Advanced Ultra-supercritical Boiler Research Articles

Microstructure and mechanical properties of a dissimilar metal welded joint of Inconel 617 and P92 steel with Inconel 82 buttering layer for AUSC boiler application

The application of the novel dissimilar metal welded (DMW) joint, utilizing Inconel 617 and P92 steel, was showcased in the advanced ultra-supercritical (AUSC) boiler. The work has been performed to investigate the effect of Inconel 82 (ERNiCr-3) buttering layer on microstructure and mechanical properties (high-temperature tensile strength, impact strength and microhardness) of gas tungsten arc welded (GTAW) dissimilar joint between Inconel 617 and P92 steel fabricated using the Inconel 617 (ERNiCrCoMo-1) filler. For optical microscopy and scanning electron microscopy (SEM), samples were machined along a transverse direction which comprised the butter layer, weld metal, and heat-affected zone of both sides. The energy-dispersive X-ray spectroscopy (EDS) was used to map the interface of the buttering layer and weld metal and butter layer and P92 steel. The high-temperature tensile testing and Charpy impact testing at room temperature were conducted for the integrity assessment of the welded joint. The examination of microstructure and hardness revealed that the buttering layer of Inconel 82 filler successfully mitigated a significant portion of the brittle martensitic microstructure from the coarse-grained heat-affected zone (CGHAZ), along with hardness peaks on the side of P92 steel. The conventional method of DMW joint fabrication, without the use of a buttering layer, has been demonstrated to be less favourable compared to the new fabrication method, which incorporates a buttering layer. The TiC/NbC carbides were identified in the Inconel 82 buttering layer, whereas M23C6 and Mo6C carbides were found in the Inconel 617 filler weld. Near the interface of the Inconel 82 buttering layer and P92 steel, the formation of peninsula and island structures, as well as Type I and Type II boundaries, were confirmed. Additionally, element diffusion of Ni, Cr, and Fe was observed. The tensile test results indicated an ultimate tensile strength of 620 ± 4 MPa and % elongation of 19 ± 4 % at room temperature, with fracture occurring in the buttering layer near the interface of the buttering layer and P92 steel. At temperatures of 550 °C and 650 °C, the ultimate tensile strength decreased to 448 MPa and 326 MPa, respectively, with fractures occurring in the P92 steel, irrespective of temperature. The hardness of the Inconel 82 buttering layer and Inconel 617 filler weld were 219 ± 10 HV and 248 ± 11 HV, respectively. The Charpy impact toughness of the Inconel 82 buttering layer and Inconel 617 filler weld were 138 ± 6 J and 115 ± 4 J, respectively. The study comprehensively investigates and discussed the correlations between microstructure and mechanical properties.

Assessment of the Structural Integrity of a Laser Weld Joint of Inconel 718 and ASS 304L

