Advanced Ultra-supercritical Power Plants Research Articles

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.

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.

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.

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.

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.

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.

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.

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.

Creep behavior of an alumina-forming austenitic steel with simple alloy design

An Fe-18Ni-12Cr-based alumina-forming austenitic (AFA) steel with simple alloy design free of Nb and C was developed for potential application in advanced ultra-supercritical power plants. Creep behavior of the material was studied at the target application temperature of 700 °C. Creep properties were analyzed in terms of stress exponent (n), Monkman-Grant relationship and creep damage tolerance factor (λ). The microstructural evolution of crept samples was investigated by scanning electron microscope (SEM), electron back scatter diffraction (EBSD) and transmission electron microscope (TEM). The minimum creep rate decreased from 5.8 × 10−8 to 1.6 × 10-9 1/s with the applied stress decreasing from 140 to 80 MPa. Precipitation of B2-NiAl during the prolonged creep process contributed to improve creep resistance of the material. The related creep mechanism was found to be dislocation creep, which was supported by the value of n = 7.0 and by the crept microstructure of dislocations tangled with precipitations. Characterization results of creep fracture morphology indicate that the creep failure was resulted from microstructural degradation of precipitation coarsening.

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.

Effects of long-term aging on microstructure and properties of a tungsten bearing heat-resistant alloy

The effects of long-term aging at 700 and 750 °C on microstructure and mechanical properties of a new developed tungsten bearing heat-resistant alloy used for advanced ultra-supercritical power plant was investigated both experimentally and thermodynamically. Experimental results showed that the mechanical properties maintained excellent stability after long-term aging at 700 °C for 10,000 h, while the impact absorbing energy decreased sharply after 1000-h aging and then kept constant till 10,000 h. The main precipitates after long-term aging at 700 and 750 °C were M23C6, MC and homogeneous γ′-phases. The mass fraction of M23C6 carbides increased with increasing aging time, and M23C6 carbides precipitated in shape of chains and lamellas on grain boundaries. The slight decrease in MC carbides during aging may be due to degradation reaction. The weight fraction of γ′-phase increased with the aging time, and then changed little after 5000 h; γ′-phase exhibited excellent microstructure stability and low coarsening rate during long-term aging at 700 °C. However, the coarsening rate of γ′-phase was much higher at 750 °C.

Creep Deformation Behavior of a Novel Precipitate-Hardened Ni-Fe-Base Superalloy at 750 °C

The creep deformation behavior of a novel precipitate-hardened Ni-Fe-base superalloy in solutionized and aged states is investigated at 750 °C/80 to 120 MPa. We found that as creep deformation proceeds, creep curves of specimens in the two states either overlap or are parallel to each other. This study provides a new strategy to design the heat treatment scheme to achieve a good compromise between the strength and fabricability of superalloys for 700 °C-class advanced ultra-supercritical power plants.

Thermal and stress analyses of a novel coated steam dual pipe system for use in advanced ultra-supercritical power plant

Improving the energy efficiency of advanced ultra-supercritical power plants, by increasing steam operating temperature up to 700 °C, can be achieved, at reduced cost, by using novel engineering design concepts, such as coated steam pipe systems manufactured from high temperature materials commonly used in current operational power plants. This paper describes a preliminary feasibility analysis of the design concept of a novel coated dual pipe system under steady-state operation, using analytical and finite element models to evaluate the possible thermal gradients and stresses generated. The results show that the protective coating layer contributes to the effective reduction in the surface temperature of the primary steel pipe. Thermal stresses generated due to the significant difference in the thermal and mechanical properties of the coating and substrate pipe are larger than the mechanical stresses generated by the combined effects of the internal steam pressure in the primary steam pipe and external pressure from the counter-flow cooling steam during steady-state operation. Compared with the stress relaxation of the coating and substrate pipe, creep has a significant impact on the stress distribution within the coating layer. Several key factors have been identified, such as the coating thickness, conductivity, thermal expansion, heat transfer coefficient of cooling steam, cooling steam temperature and cooling steam pressure, which are found to govern thermal and stress distributions during steady-state operation.

Reducing the Superheating of Extraction Stream on Advanced-Ultra Super Critical Power Plants with Regenerative Turbines in South Korea: An Economic Analysis

In this study, an advanced-ultra super critical (A-USC) simulation model was developed using the Performance Evaluation of power system efficiencies (PEPSE) software and data collected from a 500 MW ultra-supercritical (USC) coal-fired power plant in South Korea. Using the operational USC and a typical A-USC power plant steam conditions, the model analyzed the impacts of adding an additional feedwater heater (FWH) and reheater to the baseline single reheater (SR) and 8 FWH case. Through the process of introducing reheat and/or regenerative cycles, the authors found: (1) A-USC steam conditions offers an approximate 4% power plant efficiency increase in comparison to the baseline USC steam conditions and; (2) power plant efficiencies increase approximately 1.5% when a 9th FWH and double reheater are added, however; (3) this also results in an approximate 64 °C increase in the superheating of extraction stream. This excessive rise in the superheating of extraction steam was found to cause overall energy loss, reducing the overall efficiency of the power plant. Therefore, it was surmised that if the increase in the superheat degree of extraction steam from the improved steam cycle, which can effectively reduce, the efficiency of the power plant could be further improved. To determine the efficiency variations based on the reduction of the superheat degree of extraction steam, the authors applied a regenerative turbine (RT) to the model. Introducing the RT to the A-USC DR and 9 FWH was found to decrease from the average extraction steam temperature from 221 °C to 108 °C and result in an increase in power plant efficiency of approximately 0.3% to 49.5%. An economic analysis was also performed to assess the fiscal feasibility of adding an RT. Assuming the initial investment to be USD 1409 million, implementing an RT equated to a net present value increase of approximately USD 33 million as compared to that of similar life (30 years of durability) expectancy of A-USC without using an RT. The findings of this study have the potential to improve South Korea’s energy policy, reducing the superheat degree of extraction steam that rises excessively during A-USC steam condition optimization. While this study is focused on South Korea, said findings are also generalizable to worldwide energy policies, serving as an effective method to not only increase system efficiencies, but enhance the economic feasibility as well.

