Ultra-supercritical Power Plants Research Articles

Investigation of reheat cracking susceptibility in CGHAZ of modified T23 steel

T23 steel (2.25Cr–1Mo–1.6W–0.24V–0.05Nb, in wt.%) has been widely used as water walls in ultra-supercritical (USC) power plants. However, high reheat cracking susceptibility of T23 steel hazarded the safety of USC power plants. It is aimed to improve the reheat cracking susceptibility of T23 steel from the perspective of modifying chemical composition. Gleeble-3800 thermal simulator was used to simulate the coarse-grained heat-affected zone (CGHAZ) in T23 and modified T23 steels via thermal simulation of welding, and then evaluate the reheat cracking susceptibility in CGHAZ of T23 and modified T23 steels by the isothermal slow strain rate tensile test. The microstructure was systematically investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy. The results show that the reheat cracking susceptibility of modified T23 steel is extremely improved. Compared with T23 steel, there are fewer M23C6 particles at the grain boundaries in modified T23 steel, and the degree of intergranular weakening is smaller. At the same time, not only MX in the grain interiors but also the solid-solutioned C and W in the matrix decrease in modified T23 steel, leading to the decline of intragranular strengthening. In addition, small grain size is beneficial to the improvement of the reheat cracking susceptibility of modified T23 steel. The decreased number of M23C6 at grain boundaries helps to retard the formation and propagation of reheat cracks because of decreased denuded zones.

Conditioning a stable sustainability fix of ‘ungreen’ infrastructure in Indonesia: transnational alliances, compromise, and state’s strategic selectivity

This paper is the first academic attempt to critically bridge the politics of sustainability fix with social conflict theory (SCT) focusing on Indonesia’s foreign-sponsored development of Ultra Supercritical (USC) coal power plants. I examine the contradictory development of ‘green’ projects in Indonesia and specifically unpack: how transnational politico-economic forces have secured a stable sustainability fix for the USC and how they have tamed opposition forces and reshaped governance strategies for intensified accumulation. I attempt to empirically demonstrate how such process unfold through two case studies – Cirebon II developed by Japanese and Korean companies and the Java 7 project funded by China. Albeit each have different alliance formations and strategies, both cases demonstrate that the safeguarding of stable conditions for sustainability fixes of USC power plant development is primarily determined by contestation, conflicts, and compromises between socio-political forces – international fractions of capital, state apparatuses, Indonesia’s PLN and coal oligarchy as well as broader civil society actors. They reshape governing strategies that are ultimately organised through the Indonesian state and react to the selectivity of state strategies which privilege dominant forces. The paper contributes to the existing literature on the political economy of infrastructure and serves to take state transformation into account and to dispel the ‘methodological nationalism’ view that presupposes policy outcome and institutional features are inherent to the mode of capitalism of the investor’s country of origin.

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.

Evaluation and Optimization of Geometric Deviations in Abrasive Water Jet Profile Cutting of Inconel 617

Abstract Inconel 617, an extremely hard-to-machine nickel-based superalloy, is used for high-temperature applications in ultra-supercritical power plants for components like headers, pipes, and turbine blades. Conventional machining of superalloys has been widely explored and is plagued by productivity and quality issues. Nontraditional processing is seen as an effective alternative, and abrasive water jet is a capable method. Optimization involving weighted principal components and a genetic algorithm is detailed for the abrasive water jet process. Water pressure, abrasive mass flow, stand-off distance, and traverse speed are the predominant factors affecting the process. Performance characteristics are material removal rate and surface roughness. Geometric deviations like circularity, cylindricity, perpendicularity, and parallelism, which are mostly neglected, have been investigated. Analysis of variance has been reported indicating the predominant factors. Geometric deviations are consolidated into a geometric index using weighted principal components. Simultaneous multiresponse optimization is performed using multi-objective genetic algorithm and a Pareto-front. From the Pareto-front a geometric index of 0.02036 was obtained at water pressure of 266.03 MPa, standoff distance of 4.27 mm, abrasive mass flow of 0.41 kg/min, and traverse speed of 58.98 mm/min. Material removal rate of 495.42 mm3/min was obtained at water pressure of 266.29 MPa, standoff distance of 4.48 mm, abrasive mass flow of 0.40 kg/min, and traverse speed of 63.11 mm/min. Surface roughness of 6.6364 μm was obtained at water pressure of 266.43 MPa, standoff distance of 4.43 mm, abrasive mass flow of 0.42 kg/min, and traverse speed of 57.48 mm/min. The sets obtained are a compromise between the different output values and provide an optimal parameter setting for the abrasive water jet process. This tool is essential when a large database is available, and it could be used to obtain a set of solutions based on the different machining characteristics that are required from the process.