For high-temperature industries operating at nearly 750 °C (advanced ultra-super critical boilers), dissimilar welding between Inconel alloys and austenitic stainless steel (ASS) are commonly adopted. The high-temperature resistive properties of Inconel and ASS alloys are highly qualified for high-temperature applications. In this experimental study, dissimilar autogenous laser beam welding (LBW) between Inconel 718 and ASS 304L is investigated. This paper explains the detailed study on the microstructural and mechanical behavior of the LBW dissimilar joint. The microstructural study indicates the presence of laves phases in the weld zone. Additionally, the weld zone shows heterogeneous microstructural formation, owing to the non-uniform welding heat in the different areas of the weld zone. The optical images show the presence of mixed dendrites, i.e., equiaxed, cellular, and columnar morphology, in the weld zone and in the fusion zones of either side. The energy-dispersive spectroscopy (EDS) results show the presence of segregated elements (Nb, Mo, Cr, and Ti) at the weld center. These segregated elements are the reason for the occurrence of the laves phases in the weld zone. The presence of Nb and Mo may form the laves phase (Fe, Ni, Cr)2 (Nb, Mo, Ti) along with Fe, Ni and Cr. The presence of an unmixed zone is observed in the HAZ of the Inconel 718, whereas the HAZ of the ASS 304L shows the presence of an unmixed zone (UZ) and a partially mixed zone (PMZ), as observed on the optical and SEM images. To obtain the mechanical properties of the laser weld, the tensile test, microhardness test, and impact test were measured at room temperature. The tensile specimens show a brittle failure at the ASS 304L side, which was initiated from the weld top, with average tensile stress of 658.225 MPa. The reason for the ASS 304L fracture is because of the presence of UZ and PMZ, and the lower hardness value of the ASS side. The UZ and PMZ lead to the fracture of the tensile specimen along the ASS 304L side’s HAZ. The measurement of microhardness carried out along the transverse length indicates an average microhardness of 214.4 HV, and the value is 202.9 HV along the weld depth. The mixed morphology of the microstructure promotes the variation in hardness in both directions. The hardness along the length shows a high hardness value in the weld zone and uniformly decreases along the base materials. The Charpy impact test of the weld zone shows the brittle fracture of the impact specimens. From the microstructural and mechanical results, the LBW dissimilar weld between Inconel 718 and ASS 304L is qualified for safe use in high-temperature end applications, such as AUSC power plants.

Unsteady-state heat transfer characteristics of spiral water wall tube in advanced-ultra-supercritical boilers from experiments and distributed parameter model

The design and operation of water walls are crucial for ultra-supercritical boilers, particularly with the advent of advanced-ultra-supercritical (A-USC) boilers. In this study, heat transfer characteristics for a spiral water wall were analysed in a 0.3 MWt experimental facility to understand the combustion and hydrodynamics of A-USC boilers. The working pressure ranged from 13.5 to 41.0 MPa, while mass flux varied from 124.4 to 557.1 kg/(m2⋅s), along with a maximum heat flux of approximately 448 kW/m2. As the heat flux was increased, evidently enhanced and deteriorated heat transfer performances were observed near the pseudo-critical point during the unsteady operating process. The distributed parameter model was employed along with different correlations from literatures to predict the distribution of temperatures and heat transfer coefficients. Predicted temperatures were compared with the experimental measurements. The results demonstrated that distributed parameter model combining with appropriate correlation for each region, yielded good agreement with the experimental data. Particularly, for the top, middle, and bottom regions of the furnace, the correlations by Swenson et al., Jackson et al., and Mokry et al., respectively are found to be suitable. Furthermore, a criterion for heat transfer deterioration for the spiral water wall was also suggested for the A-USC units.

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.

Yield strength anomaly and dynamic strain ageing behaviour of recently developed advanced ultra-supercritical boiler grade wrought Ni-based superalloy IN 740H

ABSTRACTThe yield strength anomaly (YSA) and dynamic strain ageing (DSA) behaviour of advanced ultra-supercritical boiler grade wrought nickel-based superalloy IN 740H is studied by conducting tensile tests in temperature range 28–930°C and by employing strain rates 1 × 10−2, 1 × 10−3, 1 × 10−4 and 1 × 10−5 s−1 followed by extensive electron microscopic examination. Increase in yield strength accompanied by impairment of ductility indicates that YSA exists in alloy IN 740H in temperature range of 650–760°C. The electron microscopic observation confirms that YSA is due to pinning of dislocations by γ′ precipitates and shearing of γ′ precipitates in IN 740H. DSA is observed in the temperature range of 200–500°C and is predominant at 300°C. The nature of serrated plastic flow due to DSA is dependent on the temperature and strain rate.

