Advanced Ultra-supercritical Power Plants Research Articles

Hot deformation characteristics and processing map analysis of a new designed nickel-based alloy for 700 °C A-USC power plant

The deformation behavior of the new designed nickel-based alloy for 700 °C advanced ultra-super-critical (A-USC) power plant applications was studied by the isothermal compression tests at temperature range of 1000–1200 °C with strain rates of 0.01–10 s−1 on a Gleeble-1500 thermo-mechanical simulator in order to optimize the hot working parameters and control the microstructure under various deformation conditions. The results show that the flow stress increases with the decreasing temperature and increasing strain rate, and the activation energy of the new alloy is about 474.22 kJ/mol. Then the processing maps were developed on the basis of corrected flow stress data. The flow instability domain lies in the temperature range of 1000–1035 °C and strain rate range of 0.03–10 s−1. The microstructure in the domain is characterized with inhomogeneous microstructure resulting from flow localization which should be avoided in hot working process. The optimum hot working conditions are obtained in the temperature range of 1125–1200 °C and strain rate range of 0.01–0.3 s−1, except the grain growth domain with the strain rate and temperature range of 0.01–0.03 s−1 and 1175–1200 °C. Microstructure observations reveal that full dynamic recrystallization (DRX) occurs in the optimum conditions and the origin microstructure has been substituted by fine and equiaxed DRX grains. The nucleation mechanism of DRX in the new alloy is discontinuous dynamic recrystallization (DDRX) as to the existence of original grain boundaries bulging under the low strain rate. However, the dominant nucleation mechanism of DRX at high strain rate has turned into CDRX.

Creep-Rupture Behavior of SUS 304H – IN 617 Dissimilar Metal Welds for AUSC Boiler Applications

Advanced Ultra Super-Critical (AUSC) power plants are envisaged for achieving higher thermodynamic efficiency by operating at temperatures and pressures of 710/720 °C and 310 ata, respectively which are significantly higher than sub-critical (conventional) and supercritical power plants. This has led to tremendous research in selection of new and advanced materials to meet high temperature requirements. Ni-base superalloys having known to have excellent creep-rupture behavior were selected for high temperature sections of boiler while austenitic stainless steels were selected for moderately high temperature sections considering the economical feasibility. Since both these materials have to be fabricated for application in boiler tubes, dissimilar metal welding is inevitable. This work discusses creep-rupture evaluation of one such case of dissimilar welds, i.e., between Inconel 617 (IN 617) and SUS 304H. IN 617 is a Ni-base superalloy while SUS 304H is a Cu-precipitated austenitic stainless steel. Welding was carried out using IN 617 filler material for producing defect free welds. Creep-rupture samples were prepared incorporating the complete cross-weldment for understanding mechanical behavior as a result of prolonged exposure to elevated temperature and stresses. Creep-rupture testing was carried out at 600, 650 and 700°C at suitable stresses to obtain rupture times in the excess of 3000 hours in a few test conditions. Detailed characterization studies when carried out on ruptured samples revealed the weakest zone to be heat affected zone towards SUS 304H which led to failure. This work also provides insight into the possible creep-mechanisms that operate in various zones of weldments. Phases formed as a result of exposure to stress and temperature for a prolonged duration are also discussed. Comparison of the data obtained with data available in literature was carried out and performance of weldments was analyzed and reported.

A Newly Developed Wrought Ni–Fe–Cr-based Superalloy for Advanced Ultra-Supercritical Power Plant Applications Beyond 700 °C

Some existing wrought Ni–Cr–Co-based superalloys are being evaluated as the candidate materials for advanced ultra-supercritical power plant applications beyond 700 °C due to their high creep strength. But they are all prohibitively expensive due to the addition of Co, Mo and W. Here we developed a new Ni–Fe–Cr-based superalloy (named as HT700 alloy) with low cost and high strength. This paper reports the mechanical properties and fracture modes of HT700 alloy to support its high temperature applications and to understand prospective failure mechanism. Fractographic examinations indicate that the fracture modes shift with test condition change. In addition, the HT700 alloy has relatively stable microstructure at 750 °C. Compared with IN740 and GH2984 alloys, this new alloy has higher yield strength in the temperature range from room temperature to 800 °C. The creep life of this new alloy is much longer than that of the Ni–Fe-based superalloy GH2984. The results suggest that this new alloy is a promising material for advanced ultra-supercritical power plant applications beyond 700 °C.

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.

