 wt.% P91 alloy amongst the mixed compositions between P91 steel and 740H superalloy, proving that solidification characteristics play a major role in alloy design for additive manufacturing.

Next generation Concentrating Solar Power (CSP) plants utilizing solid particles as the heat transfer medium (HTM) are expected to achieve greater operational efficiencies. However, the erosion from the solid particles can cause significant damage to component materials. Low particle speeds (1–2 m/s) are proposed as a means of preventing excessive damage to containment materials. Within the current investigation, solid particle erosion of three potential containment materials, stainless-steel grade 316L, a nickel alloy (Inconel 740H), and a refractory material (Tufcrete 60 M), are examined at a particle impact speed of 1.6 m/s. CarboBead-HSP 40/70 particles, a candidate HTM for CSP systems, impact the specimens at a relatively high impact angle of 60° based on containment design criteria. Experiments were conducted at ambient temperature and 800 °C to examine erosion of the materials at very low impact speeds and determine how temperature effects erosion. Initial results revealed surprising outcomes inconsistent with available literature: ambient temperature erosion of SS316L is an order of magnitude lower than IN740H, but erosion (erosion-corrosion) of SS316L at 800 °C surpasses IN740H by two orders of magnitude. Results suggest removal of oxide layers (erosion-corrosion) is responsible for the significant erosion that was observed even at these low particle speeds. [Display omitted] • HSP 40/70 particles erode Inconel 740H faster than SS316L at ambient temperature. • HSP 40/70 particles erode SS316L faster than Inconel 740H at 800 °C. • Oxide layer removal affects SS316L greater than Inconel 740H at 800 °C. • HSP 40/70 particle diameter decreases at 800 °C. [ABSTRACT FROM AUTHOR]

The article discusses the effect of high temperature aluminizing processes on microstructure and corrosion resistance in 0.1 M Na 2 SO 4 solution at room temperature of the IN 740H nickel superalloy. The aluminizing processes were carried out by chemical vapor deposition (CVD): (a) in AlCl 3 vapors, (b) in a mixture of AlCl 3 +ZrCl 3 vapors under a hydrogen atmosphere as carrier gas under the same conditions (process temperature −1040 °C, time-7h under reduced pressure of 150 hPa). Observations of the surface of the produced layers showed that the additional modification of the aluminizing process with zirconium affects the morphology of the layers formed as compared to the unmodified aluminizing process. It was also found that the prepared aluminide layers unmodified and modified additionally with zirconium improve corrosion resistance of the Inconel 740H nickel superalloy. On the basis of the conducted tests, it was also found that after high-temperature aluminizing processes in the IN 740H substrate precipitates are formed at the grain boundaries containing carbide forming elements such as: Cr, Ti, Nb, which may form secondary carbides, e.g. M 6 C, M 7 C 3. • Microstructure of the NiAl coatings manufactured by CVD process. • Coatings produced by this methods exhibits good adhesion to the substrate. • The coatings exhibit positive effect on corrosion resistance of IN 740H alloy. [ABSTRACT FROM AUTHOR]

Solidification cracking is a major obstacle when joining dissimilar alloys using additive manufacturing. In this work, location-specific solidification cracking susceptibility has been investigated using an integrated computational materials engineering (ICME) approach for a graded alloy formed by mixing P91 steel and Inconel 740H superalloy. An alloy mixture of 26 wt.% P91 and 74 wt.% Inconel 740H, with high configurational and total entropy, was fabricated using wire arc additive manufacturing. Microstructure characterization revealed intergranular solidification cracks in the FCC matrix, which increased in length along with the enrichment of Nb (~27 to 56 wt.%) and Cu (~87 wt.%) in the middle and top regions. DICTRA simulations to model location-specific solidification cracking susceptibility showed that the top region with the highest cooling rate (270 K/s) has the highest solidification cracking susceptibility in comparison with the middle and bottom regions. This is in good agreement with the experimentally observed varying crack length. From Scheil simulations, it was deduced that enrichment of Nb and Cu affected the solidification range as high as ~77%, in comparison with the matrix composition. The overall solidification cracking susceptibility and freezing range was highest for the 26 wt.% P91 alloy amongst the mixed compositions between P91 steel and 740H superalloy, proving that solidification characteristics play a major role in alloy design for additive manufacturing. [ABSTRACT FROM AUTHOR]