Multidisciplinary materials and geometry optimization of superheater tubes for advanced ultra-supercritical power boilers

The recent development of the coal-fired power plant purposes to enhance environmental-friendliness and efficiency. In order to achieve these goals, many research studies are underway to develop advanced ultra-supercritical (A-USC) boilers, which requires materials that can withstand extreme conditions. Integrated materials and product design (IMPD) is a new approach for designing products able to yield effective operation in extreme conditions. Based on the IMPD approach, we optimized the geometry of the superheater tube of the A-USC boiler as well as its constituent materials. To apply the IMPD to the tube design problem, we developed a creep deformation model based on finite element analysis, a heat transfer model, and two material models constructed via artificial neural networks. The material models predict creep properties and thermal conductivity for a given heat treatment condition and weight ratio of the chemical constituents. These four models are used in combination to form an analysis model chain, which is subsequently incorporated into an optimization routine for finding optimum material constituents and shapes of the superheater tube at the same time. An optimal tube design was developed to achieve minimum creep deformation and maximum heat transfer amount under the stringent operating conditions of the A-USC boiler.

Creep-induced microstructural evolution in a nickel-based superalloy designed for advanced ultra-supercritical boilers

Long-term microstructural stability is a major concern for candidate materials used in advanced ultra-supercritical (A-USC) power plants. In the present work, precipitation behavior of a nickel based superalloy designed for A-USC boilers, with a base composition of Ni-25Cr-20Co, was investigated during long-term thermal exposure under different stresses at 1043 K. The results showed that the coarsening of γ′ precipitates in the grain interior had occurred during thermal exposure in the presence and absence of stress. The microstructure around grain boundary was altered remarkably under stress. The precipitate-free zones (PFZs) commonly existed near the grain boundary in the gauge sections of specimens after long-term creep. Coarsened and elongated particles in the PFZs were identified as γ′-Ni3 (Ti, Al) precipitates by chemical composition analysis and diffraction pattern. Furthermore, the morphology and distribution of MC-type carbides have no significant changes observed due to their good stability, but discontinuous M23C6 appeared under stress, due to formation of PFZs resulting in dissolution of the adjacent M23C6. Besides, the creep-rupture mechanism was discussed to explore a potential approach for improving creep properties.

Effects of post-welding heat treatment on microstructure and mechanical properties of welding joint of new Ni-Fe based superalloy with Haynes 282 filler metal

The gas tungsten arc welding process was employed to weld a new wrought Ni-Fe based superalloy with the Haynes 282 filler metal, which was designed for advanced ultra-supercritical boiler tubes operated beyond 700°C. In this study, the evolution of microstructure and mechanical properties of the welding joint prior to and following the post-welding heat treatment (such as annealing treatment at 1010°C for 2h and 788°C for 8h) were investigated. The microstructure stability and hardness were also studied following 500h, 1000h and 3000h at the service temperature (750°C). An epitaxial growth of the grains near the fusion line was observed, whereas the partition coefficients and solidification temperature range during the thermodynamic state of equilibrium were calculated in the weld metal zone. The welding heat affected zone of the as-welded welding joint was the weakest zone. In contrast, following the post-welding heat treatment, the segregation was significantly eliminated in the weld metal zone. Also, the mechanical properties of the welding heat affected zone were restored to the property levels of the base metal, with the weld strength reduction factor of 4%. Therefore, the effects of post-welding heat treatment on the microstructure and properties of welding joint were also investigated. Following 3000h thermal exposure at 750°C, the coarsening rates of the spherical γ′ precipitates of both the as-welded and post-welding heat treatment samples were consistent with the Lifshitz-Slyozov-Wagner (LSW) model. In addition, the microstructure stability of the welding joint was excellent, without any topologically closed packed phase precipitation. The Haynes 282 filler metal could be a potential candidate material for the welding of this new Ni-Fe based superalloy.

