Advanced Ultra-supercritical Research Articles

Microstructure and mechanical properties of friction stir welded Haynes 282

Advanced ultra-supercritical (A-USC) steam plants are designed to operate at high temperatures and pressures due to the necessity for higher operational efficiency. The extreme operating conditions of A-USC requires the deployment of precipitation strengthened Ni-base alloys that exhibit elevated temperature strength and good fabricability. Fusion welding of precipitation strengthened nickel alloys lead to solidification cracking in fusion zone and/or liquation cracking in heat affected zone. Therefore, an alternative non-melting welding technique is a necessity to efficiently join nickel alloys. In the current study, friction stir welding, a solid-state joining technique was implemented on a precipitation strengthened nickel-based superalloy, Haynes 282. Detailed microstructural and mechanical properties characterization was carried out. The processed region exhibited wrought, fine-grained microstructure, absence of weld defects such as voids and cracks and absence of elemental segregation. Both hardness and cross-weld tensile tests demonstrated that the weld region was stronger than the base material. And the cross-weld tensile samples failed in the base material. Based on joint efficiency analysis, friction stir welded Haynes 282 outperformed fusion welds.

Effect of heterogeneous microstructure on the tensile and creep performances of cast Haynes 282 alloy

Precipitation-strengthened Ni-based superalloys are leading candidate materials for advanced ultra-supercritical (A-USC) plants with steam conditions up to 760 °C (1400 °F) and 35 MPa (5 ksi). This study evaluates representative specimens from a large casting of Haynes 282 to study the effect of microstructural heterogeneity on the mechanical behavior of this alloy. The tensile test results of cast Haynes 282 over the temperature range 20–816 °C exhibited lower tensile strength and ductility in comparison with the reference wrought Haynes 282. However, the creep rupture tests of the cast alloy below 704–788 °C and 190–431 MPa presented a similar stress-Larson-Miller parameter to that of the wrought material. Microstructural and dislocation characterizations using scanning electron microscopy, conventional transmission electron microscopy, and scanning transmission electron microscopy in conjunction with energy dispersive X-ray spectroscopy were performed to understand the microstructural evolution before and after the mechanical tests. The heterogeneous microstructures of the cast Haynes 282 material, including the coarse grains, potential casting defects, and a bimodal size distribution of γ′ precipitates, were detrimental to the tensile behavior, whereas the coarse-scale grains had a positive effect on the creep performance because diffusional creep was the dominant creep mechanism.

Comprehensive technical review of the high-efficiency low-emission technology in advanced coal-fired power plants

Abstract Advancements in supercritical (SC), ultrasupercritical (USC), and advanced USC coal-fired power plants have been achieved through the development of enhanced materials utilized in advanced steam cycles and through the deployment of advanced emission control systems. These are referred to as high-efficiency low-emission (HELE) technologies, which may solve numerous issues associated with coal-based power generation. There is a clear global transition from subcritical to advanced power plant types and significant R&D work on HELE technologies. Therefore, this comprehensive review covers the latest HELE technology deployment in major coal-consuming countries and their R&D roadmaps to advance HELE technologies. In spite of the various advantages of HELE technologies, there have been numerous technical challenges relevant to achieving the HELE steam conditions and deploying low emission control technologies in the HELE systems. Hence, this review covers the technical challenges and the relevant recent research by using various coal combustion test facilities. The current focus for the progression from USC boilers to advanced USC boilers is a successful demonstration of the developed high-performance alloys under the advanced steam conditions. This review covers the current status of research and development of advanced USC (A-USC) materials and challenges based on the major material research programs.