• Coupled erosion-oxidation wear of particles is complex at high temperatures. • Wear rates are highly material, particle, leading edge area, and oxidation dependent. • High nickel alloys exhibited greater resistance to erosion-oxidation wear. Solid particles represent an attractive heat transfer and energy storage media for next generation solar thermal power facilities for their ability to operate at high temperatures without concerns for freezing or corrosion. A potential outstanding issue that still remains for particles as heat transfer media is the wear on components operating with particles flowing (sliding) in, along, or over them. Prior work has shown that even at the low velocities (∼1 cm/s to ∼ 1 m/s) expected in solar thermal facilities, material wear will still occur, and the lifetime and performance of components undergoing wear needs to be well understood. Abrasion erosion occurs when particles come into contact with containment materials at extremely shallow angles, and is likely to be seen in heat exchangers, storage bins, and flow control mechanisms. Additionally, because of the open nature of most of these systems to air, oxidation of metals is also of concern. Here, we show how critical the coupled erosion-oxidation mechanism is to understanding particle/material durability and therefore need for greater in-depth analysis. Abrasive wear experiments are conducted on metals such as SS316 (L and H grades), Inconel 740H, Haynes 230 with different particles (HSP 40/70, Wedload 430, and CarboMax HD) for different operating conditions. Results clearly indicate that oxide spallation and removal is critical to the overall wear seen at high temperatures. [ABSTRACT FROM AUTHOR]

Concentrated Solar Power (CSP) receivers, especially those adopted in Solar Tower (ST) configurations, operate at high temperatures and low stresses making them susceptible to creep damage. Creep modelling used to predict components' life and therefore the accuracy of creep modelling can significantly impact life estimation, prediction of failures, and ultimately the design. The accuracy of creep modelling is highly variable and depends on the choice of model, the choice of fitting functions, and sometimes depends on 'judgement' decisions by the modeller. In this article the Larson-Miller and Wilshire creep models are fitted to three common power generation materials (Grade 22, SS316 and Inconel 740H) with differently sized datasets. Region splitting via activation energy is also examined. The fits are examined for accuracy and bias for stress, temperature and rupture time by a statistical process of bootstrapping to provide a better understanding of the impact of the creep model and dataset size on the design and lifing of CSP receivers. • Considers the impact of dataset size on creep model accuracy and biases. • Examines model accuracy w.r.t. rupture time, stress and temperature. • First paper to use boostrapping to examine the stability and biases of creep. • Traditional creep models and region splitting examined. • Provides authors with awareness of the impact of creep model selection. [ABSTRACT FROM AUTHOR]

Three Ni-based superalloys, Haynes 214, Haynes 230 and Inconel 740H, have been analyzed for their mechanical behavior under conventional industrial hot-processing conditions. These and other comparable alloys are expected to be used in sCO 2 heat exchanger cycles, as proposed in the Advanced Ultra-Supercritical (A-USC) program, due to their unique combination of mechanical, corrosion and creep properties. However, these alloys are difficult to form, specifically for sCO 2 components, such as header pipes and manifolds, requiring optimal forming conditions. In this study, hot-deformation testing was conducted using a Gleeble 3500, to simulate temperatures and strain rates expected during industrial forming operations. The effect of using as-received material on the deformation behavior was analyzed, emulating microstructures under real conditions more accurately. The flow-stress curves for all three alloys showed the occurrence of dynamic recrystallization (DRX) with some signs of dynamic recovery, for the slowest strain rates and higher temperatures. Additionally, the influence of secondary phases on the hot deformation behavior was analyzed. The M 6 C-carbide in Haynes 230 showed the strongest effect, promoting recrystallization necklacing and inhibiting DRX growth. The activation energies for plastic flow were 246, 302 and 217 kJ mol−1, for Haynes 214, Haynes 230 and Inconel 740H, respectively. Hot-processing maps were generated for all alloys and suggested specific domains for optimal workability under the tested conditions. For Haynes 214, that is ε ˙ ≤ 0.1/s and T ≥ 1050 ∘ C. For Haynes 230, it is 1050 ∘ C ≤ T ≤ 1150 ∘ C and 0.01/s ≤ ε ˙ ≤ 0.5/s. For Inconel 740H, it is 1010 ∘ C ≤ T ≤ 1075 ∘ C and 0.01/s ≤ ε ˙ ≤ 0.1/s. ● Haynes 214 needs low force to process, showing little work softening. ● Inconel 740H and Haynes 230 need higher forces, showing strain softening. ● Haynes 214 fully recrystallized and large grain growth earlier than other alloys. ● Haynes 230 shows the most sluggish recrystallization kinetics. ● Haynes 230 has most secondary phases present after deformation. ● Inconel 740 and Haynes 214 show dissolution of precipitates with temperature. [ABSTRACT FROM AUTHOR]