Numerical investigation of oxy-fuel combustion in 700 °C-ultra-supercritical boiler

The combination of the advanced ultra-supercritical (AUSC) and oxy-fuel combustion technologies is considered the most feasible and promising choice for carbon capture and storage. The high efficiency of AUSC boilers compensates for the cost incurred in controlling CO2 emissions. In this study, a new improved model was proposed for gas-radiative properties as a polynomial function of the continuous molar ratio of H2O to CO2 and temperature, which was compiled as user-defined functions in C language. The modified radiative model was employed in the computational fluid dynamics analysis to simulate a tangentially fired 700°C-USC boiler with a capacity of 300MW under the oxy-fuel condition. The results show that the improved model has a good accuracy for gas-radiative properties compared to other classical models. In the oxy-fuel cases, the heat transfer obtained in the simulation is approximately 4–11% lower than that in the thermodynamic calculation because of the char gasification. The radiative capability of the flue gases in the oxy-fuel case is much greater than that in the air–fuel case with the increase in the molar fraction of oxygen. The heat transfer via the water wall is approximately 4–8% higher than that via air, when the molar fraction of oxygen increases from 26% to 29%. The increase in the heat absorption helps in better arrangement of the heat-exchanger surfaces in the furnace of the AUSC boiler.

Studies on Hot Cracking Susceptibility and Establishment of Welding Procedure of Austenitic Stainless Steel Grade UNS S31035 for Power Plant Application

Austenitic stainless steel Sandvik Sanicro 25 (UNS S31035) was developed in European Advanced ultra super critical boiler program called Thermie-AD700 project. Compared to all other commercial heat resistant austenitic stainless steels, Sanicro 25 has the highest creep strength. Hence, this material has been identified as one of the candidate materials in reheater and super heater lines of advanced ultra super critical boiler for up to 700 °C temperature usage. In this austenitic stainless steel, along with Fe, Ni and Cr, solid solution hardening element, like W, Co & N and precipitation hardening elements like Nb and Cu are added to enhance the creep and high temperature oxidation resistance. Addition of more alloying elements and full austenitic microstructure may lead to the possibility of solidification cracking susceptibility during welding. Hence, investigation on the weldability of this material becomes inevitable to establish the welding procedure. In this paper, solidification cracking behaviour of UNS S31035 has been studied with varestraint test. From the test results, it is concluded that solidification cracking susceptibility is higher in this steel and similar composition filler cannot be used for welding. Based on this conclusion, Ni based super alloy filler ER NiCrCoMo-1 is used for establishing welding procedure in gas tungsten arc welding process. Test results are presented and discussed in this paper.

Effects of long term thermal exposure on microstructure and mechanical property evolution of a new wrought Ni–Fe based superalloy

A new wrought Ni-Fe based superalloy was designed for advanced ultra-supercritical boiler tubes beyond 700°C. In this study, the recrystallization treatment of the alloy following cold rolling and the microstructural evolution following 500h, 1000h and 3000h at 750°C were studied. The optimization of heat treatment was performed by thermodynamic calculation and the optimum combination was determined, i.e. annealing treatment at 1150°C for 45min, followed by a two-step aging treatment, at 810°C for 1h and at 770°C for 16h. Following this process, the M(C,N) precipitates and Cr-rich M23C6 carbides were formed at grain boundaries. Following 3000h thermal exposure at 750°C, the coarsening rate of the spherical γʹ precipitates was consistent with the LSW model and similar to that of the IN 740H alloy, while the variation in hardness values was relatively low, as the exposure time was increased. The α-Cr phase was observed after 1000h at 750°C and during the long term thermal exposure the coarsening of α-Cr phase occurred. The formation of α-Cr phase enhanced the intragranular yield strength and hardness values at room temperature and the stability of the mechanical properties was partially improved.

Strain rate effect on cyclic deformation behaviour of advanced ultra-supercritical boiler grade wrought Ni-based superalloy IN740H at 760 °C

In order to understand the simultaneous operation of fatigue and creep mechanisms, strain rate (ε̇) effect on low cycle fatigue (LCF) behaviour of recently developed power plant grade wrought Ni-based superalloy IN740H was studied. Fatigue experiments were conducted in air, at 760°C at ε̇ values 10−2, 10−3 and 10−4s−1. The fatigue life significantly decreased at lower ε̇ values. Bilinear Coffin-Manson slopes were obtained for all ε̇ values. Through microstructural studies, it was ascertained that decrease in fatigue life at lower ε̇ value can be attributed to ingress of oxygen by grain boundary diffusion and intergranular crack initiation and growth.