Study on hot cracking susceptibility of alloy 617M and an analysis on parameters used for its evaluation

Hot cracking study on nickel base superalloy, alloy 617M, a candidate material for Advanced Ultra Supercritical (AUSC) power plant applications, was carried out. The susceptibility of this alloy to solidification cracking was evaluated using varestraint (variable restraint) test set up and parameters like Brittleness Temperature Range (BTR) and Critical Strain rate per Temperature drop (CST) were estimated from the results. The BTR and CST for alloy 617M indicate higher susceptibility to solidification cracking than stabilized and un-stabilized austenitic stainless steels. However, mock-up welding of 400 mm diameter alloy 617M hollow forgings with weld thickness of 95 mm and alloy 617M boiler tubes of ~12 wall thickness made from this alloy did not reveal any significant cracking in the welds (except some isolated micro fissures) and hence, a detailed analysis of the results from varestraint tests was taken up. The study revealed the dual hot cracking behaviour of alloy 617M and the reasons behind such observation is elucidated. The microstructural characterisation of the hot crack tested specimens of alloy 617M revealed eutectics of austenite + carbides rich in Mo and Cr in the backfilled regions of the hot cracks in the fusion zone. The formation of significant fraction of eutectic phases along the interdendritic regions at the end of solidification is the primary reason for the high BTR exhibited by alloy 617M. Further, backfilling of the solidification cracks by this eutectic liquid is enabled under low augmented strain and the same is insufficient to aid backfilling in cracks at high restraints. The mechanism mentioned above was found to be the underlying reason for the dual behaviour revealed by alloy 617M. Hence, it was deduced that BTR and CST determined at saturated strain cannot be effectively used to predict the actual hot cracking behaviour of alloy 617M. A refined criterion-BTR at threshold strain is proposed to be an appropriate solidification cracking susceptibility index for this alloy.

Concentrated Solar Energy with Thermal Energy Storage for Hydrogen Production by Three-Step Thermochemical Water-Splitting Cycles

The high-temperature thermochemical water splitting (TWS) cycles utilizing concentrated solar energy (CSE) and water are the most promising alternatives to produce renewable hydrogen. Here we couple CSE with thermal energy storage (TES) and TWS cycles to best levelize the cost of hydrogen by 2030, due to the synergies with concentrated solar power (CSP), the high technology-readiness-level (TRL) for the upstream thermal energy production and storage, and the medium TRL for the downstream TWS cycles. A similar design of the solar field and tower and TES for CSP advanced ultrasupercritical (AUSC) or supercritical CO2 (sCO2) power cycles for 100 MW of continuous production of electricity, permit the continuous production of hydrogen at ∼2,750 kg/h a day. The predicted cost is 1.058 to 1.197 $/kg H2 by 2030.

Development and Deployment of Welding Technologies for the Indian Sodium-Cooled Fast Reactor and Advanced Ultra-supercritical Thermal Power Programmes

Development and Deployment of Welding Technologies for the Indian Sodium-Cooled Fast Reactor and Advanced Ultra-supercritical Thermal Power Programmes

Development of the matching filler metal for MARBN—new advanced creep resisting alloys for thermal power plant

MARBN alloys, which are anticipated an estimated 25 °C increase in advanced ultra-super critical (A-USC) power plant operating temperature, are expected to be commercially available soon and will partially displace some older materials and lead the market. Two examples are the Japanese alloy, 9Cr-3W-3Co-Nd-B material, which is considered a strong contender in wrought pipework, and the UK’s IBN-1 alloy which currently leads the development in cast steels. Through the courses of two consecutive UK collaborative projects IMPEL and IMPULSE, a matching composition filler metal for welding MARBN alloys in the form of shielded metal arc SMAW electrode has been developed. The design of this filler metal was aimed to optimize the deposit chemical composition hence to provide creep resistance properties matching the base alloys. The weld metal was specifically intended for high integrity structural service at expected temperatures. Accordingly, the minor alloy additions responsible for its creep properties were kept at the middle of the base alloys or at least above the minimum considered necessary to ensure a satisfactory performance. This paper introduces the design, investigation and test results of the matching filler metal. Findings relevant to the microstructure, mechanical properties at ambient and elevated temperatures, including creep properties of the all-weld metal and weld joint made with IBN-1 base alloy, are presented.