Nickel alloy Inconel 740H, a candidate material for use in ultra-supercritical power plants, is susceptible to solidification cracking during high power deep penetration laser welding. Here we examine how cracking is affected by welding variables and determine the locations where the cracks occur experimentally and theoretically. We use a solidification cracking model to calculate the effects of welding variables on cracking and the locations where the cracks form during high power laser keyhole mode welding of IN 740H. The parameters needed for the cracking model are obtained from a well-tested numerical heat transfer and fluid flow model for keyhole-mode welding. Model predictions of cracking and their locations for different welding conditions are verified by experiments. [Display omitted] • Solidification cracking of nickel alloys during laser welding is poorly understood. • We propose a novel methodology to understand why, when, and where cracking occurs. • A heat and fluid flow and solidification model, and experimental data are used. • Model predictions of cracking locations and conditions are verified by experiments. • The work can be used to prevent weld failures in thick section laser keyhole welds. [ABSTRACT FROM AUTHOR]

A new medium-entropy superalloy was produced based on the compositions of equiatomic CrCoNi and Ni-base superalloy Inconel 740H. Initial alloy design was performed using Thermo-Calc. The aging response and microstructural stability were assessed following heat treatment at temperatures between 600 and 900 °C and durations up to 100 h. Aging from a fully recrystallized state resulted in negligible grain growth and produced γ' and σ phases. The same phases were present after aging from a cold-rolled state, but partially recrystallized microstructures resulted in multi-modal size distributions and heterogeneous spatial arrangements. Room temperature hardness measurements were used to correlate aging conditions with quantitative precipitate measurements and mechanical properties. • New alloy combines medium- and high-entropy alloys with Ni-base superalloys. • γ′ and σ phases precipitate between 700 and 900 °C, grain size stable up to 100 h. • Recrystallization significantly inhibited compared to equiatomic CrCoNi. • Unlike CrCoNi, no deformation twinning observed in precipitate-free condition. • Diffusion pathway differences explain directional coarsening of γ′ precipitates. [ABSTRACT FROM AUTHOR]

Abstract: Creep–rupture experiments were conducted on multiple heats of the nickel-based superalloy Inconel® 740 at temperatures between 923 and 1123K (650 and 850°C). The interactions between chemistry, microstructure, and creep performance were evaluated by analysis of creep data, optical microscopy, electron microscopy, and computational thermodynamics. The data show that grain size has a modest effect on the creep–rupture strength. Computational thermodynamics verified experimental observations of the formation of eta phase as a function of temperature and alloy chemistry, but the kinetics for the precipitation of eta phase did not agree with the experimental findings. Despite the formation of eta phase and the concomitant reduction in volume fraction of gamma prime, the creep resistance of the alloy is insensitive, within the range of chemistries tested, to the volume fraction of gamma prime. The creep ductility was found to increase with test temperature. Precipitation of a large volume fraction of eta phase (greater than 7%) appears to reduce the creep–rupture ductility, but smaller amounts do not produce adverse effects. [Copyright &y& Elsevier]