Development and evaluation of SUS 304H — IN 617 welds for advanced ultra supercritical boiler applications

At moderately high temperature sections of Advanced Ultra Super Critical (AUSC) boilers, welding of superalloys to austenitic steels is inevitable owing to economic aspects of boiler. Welding of SUS 304H and Inconel 617 (IN 617) was attempted using IN 617 filler material employing conventional Gas Tungsten Arc Welding (GTAW) process and the procedure was successfully established along with optimized welding parameters. Microstructural characterization was carried out to identify various zones on either side of the fusion boundaries. Unmixed Zone and Heat Affected Zone (HAZ) were observed towards SUS 304H fusion boundary while no distinct HAZ was observed towards IN 617 fusion boundary. Micro-hardness profiling indicated decrease in hardness at the HAZ towards SUS 304H fusion boundary. Mechanical properties evaluation at both ambient and elevated temperatures was carried out and data obtained was compared with those of base metals. The tensile strength of the cross weld specimens at high temperatures were observed to be marginally lower than that of IN 617 but significantly more than that of SUS 304H, hence, tolerable. Stress-rupture properties of the cross-weld specimens as tested in this study were found to be intermediate to the base metals’ data, thus, suitable for AUSC power plants' boiler applications. Hence, this work gives an insight into welding procedure establishment, microstructural development, variation of mechanical properties at elevated temperatures and stress-rupture properties of the dissimilar metal welds at elevated temperatures.

Tensile and creep deformation of a newly developed Ni-Fe-based superalloy for 700 °C advanced ultra-supercritical boiler applications

A new Ni-Fe-based superalloy, HT-X, has been developed for applications in 700 °C advanced ultra-supercritical (A-USC) boilers. The HT-X alloy is subjected to various heat treatments. Tensile tests are conducted at room temperature (RT), 700 °C and 750 °C. Creep tests are carried out under conditions of 700 °C/300 MPa and 750 °C/150 MPa. After aging treatment, the yield strength of the HT-X alloy at RT and 750 °C is 787 MPa and 624 MPa, respectively. When additional thermal exposure at 750 °C for 5400 h is applied, the yield strength is decreased to 656 MPa at RT and 480 MPa at 700 °C. For an aged specimen, the a/2 dislocation shearing process occurs when tensile testing is conducted at RT and 750 °C. As the γ’ precipitate size increases in the specimen that is thermally exposed at 750 °C for 5400 h, Orowan bowing is the dominant dislocation process, and stacking faults develop in the γ’ precipitates at both RT and 700 °C. Dislocation slip combined with climb is the dominant mechanism under the creep testing conditions. The factors that affect the mechanical properties and deformation mechanisms are discussed.

J0310301 A-USCボイラ用具材溶接継手のクリープ強度

The commercialization of a 700℃ class pulverized coal power system, advanced ultra-supercritical (A-USC) pressure power generation, is the target of an ongoing research project in Japan. In the A-USC boiler, Ni or Ni-Fe base alloys are used for high-temperature parts at 650-700℃, and advanced high-Cr ferritic steels are planned to be used at temperatures lower than 650℃. In the dissimilar welds between Ni base alloys and high-Cr ferritic steels, Type-IV failure in the heat-affected zone is a concern. The high B-9Cr steel developed at the National Institute for Materials Science, which has improved creep strength in both base metal and weldment, is a candidate material for the Japanese A-USC boiler. In the present study, long-term creep tests were conducted on the dissimilar welded joints between Ni base alloys and high B-9Cr steels. Microstructures and creep damage in the dissimilar welded joints were investigated. The creep rupture life of the dissimilar welded joints using high B-9Cr steel was 5-10 times longer than that of the conventional 9Cr steel welded joints at 650℃. The effect of welding procedure was also investigated.