Research and Development of Heat-Resistant Materials for Advanced USC Power Plants with Steam Temperatures of 700 °C and Above

Materials-development projects for advanced ultra-supercritical (A-USC) power plants with steam temperatures of 700 °C and above have been performed in order to achieve high efficiency and low CO2 emissions in Europe, the US, Japan, and recently in China and India as well. These projects involve the replacement of martensitic 9%−12% Cr steels with nickel (Ni)-base alloys for the highest temperature boiler and turbine components in order to provide sufficient creep strength at 700°C and above. To minimize the requirement for expensive Ni-base alloys, martensitic 9%−12% Cr steels can be applied to the next highest temperature components of an A-USC power plant, up to a maximum of 650°C. This paper comprehensively describes the research and development of Ni-base alloys and martensitic 9%−12% Cr steels for thick section boiler and turbine components of A-USC power plants, mainly focusing on the long-term creep-rupture strength of base metal and welded joints, strength loss in welded joints, creep-fatigue properties, and microstructure evolution during exposure at elevated temperatures.

Thermal Stability and Tensile Properties of GH984G Alloy for A-USC Power Plants

GH984G is a new Ni-Fe-based alloy which has been designed for use as superheater, reheater, and header materials for boilers in 700°C advanced ultra-supercritical (A-USC) coal-fired power plants. In this paper, the tensile properties and microstructure evolutions of GH984G were investigated during long-term thermal exposure at 700 and 750°C up to 15000h. The results show that the major precipitates were MC, M23C6 and γ ́, and no detrimental TCP or GCP phase appeared during long-term thermal exposure. The morphology of γ ́ was spherical and had no obvious change, but the size increases. After thermal exposure at 700°C for 15000h, the 700°C yield strength slightly increased, but the yield strength decreased after thermal exposure at 750°C for 15000h. The variation of strength is attributed to the coarsening of γ ́ precipitation. The deformation mechanism is the moving dislocations shear γ ́ precipitates and the stacking faults form in γ ́ precipitates at room temperature and 700°C. At 750°C, the deformation mechanism is characterized by the formation of dislocation loops and tangles.

Influence of Thermal Exposure on the Microstructure, Surface Stability and Strength of a New Fe-Ni-Base Superalloy for A-USC Applications

A low-cost, high-creep strength wrought Fe‒Ni-base superalloy for 700 °C advanced ultra-supercritical coal-fired power plant applications has been developed. The microstructural stability of this alloy during long-term exposures at 700 °C‒750 °C for up to 10000 h was investigated by SEM and TEM. The correlation between the microstructure evolution and the mechanical properties was studied. The experimental results showed that the major precipitates in the alloy were spherical γ′, MC and discrete M23C6 after the long-term exposure at 700 °C and 750 °C and no harmful phases, such as σ phase and η phase, were observed. The amount of M23C6 increased with exposure time at 700 °C while it decreased at 750 °C. During thermal exposure γ′ coarsened with increasing the exposure time and exposure temperature. Alloy showed good oxidation resistance due to the formation of external layer of Cr2O3. The yield strength of the alloy increased at first and then decreased with increasing exposure time at 700 °C, but it decreased gradually with increasing exposure time at 750 °C. The relationship between yield strength and the dominant deformation mechanisms of the alloy was discussed.

Oxidation Behavior and Mechanism of Inconel 740H Alloy for Advanced Ultra-supercritical Power Plants Between 1050 and 1170 °C

The oxidation behavior and mechanism of as-cast Inconel 740H superalloy, newly developed for advanced ultra-supercritical power plants superheater and reheater pipes, were investigated in the temperature range of 1050–1170 °C in air. Isothermal oxidation testing, X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy were used in this study. The results showed that the oxidation of Inconel 740H was a diffusion-controlled process and controlled by the outward diffusion of metallic elements and inward penetration of oxygen. Owing to the complicated chemical composition of the alloy, different oxides tended to form in the oxidation process. Cross-sectional characterization investigation indicated that the oxide-scale layer consisted of three parts when the temperature was high or the oxidation time was long: an internal oxidation zone composed of Al2O3 and TiO2, a continuous and dense middle layer of Cr2O3 containing precipitates of TiO2 and an outmost layer of NiCr2O4 spinel. In the formation process of NiCr2O4, NiO was found in the early transient stage. With further oxidation, TiO2 finally could not be detected in the internal oxidation zone as a result of outwards diffusion of Ti and its limited concentration in the matrix.

Martensitic and austenitic creep resistant steels for application in advanced ultra-supercritical thermal power plants

The paper represents the results of investigation of martensitic and austenitic steels and their development in manufacturing of steam pipes and heating surface for advanced ultra-supercritical (A-USC) power plants with service steam temperature up to 680C and pressure up to 32 MPa. There was shown that alloying by 3% Co of martensitic steels such as 9Cr-2W-Mo-VNb contributed to obtaining a high degree of dispersion of structure and improve creep resistance on 15-20% compared to steel type T/P92. Optimization of chemical composition and heat treatment parameters developed austenitic heat-resistant steel have provided a growth of long-term strength up to ~ 1.5 times (~ 96 N/mm2 at 680C, 10h) in comparison with steels such as TP347. Steels were developed in manufacturing of parts of TPP equipment (headers, reheaters, steampipes etc.).