A mathematical model of the double-pipe system with TBCs application

Following Rankine’s cycle efficiency, steam with ever-higher parameters is used to improve the efficiency of advanced ultra-supercritical power plants. The high steam parameters require the use of expensive high-alloy steels. Therefore, design concepts with reduced investment costs are more and more popular. In the power industry, the use of thermal barrier coatings to protect components exposed to high temperatures is becoming ever more common. The innovative concept is a double-pipe system with a thermal barrier that provides insulation for the primary pipe, in which ultra-supercritical steam flows. On the outside, the pipe is cooled by lower performance steam. The following paper presents a two-dimensional mathematical model of the proposed solution. A set of heat transfer equations allows the determination of the temperature field in the steady and transient-state operation of such a system. The numerical model is compared with the CFD one. The temperature gradient in the inner pipe wall with and without coating was determined. In addition, the response of the wall temperature to the step-change of the steam temperature was investigated. The paper shows that the use of TBCs allows reducing high-alloy steels and improving the handling properties of thick-walled components.

초초임계압 발전소 확대에 따른 CO2 감축잠재량 연구

In this study, the CO₂ emission factors of supercritical and ultra-supercritical power plants considering the transmission stage efficiency were calculated. The CO₂ reduction potential was calculated for the ultra-supercritical power plant, which was introduced up to 40%. The CO₂ emission factor of the supercritical power plant was 1.017 kg CO₂/kWh, while the emission factor of the ultra-supercritical power plant was 0.947 kg CO₂/kWh. Thus, the emission factor of the ultra-supercritical power plant was estimated to be about 7% lower. By calculating the CO₂ emissions according to the supercritical and ultra-supercritical power generation ratio, we found that when the power generation ratio of the ultra-supercritical power generation increased to 40%, the CO₂ emissions decreased by 1.74% (4,906,117 tons of CO₂).

Effect of applied stress on creep properties of Sanicro 25 welded joint made with a composition-matched weld filler metal

Sanicro 25 is a candidate material for reheaters and superheaters used in 650–700 °C advanced ultra-supercritical power plants. A composition-matched weld filler metal has been developed to join Sanicro 25 tubes through tungsten inert gas welding. Creep tests of the Sanicro 25 welded joint were conducted at 973 K under a stress of 180–240 MPa. The creep lifetime decreased with an increase in applied stress. The welded joint crept at 180 MPa fracture at the base metal (BM) in a mixed fracture type, and voids were the main damage form in the BM, which had low ductility in the welded joint. Creep fractures occurred in the weld metal (WM) in an intergranular manner at 200–240 MPa, with wedge cracks representing the main damage form and a lower creep ductility in WM. The distribution and strengthening mechanism of the nano-MX and nano-Cu phases in the WM of joint were close to those of pure BM after creep. The growth of carbides in the WM was restrained by element B during the 180 MPa creep test, which caused increasingly serious carbide coarsening in the BM. The amount of low misorientation angle boundaries in the WM following the creep substantially increased. Besides, the creep deformation of the welded joint at 180 MPa is dominated by BM, and the change in the weld grain shape is small. For tests under 200–240 MPa, the creep deformation of the welded joint was dominated by the weld, and the shape of the weld grain changed significantly.

Effects of nitrogen content on microstructures and tensile properties of a new Ni–Fe based wrought superalloy