Advanced Ultra-Supercritical Coal-Fired Power Plant with Post-Combustion Carbon Capture: Analysis of Electricity Penalty and CO2 Emission Reduction

This article presents the performance analysis of a 700 MW future planned advanced ultra-supercritical (A-USC) coal-fired power plant fitted with post-combustion carbon capture and storage (CCS) technology. The reference A-USC unit without CCS achieves a net efficiency of 47.6% with CO2 emissions of 700 kgCO2/MWh. Relatively to subcritical units, the net efficiency of the A-USC is 8%-pts higher while CO2 emissions are 16.5% lower. For a CO2 removal rate of 90%, the net efficiency of the CCS integrated A-USC unit is 36.8%. The resulting net efficiency loss is 10.8%-pts and the electricity output penalty is 362.3 kWhel/tCO2 for present state CCS technology. The study continues with the assessment of interface quantities between the capture unit and the steam cycle affecting the performance of the A-USC. Improved CO2 absorbents could alleviate the net efficiency loss by 2–3%-pts, and enhanced CO2 compression strategies and advanced heat integration could further reduce the efficiency loss by 0.5–1.2%-pts and 0.4–0.6%-pts, respectively. The total efficiency gain from CCS technology upgrades is estimated at 3.6%-pts, thus bringing down the net efficiency loss to 7.2%-pts and the electricity output penalty to 241.7 kWhel/tCO2.

Microstructure and Precipitation Studies of Gas Tungsten Arc Welded Haynes 282 Superalloy

Improvement in efficiency of energy conversion requires the use of high temperature materials in thermal power plants. This has led to the development of new γ' strengthened nickel based superalloy (Haynes 282). This alloy is used for advanced ultra-supercritical (AUSC) plants which are operated under the service conditions of 760 oC temperature and 35 MPa pressure. Bead on plate gas tungsten arc welding experiments were done with optimized process parameters. Thermal cycle in heat affected zone was measured by K-type thermocouple attached to a data acquisition system. Welding simulations were carried out in simufact welding® by using experimental parameters and thermal field was established. Base metal is characterized with γ solid solution and randomly distributed MC carbides. SEM results showed that the carbides are of MC type. The carbide precipitate distribution correlates with the segregation pattern during solidification of the weld.

Dissimilar Metal Welds used in AUSC Power Plant, Fabrication and Structural Integrity Issues

ABSTRACTIn the current era, the Advanced Ultra Super-Critical (AUSC) plants have been used to substantially increased efficiency with environmental protection. AUSC components boiler header, re-heater and super-heater tube etc. are fabricated by using advanced materials to join by the welding process. Any failure of the power plant can lead to grave consequences, not only of substantial financial losses but additionally immensely treasured and irreparable human life loss. In this review, the dissimilar metal welds (DMWs) have many issues such as metallurgical issue, cracking, carbon migration, welding electrode selection, creep properties, suitable welding process and structure integrity issues which enhanced the failure of weldments. The results of our review showed that DMWs of AUSC power plant are joined by activated flux Tungsten Inert Gas (A-TIG) and Pulse A-TIG process to increase the productivity and cut the significant fraction of cost. However, A-TIG process with Post Weld Heat Treatment (PWHT) improved the mechanical properties and high-temperature creep resistance properties of DMWs. This state of the art is encouraged the researchers about the enormous future scope.

Effect of interlayer on keyhole plasma arc butt welded AISI 304H austenitic stainless steel high temperature boiler material

Advanced Ultra Super Critical (AUSC) technology in power plant and boiler industries were the solutions concerning to exhaust gas emission and conservation of energy. Austenitic stainless steels are used in parts of the boilers with moderately elevated temperatures. In the current research the weldability and mechanical attributes of keyhole plasma arc welded (K-PAW) similar 6 mm AISI 304H plates with nickel interlayer was investigated. Keyhole Plasma Arc Welding (KPAW) is the most common PAW technique which offers complete penetration in single pass welding. X-ray radiographic technique used to assess the weldments and the upshott of interlayer on the mechanical attributes of the weldment are analysed with impact test, tensile test and bend test as per ASTM standards. The results showed that satisfactory weld joints were obtained without any defect formation. Notable enhancement in average absorbed energy and lateral expansion observed in weldment with the addition of interlayer. Ductility and soundness of weldment confirmed by bend test. Ultimate tensile strength of the similar joint observed as 695 MPa which has significant improvement when compared to the base metal.