Detrimental effect of cellular precipitation on the creep strength of Inconel740H

Cellular precipitation (also known as discontinuous precipitation) has been observed at the grain boundaries of a newly developed nickel-based Inconel740H alloy designed for use at 700 °C in advanced ultrasupercritical coal-fired power plants. By means of element mapping and selected area diffraction, the cellular precipitates were identified as Cr-rich M23C6 carbides. The onset of cellular precipitation was found to follow a pucker mechanism in Inconel740H. The cellular precipitates at the grain boundaries, even at low volume fractions, were severely detrimental to the creep strength at 750 °C. The creep rupture life of Inconel740H containing cellular precipitates at grain boundaries was only one-tenth of that for the alloy without cellular precipitates. The reason for the drastically decreased creep rupture life is attributed to the poor resistance of cellular precipitates to crack propagation during creep.

A new wrought Ni–Fe-base superalloy for advanced ultra-supercritical power plant applications beyond 700 °C

Ni-base superalloys are the best candidate alloys for advanced ultra-supercricial (A-USC) plant applications in terms of creep strength but they are prohibitively expensive. Here we developed a new low-cost, high creep strength Ni–Fe-base alloy. It also has excellent workability, good microstructural stability and oxidation resistance, and high yield strength. The creep rupture resistance of this new alloy was comparable to the levels exhibited by Ni-base candidate alloys CCA617 and Nimonic 263 and was much better than the state-of-the-art Ni–Fe-base alloys HR6W and GH984. The results suggest that this alloy is a promising candidate for A-USC power plant applications beyond 700°C.

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.

VM12 Steel for Advanced Power Generation Plants – Metrology of the Precipitates by Electron Microscopy

A new high chromium martensitic steel, VM12, was recently developed for advanced coal-fired ultra-supercritical steam power plants. A very important factor in maintaining high creep resistance during service exposure is its stable microstructure, in particular high dispersion particle strengthening, which slow down the recovery and softening processes of the matrix. Quantitative characterisation of the precipitates in the VM12 steel as received condition and after creep tests up to about 30000 h at 625 °C using TEM was carried out.

Processing of Advanced Cast Alloys for A-USC Steam Turbine Applications

The high-temperature components within conventional supercritical coal-fired power plants are manufactured from ferritic/martensitic steels. To reduce greenhouse-gas emissions, the efficiency of pulverized coal steam power plants must be increased to as high a temperature and pressure as feasible. The proposed steam temperature in the DOE/NETL Advanced Ultra Supercritical power plant is high enough (760°C) that ferritic/martensitic steels will not work for the majority of high-temperature components in the turbine or for pipes and tubes in the boiler due to temperature limitations of this class of materials. Thus, Ni-based superalloys are being considered for many of these components. Off-the-shelf forged nickel alloys have shown good promise at these temperatures, but further improvements can be made through experimentation within the nominal chemistry range as well as through thermomechanical processing and subsequent heat treatment. However, cast nickel-based superalloys, which possess high strength, creep resistance, and weldability, are typically not available, particularly those with good ductility and toughness that are weldable in thick sections. To address those issues related to thick casting for turbine casings, for example, cast analogs of selected wrought nickel-based superalloys such as alloy 263, Haynes 282, and Nimonic 105 have been produced. Alloy design criteria, melt processing experiences, and heat treatment are discussed with respect to the as-processed and heat-treated microstructures and selected mechanical properties. The discussion concludes with the prospects for full-scale development of a thick section casting for a steam turbine valve chest or rotor casing.

Creep strength and ductility of 9 to 12% chromium steels

The present paper focuses in on long-term creep properties of parent material of the new 9-12%Cr creep resistant steels, P91, E911 and P92 developed for use in advanced ultrasupercritical power plants. These steels have been at the center of activities in the ECCC Working Group 3A (WG3A) “Ferritic Steels”, which covers creep data development and analysis for parent materials and welds of all ferritic creep resistant steels ranging from low alloy steels up to 12%Cr steels. The opinions stated in the paper represent the views of the author rather than the whole ECCC WG3A group.

Creep strength and ductility of 9 to 12% chromium steels

The present paper focuses in on long-term creep properties of parent material of the new 9-12%Cr creep resistant steels, P91, E911 and P92 developed for use in advanced ultrasupercritical power plants. These steels have been at the center of activities in the ECCC Working Group 3A (WG3A) “Ferritic Steels”, which covers creep data development and analysis for parent materials and welds of all ferritic creep resistant steels ranging from low alloy steels up to 12%Cr steels. The opinions stated in the paper represent the views of the author rather than the whole ECCC WG3A group.