The influence of the nitrogen (N) content (0.0013–0.034 wt%) on the microstructures and 750 °C tensile properties of a newly developed Ni–Fe based wrought superalloy (named as HT700T alloy) for ultra-supercritical power plant applications beyond 700 °C was investigated using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) techniques. The results show that the increase in the N content promotes the morphological transformation of the intergranular Cr23C6 carbides from semi-continuous rods to discrete granular particles, and leads to the decrease of the amount of the Cr23C6 carbides. Meanwhile, the increase in the N content promotes the formation of the MX phase and the MX phase more preferentially forms at the grain boundary. The N content has no obvious effects on the morphology of the γ′ phase, but slightly decreases the γ′ fraction. The average grain size continuously decreases with the increase of N content and the proportion of low Σ coincident site lattice (CSL) boundary significantly increases at the 0.0077% N content. The 750 °C tensile properties show that the increase of N content has little effect on the yield and tensile strength of the alloy, while the ductility increases remarkably. A mixture fracture mode of intergranular and transgranular occurs in all samples regardless of the N content. The dominant deformation mechanism is γ′ bypassing via Orowan looping regardless of the N content, while deformation microtwinning and stacking faults shearing the γ′ precipitates are confirmed to be the assistant deformation mechanisms at the 0.0077% N content.

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.

Microstructural change and stress rupture property of Nimonic 105 superalloy for advanced ultra-supercritical power plants

Microstructural change, stress rupture property, deformation and fracture mechanisms of Nimonic 105 superalloy at 750 °C have been studied. Experimental results showed that the stress rupture strength of the alloy at 750 °C for 105 h is about 200 MPa. γ′ precipitates and M23C6 carbides grew gradually with prolonging the rupture time, while no significant change was observed in MC carbide morphology. After stress rupture test at 750 °C and 250 MPa for 23,341 h, a transition from spherical to cuboidal morphology of γ′ precipitates was found, and nearly continuous chains of M23C6 carbides formed on the grain boundary. Orowan looping and strongly coupled dislocation pairs cutting and microtwinning were the dominant deformation mechanisms at 750 °C and 350–450 MPa, while the main deformation mode was Orowan looping at 750 °C and 250 MPa. The failure of the alloy was mainly attributed to the nucleation, growth and interlinkage of voids.

Elastoplastic Fracture Analysis of the P91 Steel Welded Joint under Repair Welding Thermal Shock Based on XFEM

P91 steel is a typical steel used in the manufacture of boilers in ultra-supercritical power plants and heat exchangers in nuclear power plants. For the long-term serviced P91 steel pressurized structures, the main failure mode is the welded joint failure, especially the heat affected zone (HAZ) failure. Repair welding technique is an effective method for repairing such local defects. However, the thermal shock composed of high temperature and thermal stress in the repair welding process will pose a critical loading condition for the existing defects near the heat source which cannot be detected by conventional means. So, the evaluation of structural integrity for the welded joint in the thermal-mechanical coupling field is necessary. In this work, the crack propagation law in the HAZ for the P91 steel welded joint was investigated under repair welding thermal loads. The weld repair model of the P91 steel welded joint was established by ABAQUS. The transient temperature field and stress field in repair welding process were calculated by relevant user subroutines and sequential coupling simulation method. The residual stress was determined by the impact indentation strain method to verify the feasibility of the finite element (FE) model and simulation method. In order to obtain the crack propagation path, the elastoplastic fracture analysis of the welded joint with initial crack was performed based on the extended finite element method (XFEM). The influence of different welding linear energy on the crack propagation was analyzed. The results show that the cracks in the HAZ propagate perpendicular to the surface and tend to deflect to the welding seam under repair welding thermal loads. The crack propagation occurs in the early stage of cooling. Higher welding linear energy leads to larger HAZ and higher overall temperature. With the increase of welding linear energy, the length and critical distance of the crack propagation increase. Therefore, low welding linear energy can effectively inhibit the crack propagation in the HAZ. The above calculation and analysis provide a reference for the thermal shock damage analysis of repair welding process, which is of great significance to improving the safety and reliability of weld repaired components.

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.

Fuzzy K-Means Cluster Based Generalized Predictive Control of Ultra Supercritical Power Plant

This article proposes a fuzzy k-means cluster based generalized predictive control (GPC) method for a 1000 MW ultra supercritical power plant to improve of the boiler combustion efficiency. First, to fully use the statistic characteristic of the historical data, a fuzzy k-mean cluster network (FKN) is well constructed to derive the local linear models, and the nonlinear dynamic process of studied system is elaborately approximated by the fuzzy combination of the local linear models. Then, a global GPC method is proposed to improve the control performance by using the membership of the current FKN. Different from the traditional GPC, the advantage of proposed GPC is that local GPC is fuzzily combined together to achieve the purpose of global GPC by a scheduling algorithm. Finally, an example illustrates that the proposed control strategy can achieve the satisfactory performance.