Precipitation Behavior of Ni-Based Superalloy Alloy 625 for A-USC Power Plants

The age-hardening behavior of the wrought Ni-based superalloy Alloy 625 was investigated in the temperature range between 923 and 1173 K for the application to advanced ultra-supercritical (A-USC) power plants. The carbon content of the alloy was controlled as low as possible to minimize the precipitation of carbides during aging. A two-step increase of hardness was detected for the alloy at temperatures between 1000 and 1100 K; the first increase of hardness results from the precipitation of the metastable γ′′ phase, and the second increase corresponds to the precipitation of the orthorhombic δ phase. In contrast, a single-step increase of hardness was detected below 1000 K derived from the precipitation of γ′′ phase and above 1100 K derived from the precipitation of δ phase. The TTP (time–temperature–precipitation) diagram for the alloy was established on the basis of the results of hardness measurements and microstructure observations, where the nose temperatures of γ′′ and δ phases are determined as 1050 and 1123 K, respectively. The γ′′ particle coarsened along the Ostward ripening. The activation energy for the γ′′ coarsening was evaluated as 202 kJ/mol, which is very close to that for the inter-diffusion of Nb in Ni.

Physical modeling of the M-shaped boiler furnace aerodynamics

In order to increase the thermodynamic efficiency of the Rankine cycle, in Russia, China, the USA, and EU countries, new-generation power units with advanced ultra-supercritical (A-USC) steam parameters are being developed. Using A-USC steam parameters (temperature – 700-760 °C, pressure – 35 MPa) will lead to the need to use expensive nickel steels. A single-shell design of the M-shaped boiler for burning Kuznetsk coal grade TP with solid slag removal (SSR) was proposed to reduce the total length of the main steam pipelines. A solid fuel combustion scheme with direct-flow burners, nozzles and system of vertical-horizontal tangential flames (VHTF) has been developed for this boiler, which will ensure: emissions of nitrogen oxides into the atmosphere at a level lower than standard; continuous operation of the boiler in the absence of slagging of the furnace walls and overheating of the pipe metal; a high degree of burnout of coal dust, with the provision of the standard value of the mechanical underburning of fuel; long minimum boiler load at a level not exceeding 40% of the nominal one according to the conditions of coal dust combustion stability.

Basic Microstructural Characterization of Second Phases in Homogeneous Weld Joint Made of X6CrNiNbN25-20 Steel After Long-Term Exposure Time at 973 K