On the analysis of post weld heat treatment residual stress relaxation in Inconel alloy 740H by combining the principles of artificial intelligence with the eigenstrain theory

Post weld heat treatment (PWHT) process has an important role on fabrication of advanced ultra-supercritical power plant turbines. This process relieves the residual stresses formed as a result of welding by converting elastic strains into creep strains. In order to analyse the residual stress relief mechanism during the PWHT process, a novel simulation approach based on experimental data was developed for the analysis of residual stress states from complex manufacturing processes which are welding and heat treatment. This model uses permanent plastic strains (eigenstrains) formed as a result of welding process to set the initial mechanical state of the sample. The distribution of eigenstrains in the whole body was determined using displacement data obtained from contour measurements. The use of eigenstrains to set the initial residual stress state of the creep model reduced the number of uncertainties. This allowed the use of the principles of artificial intelligence for the development of a new fuzzy finite element model (fFEM) that determines the eigenstrain-creep model parameters through an evolution process. Subsequent to the determination of the model parameters, conditions of the PWHT process are investigated to analyse residual stress relaxation in Inconel Alloy 740H weldments.

Behaviour of Alloy 617 OCC Under Hot Corrosion Conditions Encountered in Boilers in A-USC Power Plants

Superheater and reheater tubing constitute very critical parts of the boilers operating in advanced ultra-supercritical (A-USC) power plants. They are subjected to severe hot corrosion conditions during operation. The operating temperature and pressure in these power plants are very arduous, and materials with high resistance to hot corrosion and creep are required to produce the said tubing. Increased use of superalloys comes into picture in A-USC power plants, because of the demanding operating conditions prevailing there. Inconel 617 has been a superalloy developed for use in the form of superheater and reheater tubing seen in the power plants operating in A-USC environments. There have been researches aiming at improving its creep resistance, and versions with optimum chemical composition have been developed. Alloy 617 OCC, an optimized version of the superalloy 617, has been produced as a candidate material for superheater and reheater tube manufacturing in the context of development of A-USC power plant technology in India. However, hot corrosion issues are equally important for such applications. In light of the foregoing, studies have been carried out to analyse the performance of Alloy 617 OCC in hot corrosion conditions in simulated A-USC power plant environments. Oxidation tests for the alloy were conducted in the laboratory air environment under identical temperature/time conditions for the sake of comparison. Hot corrosion process was examined using thermo-gravimetry, and the corrosion products were characterized using a variety of material characterization techniques. The study reported the comparative corrosion performance of the superalloy in the two environments.

Assessment of creep deformation and rupture behaviour of 304HCu austenitic stainless steel

Reduction in CO2 gas emission and decrease in fuel consumption can result in increase in efficiency of the thermal power plants. Increase in efficiency is directly linked to the steam temperature and pressure which requires materials having high creep strength. This resulted in development of Advanced Ultra Super Critical (AUSC) power plant which aims to increase efficiency to more than 45 % and significant decrease in CO2 emission. The 304HCu stainless steel is one of the candidate materials to be used in AUSC power plant. It contains around 3 wt. % of copper, certain amounts of niobium and nitrogen and increased carbon content for enhancing creep strength. Creep tests are conducted for 304HCu stainless steel at 923K, 973K and 1023K over a stress range of 100-240MPa. The creep curve exhibited shorter primary regimes followed by marginal secondary regimes and extended tertiary regime. The variation of steady state creep rate with applied stress exhibited Norton’s power law relationship (εs = Aσn). The value of n (stress exponent) decreased with increase in temperature but decrease was more pronounced at 923K. The product of steady state creep rate and rupture life obeyed Monkman-Grant relation. The contribution of tertiary creep was found to increase with temperature. Microstructural degradation in the form of coarsened precipitates, dislocation cell formation and deformation bands was the primary reason for increase in tertiary regime of creep curve and increase in the value of damage tolerance factor (λ). A mathematical model based on finite element analysis coupled with continuum damage mechanics has been used to predict the creep deformation and rupture life of 304HCu SS. The prediction of creep curve based on this model was found to be in good agreement with experimental results.

Hot Corrosion of Alloy 617 OCC in Simulated USC Power Plant Environment

Alloy 617 OCC, a variant of INCONEL 617 with optimised chemical composition, has been produced in India for manufacture of superheater and reheater tubing in boilers operating in advanced ultrasupercritical (A-USC) power plants. The tubing encounters intense hot corrosion conditions during service. The present study deals with hot corrosion behaviour of 617 OCC in A-USC environment. The environment occurring in A-USC plants was simulated in the laboratory by exposing the material coated with a mixture of salts at 700°C in a flowing gas mixture. For use in A-USC boiler technology, the metal loss due to fireside corrosion of the material should be less than 2 mm in 200,000 hours. The loss obtained in the present study was nearly 5 times this value. The corrosion processes were studied using SEM/EDAX, XRD and thermogravimetry. The degradation mechanisms coming into play, disqualifying the material for use in A-USC plants, would be discussed.