A critical review on dissimilar welds joint between martensitic and austenitic steel for power plant application

Abstract In this review article, microstructure and mechanical behavior of the dissimilar welded joint (DWJ) between ferritic-martensitic steel and austenitic grade steel along with its application have been summarized in Ultra Super Critical (USC) power plant. Creep-strength enhanced ferritic-martensitic (CSEF/M) P91 steel was developed to sustain at extreme operating conditions of ultra-supercritical (USC) power plants, and later, P92 was developed to achieve better mechanical properties, higher creep-rupture strength and high operating temperature with the reduction in wall thickness as compared to P91 steel. The most common application of P91/P92 material in power plants includes high pressure and high-temperature steam piping, headers, super-heater tubing, and water-wall tubing. The other most commonly used material in the power plants is austenitic stainless steel, i.e., SS 304 L. The austenitic grade stainless steel offers high resistance to corrosion due to the high wt. % chromium and nickel content (18–20 and 8–12, respectively). Due to the low carbon content, the SS 304 L is less sensitive to the sensitization problem and offers excellent weldability. The joining of these dissimilar materials is frequently required in the power generation industry. The current review focuses on the main difficulty associated with dissimilar welding of martensitic P91/P92 and austenitic grade stainless steel. The different chemical composition, mechanical, physical and metallurgical properties of the martensitic P91/P92 and austenitic grade stainless steel leads to the problems such as hot cracking and carbon migration. The other weldability issues are the formation of a brittle intermetallic compound, the formation of soft transaction heat affected zone along with martensitic steel, δ ferrite formation in fusion zone, diffusion related problem, and residual stresses, which necessitates thorough study and qualification of welds. The effect of coarsening of various precipitates such as M23C6 carbides, MX carbonitrides, and effect of laves phase, z-phase, and sigma phase on mechanical property, and creep-rupture strength of DWJ are also discussed in detail. Based on the literature reviewed, it has been found that some of the above-stated problems can be solved by using nickel-based filler wire due to its intermediate physical and mechanical properties. The selection of the proper filler metal is another vital issue in dissimilar welds joint that is also covered in this review article. The reason behind the formation of the unmixed zone, filler deficient region, peninsula, island, beach, migrated grain boundaries, solidified grain boundaries, and solidified subgrain boundaries during DWJ of martensitic P91/P92 and austenitic grade stainless steel is also discussed. The heat treatment is required to eliminate the heterogeneous microstructure during the dissimilar welding. The effect of post-weld heat treatment (PWHT) on the microstructure and mechanical behavior of the DWJ also reviewed. The residual stress developed during the DWJ may cause the premature failure of the components under service, has also been discussed in detail. The effect associated with the residual stress deformation has been reviewed in the different conditions of the DWJ.

The Effects of Co and W on Structural Stability and Mechanical Properties of Austenitic Heat-Resistant Steel Sanicro 25: A First-Principle Study

Sanicro 25 austenitic heat-resistant steel is expected to be used in superheaters and reheaters for ultra-supercritical power plants above 600 °C due to its excellent structural stability and high temperature mechanical properties. In this paper, the effects of Co and W on the structural stability, thermodynamic stability and mechanical properties of Sanicro 25 steel are analyzed by calculating the formation energy, binding energy, Gibbs free energy, elastic constant, Peierls stress and generalized stacking fault energy (GSFE) with first-principles calculation method. By calculating the formation energy, binding energy and Gibbs free energy, it concludes that alloying elements Co and W in Sanicro 25 steel can improve the structural stability and thermodynamic stability. It indicates that W and a small amount of Co can improve the plasticity and ductility of Sanicro 25 steel by calculating the bulk modulus (B), shear modulus (G), Young’s modulus (E), the B/G ratio, Poisson’s ratio and Peierls stress. It is found that when Co and W are far from the stacking fault region, it will promote the formation of partial dislocations and twins in the system, thereby improving its plastic deformation ability and mechanical properties.