Creep Strength of Dissimilar Welded Joints Using High B-9Cr Steel for Advanced USC Boiler

The commercialization of a 973 K (700 °C) class pulverized coal power system, advanced ultra-supercritical (A-USC) pressure power generation, is the target of an ongoing research project initiated in Japan in 2008. In the A-USC boiler, Ni or Ni-Fe base alloys are used for high-temperature parts at 923 K to 973 K (650 °C to 700 °C), and advanced high-Cr ferritic steels are planned to be used at temperatures lower than 923 K (650 °C). In the dissimilar welds between Ni base alloys and high-Cr ferritic steels, Type IV failure in the heat-affected zone (HAZ) is a concern. Thus, the high B-9Cr steel developed at the National Institute for Materials Science, which has improved creep strength in weldments, is a candidate material for the Japanese A-USC boiler. In the present study, creep tests were conducted on the dissimilar welded joints between Ni base alloys and high B-9Cr steels. Microstructures and creep damage in the dissimilar welded joints were investigated. In the HAZ of the high B-9Cr steels, fine-grained microstructures were not formed and the grain size of the base metal was retained. Consequently, the creep rupture life of the dissimilar welded joints using high B-9Cr steel was 5 to 10 times longer than that of the conventional 9Cr steel welded joints at 923 K (650 °C).

Structural stability and mechanical properties of phosphorus modified Ni–Fe based superalloy GH984

GH984 is a Ni–Fe based superalloy that has been used as superheater tube material in high pressure boilers for more than 10 years, and is now being considered for use as superheater, reheater and header material for boilers in 700°C advanced ultrasupercritical (A-USC) coal fired power plants. In order to meet the requirements of 700°C A-USC boilers, the alloy composition must be optimised to enhance thermal stability and improve mechanical properties. In this paper, the influence of phosphorus (P) on the structural stability and mechanical properties of the GH984 alloy was investigated. The results show that P has no influence on the major precipitates, which are spherical γ′, blocky MC and discrete M23C6 at the grain boundary after standard heat treatment, except decreases in the size of M23C6. After the thermal exposure at 700°C for 1000 h, at the grain boundary, the morphology of M23C6 changes from a blocky and irregular shape to a blocky and nodular shape, and the size decreases with the increase in P content. Additionally, the growth rate of M23C6 increases with increasing the thermal exposure temperature from 700 to 800°C. The P content has no obvious influence on the tensile strength and ductility. However, at 700°C/350 MPa and 750°C/300 MPa, the rupture life obviously increases with increasing the P content due to the improvement of grain boundary strength when the content of P is lower than 0·034 wt-%. It can be concluded that the addition of P can increase the rupture life and improve the structural stability of GH984.

Superalloys and the Development of Advanced Ultra-Supercritical Power Plants

A 700 advanced ultra-supercritical (A-USC) power plant technology project was initiated in China in the year of 2011. The highest temperature components in A-USC boiler are superheater and reheater tubes. The fire-side metal temperature can reach 750 (even higher). Based on the very long time service (30-40 years) these important high temperature tubes require 105h long time stress rupture strength higher than 100MPa and the corrosion/oxidation layer loss less than 2mm for 2×105h. The highest temperature components in 700 steam turbine are the buckets of different stages and require very long service time. At this severe condition only superalloys can fulfill these unusual requirements. This paper reviewed Fe-Ni and Ni-base superalloys from the view point of structure stability and long term mechanical properties and corrosion/oxidation resistance for 700 A-USC power plant materials selection.

Satisfaction of Low Carbon Society - Latest Technical Approach on Coal-Fired Power Plant

IHI Corporation (IHI), as a supplier of power plant equipment including boiler, is facing to CO2 emission issue by various technical approaches. Latest status of advanced technologies, which is Advanced Ultra Super Critical Boiler (A-USC) — Improvement in Efficiency, Oxy-fuel Combustion Technology — CO2 Capture and Use of Biomass Fuel — Renewable Energy, is introduced in this paper. Finally, these advanced technologies certainly contribute to reduce CO2 emission economically.