Abstract New blocks of fossil fuel power plants designed for steam temperatures above 873 K require advanced stainless steels as material for superheater or reheater systems. Weld joints are critical parts in fossil power units. Great attention is paid to the exploitation of new steel grades with higher material properties. In the austenitic steels family, the superior grade is undoubtedly HR3C steel (X6CrNiNbN25-20). A detailed knowledge on stability and microstructure composition during thermal exposure of the weld joints made from HR3C is necessary in order to use them in fossil fuel power plants with ultrasupercritical (USC) and new advanced ultrasupercritical (A-USC) steam parameters. The aim of the paper is to identify critical minor phases in HR3C steel, which allow acceleration of creep damage. The σ-phase and rough carbides M23C6 type is considered as such a phases in this steel. In this study, the σ-phase is identified and studied in more detail in relation to the development of creep damage at 973 K. Experimental material of the homogeneous HR3C weld joints in two states: in the as-welded state (AW) and after the postweld heat treatment (PWHT). Weld joints were manufactured by orbital welding using the gas tungsten arc welding (GTAW) method, heat input Qs = 1600 J/mm, interpass 423 K, three beads. Nickel-base alloy UTP A6170 Co (equivalent to Thermanit 617) was used as a filler material. The PWHT was carried out at the temperature of 1503 K for 15 min. Stress rupture tests were performed on the cross-weld (CW) joints of tubes ø 38 × 6.3 mm at 973 K with times to rupture up to nearly 22,000 h. The polished surface of the longitudinal sections was subjected to color etching in Murakami (30 g K3(Fe(CN)6), 30 g KOH, 60 ml H2O) in order to highlight the σ-phase. Several microscopic techniques were used for the study. The results were supplemented by creep, grain size, and microhardness data hardness vickers (HV) 0.5. The PWHT specimens exhibited an average σ-phase size of approximately 5 μm as well as AW specimens in specimens with short time to rupture (tr). However, tr such as 20,000 h, the average σ-phase size already reached dangerous border 10 μm. The AW specimens as opposed to the PWHT specimens did not show a noticeable growth of austenitic grains in the heat-affected zone (HAZ). In specimens after PWHT, the average grain size in HAZ was more than twice that of the body material (BM). It is worth noting that creep ductility values of specimens in the state after PWHT are very low, which is the result of coarse-grained structure in the HAZ and accelerated precipitation of σ-phase particles along grain boundaries during creep at 973 K.

The effect of η phase precipitates on the creep behavior of alloy 263 and variants

Phase stability is an important design parameter for Ni-based superalloys to be used in future advanced ultra-supercritical (AUSC) power plants as exposure times in this type of environment are considerable. In this investigation, microstructures based on candidate alloy 263 were obtained with varying amounts of η precipitates using isothermal exposure at 800 °C for times ranging from 1000 h to 10,000 h. The effect of η phase stability on the creep properties was determined using creep specimens isothermally aged at 800 °C for 8 h, 3000 h, 5000 h and 10,000 h prior to creep screening. The creep life was found to exponentially decrease with increasing density of η phase while the elongation to failure was found to increase. Furthermore, the minimum creep rate was related to the density of η phase; a relationship that did not depend on the alloy formulation. Modification to the Ti and Al concentrations slowed down the γ′ to η transformation while doubling the γ′ fraction after standard heat treatment. By modifying the Ti and Al content, and thereby improving γ′ stability over η, the creep lives of specimens isothermally aged for up to 5000 h were greater than that of the nominal alloy in its standard aged condition.

SPECIAL METALS INCONEL 740H

Abstract Special Metals Inconel 740H (UNS N07740) is a precipitation hardenable, nickel-base superalloy that offers a unique combination of high strength and creep resistance at elevated temperatures along with resistance to coal ash corrosion. This alloy was originally targeted for use in advanced ultra-supercritical (A-USC) power plants. This datasheet provides information on composition, physical properties, elasticity, and tensile properties. It also includes information on corrosion resistance as well as forming, heat treating, and joining. Filing Code: Ni-762. Producer or source: Special Metals Corporation.

Effect of Nb/(Ti + Al) ratio on the phase stability and tensile properties of GH984G alloy