An Efficient Robust Predictive Control of Main Steam Temperature of Coal-Fired Power Plant

Regulating performance of the main steam temperature (MST) system concerns the economy and safety of the coal-fired power plant (CFPP). This paper develops an offset-free offline robust model predictive control (RMPC) strategy for the MST system of CFPP. Zonotope-type uncertain model is utilized as the prediction model in the proposed RMPC design owing to its features of higher accuracy, compactness of representation and less complexity. An offline RMPC aiming at the system robustness and computational efficiency is then developed to maintain the desired steam temperature in case of wide operating condition change. The proposed RMPC is realized by two stages: in the first stage, the RMPC law set, which is the piecewise affine (PWA) of the MST system state is designed offline; then in the second stage, the explicit control law is selected online according to the current state. To achieve an offset-free tracking performance, a manipulated variable target observer is employed to update the chosen RMPC law. The control simulations using on-site operating data of a 1000 MW ultra-supercritical power plant show that the proposed approach can achieve satisfactory control performance and online computation efficiency even under complicated operating conditions.

Optimisation and thermo-mechanical analysis of a coated steam dual pipe system for use in advanced ultra-supercritical power plant

Improving the energy efficiency of power plants by increasing steam operating temperature up to 700 °C can be achieved using novel engineering design concepts such as coated steam pipe systems. This paper presents an optimised design for a novel coated dual pipe system to be used in advanced ultra-supercritical power plant. The approach developed in this study uses a combination of an optimisation algorithm and FE simulation, based on the reduction of the hoop stress at top coat/bond coat interface generated by the thermal and mechanical stresses. This allows determination of the optimum dimensions and material properties of the system. A unified viscoplastic model which combines a power flow rule with non-linear anisothermal evolution of isotropic and kinematic hardening has been used for the thermo-mechanical analysis of the coated dual pipe system under the cyclic loading. The results of the optimisation show that the value of the hoop stress at the top coat/bond coat interface is reduced significantly, compared with that in the baseline model. Finally, the potential technical challenges and future works for the proposed steam dual pipe system are discussed.

Chromium-Dependent Effect on Oxidation Behavior of Ni-Fe-Based Superalloy for Ultra-Supercritical Steam Turbine Applications: Influence of Temperature and Pure Steam

Three Ni-Fe-based superalloys with 16 wt% to 18 wt% Cr were designed for steam turbine components in advanced ultra-supercritical power plants. The oxidation behavior of these alloys was evaluated at 750°C to 850°C for 1,000 h to explore the chromium-dependent effect on the oxidation behavior in static air and pure steam. The results reveal that the oxidation rate of the alloys decreased as the Cr content increased in both the air and steam environment. However, such a beneficial effect of Cr was found to be temperature-dependent, which became less significant when the exposure temperature increased from 750°C to 850°C, leading to the formation of an additional NiFe2O4 layer and severe internal Al2O3/TiO2 oxide. The impact of temperature and pure steam on the oxidation resistance of the alloys were discussed based on the calculation of critical Cr concentration.

Effect of tempering time on the microstructure and stress rupture properties of P92 steel

Abstract P92 steels (9 Cr – 0.5 Mo – 1.8 W) are one of the candidate materials for Ultra Super Critical (USC) power plants. Since normalization of P92 results in the formation of martensitic microstructure, the steel is generally used in the normalized and tempered condition. In the present work, the material is subjected to normalization at 1060 °C and the effect of tempering time has been studied at constant tempering temperature of 780 °C. Decrease in dislocation density, increase in lath width, and precipitate size are the predominant microstructural changes. Short term (∼500 h) stress rupture tests are conducted at 600 °C temperature and structure – property correlation is attempted.

Microstructure and Charpy Impact Toughness of a 2.25Cr-1Mo-0.25V Steel Weld Metal.

The demand for heat-resistant steel has increased owing to its utility in numerous devices that must withstand high steam pressures and high temperatures, such as turbine rotors and blades in ultra-supercritical power plants. It is inevitable to join heat-resistance steel part by welding method, so it is important to maintain the toughness of the weld metals. In this study, the microstructure, low-temperature impact toughness, and fracture surface of as-welded and post-weld heat treatment (PWHT) of 2.25Cr-1Mo-0.25V weld metal were investigated. The microstructures of the as-welded and PWHT specimens are granular bainite and ferrite, respectively. This work revealed the relationship between effective microstructure nearby crack initiation origin and low temperature impact toughness for both the as-welded and PWHT specimens. The evolution of the microstructure and prior austenite was then investigated using confocal laser scanning microscopy (CLSM) to observe the formation of coarse ferrite grain structures. A suggestion for enhancing the low-temperature toughness was provided based on the effect of adjusting Mn content and forming acicular ferrite.