GH984G alloy is a precipitate-strengthened Ni–Fe based alloy considered as boiler materials in next generation of advanced ultra-supercritical (A-USC) coal fired power plant. In this paper, the effect of Nb/(Ti + Al) ratio on the phase stability and tensile properties of GH984G alloy was investigated. Detailed investigations reveal that the major precipitates of the alloys with different Nb/(Ti + Al) ratios are all γ', MC and M23C6. The higher Nb/(Ti + Al) ratio retards γ′ nucleation. During thermal exposure, γ' coarsening behavior of the alloys with different Nb/(Ti + Al) ratios all follows Ostwald ripening kinetics. However, the higher Nb/(Ti + Al) ratio accelerates γ' coarsening at early stage, but its influence decreases with increasing thermal exposure time. The reasons are attributed to the influence of Nb/(Ti + Al) ratio on γ/γ' misfit and the diffusion behavior of γ' forming elements. The thermal stability of MC carbide and the precipitation and growth of M23C6 carbide are no dependence on Nb/(Ti + Al) ratio. During thermal exposure, the higher Nb/(Ti + Al) ratio has a negative effect on yield strength, but its influence decreases with increasing thermal exposure time. The reasons are attributed to the influence of Nb/(Ti + Al) ratio on the precipitation behavior of γ′ phase. In addition, Nb/(Ti + Al) ratio has no notable influence on ductility and fracture behavior.

An Alternative Casting Technique to Improve the Creep Resistance of Cast INCONEL Alloy 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.

Solving Recent Challenges for Wrought Ni-Base Superalloys

This paper reviews the status of technology in design and manufacture of new wrought polycrystalline Ni-base superalloys for critical engineering applications. There is a strong motivation to develop new alloys that are capable of operating at higher temperatures to realize improvements in thermal efficiency, which are necessary to achieve environmental targets for reduced emissions of harmful green-house gases. From the aerospace sector, the development of new powder metallurgy and ingot metallurgy alloys is discussed for disk rotor and static applications. New compositions for powder metallurgy contain about 50 to 55 pct of gamma prime (γ′) strengthening precipitates to ensure components operate successfully at temperatures up to 788 °C (1450 °F). In contrast, new compositions for ingot metallurgy aim to occupy a design space in temperature capability between Alloy 718 and current powder alloys that are in-service, and show levels of γ′ of about 30 to 44 pct. The focus in developing these alloys was design for manufacturability. To complement the aerospace developments, a review of work to understand the suitability of candidate alloys for multiple applications in Advanced-Ultra Supercritical (AUSC) power plants has been undertaken by Detrois, Jablonski, and Hawk from the National Energy Technology Laboratory. In these power plants, steam temperatures are required to reach 700 °C to 760 °C. The common thread is to develop alloys that demonstrate a combination of high-temperature properties, which are reliant on both the alloy composition and microstructure and can be produced readily at the right price. For the AUSC applications, the emphasis is on high-temperature strength, long-term creep life, phase stability, oxidation resistance, and robust welding for fabrications. Whereas for powder disk rotors in aircraft engines, the priority is enhanced resistance to time-dependent crack growth, phase stability, and resistance to environmental damage, while extending the current strength levels, which are shown by existing alloys, to higher temperatures.

Development of Test Protocol for Accelerated Creep and Transient Thermo-mechanical Testing of AUSC Steam Turbine Rotor in High-Temperature Spin Test Rig

Rotor constitutes one of the critical components of steam turbine. Steam turbine rotor undergoes severe thermal transients during start up and shut down. When steam turbine starts up, temperature of rotor surface gradually rises with the incoming steam temperature. During this process temperature rise of central part lags behind that of rotor surface, resulting in extreme compressive stresses developed within the rotor. Similarly, the rotor surface is subjected to tensile stresses during the cooling down in the shutdown process. After a certain number of cycles, the surface or core of rotor may generate cracks due to low cycle fatigue (LCF) phenomena. Steam turbine unit for advanced ultra-super-critical (AUSC) power plant works under very high pressure and temperature, constituting a severe working environment. For steam turbine rotor, Alloy 617 M is planned to be used for the first time. Hence, it is required to test the rotor in rotating condition in a high-temperature spin rest rig before putting into actual operation. To test the AUSC turbine rotor in a high-temperature test rig, it is required to simulate the radial temperature gradient to achieve the required thermal stress in the rotor. No standard test protocol exists for the accelerated LCF testing for the rotor. A test protocol is developed for Accelerated Creep and Transient Thermo-mechanical testing of the AUSC steam turbine rotor at high temperature taking the guideline from the existing ASME standard.