Search Results for thermodynamics calculation

Inconel alloy 740H is designated for boiler sueprheater/reheater tubes and main steam/header pipes application of advanced ultra-supercritical (A-USC) power plant at operating temperatures above 750°C. Microstructure evolution and precipitates stability in the samples of alloy 740H after creep-rupture test at 750°C, 800°C and 850°C were characterized in this paper by scanning electron microscopy, transmission electron microscopy and chemical phase analysis in details. The phase compositions of alloy 740H were also calculated by thermodynamic calculation. The research results indicate that the microstructure of this alloy keeps good thermal stability during creep-rupture test at 750°C, 800°C and 850°C. The precipitates are MC, M 23 C 6 and γ′ during creep-rupture test. The temperature of creep test has an important effect on the growth rate of γ′ phase. No harmful and brittle σ phase was found and also no γ′ to η transformation happened during creep. Thermodynamic calculations reveal almost all the major phases and their stable temperatures, fractions and compositions in the alloy. The calculated results of phase compositions are consistent with the results of chemical phase analysis. In brief, except of coarsening of γ′, Inconel alloy 740H maintains the very good structure stability at temperatures between 750°C and 850°C.

Materials are developed and improved by adjusting both the alloy chemistry and the processing conditions to achieve desired microstructures and properties. Traditionally, these improvements have been made by a slow and labor-intensive series of experiments. But it is now possible to replace this expensive trial and error process by carrying out only a few ‘key’ experiments in conjunction with thermodynamic calculations. These calculations are powerful tools for alloy design, enabling improvement in the selection of alloy chemistry and the parameters used for fabrication steps such as heat treatments. In order to have the utmost confidence in the results obtained from the calculations, it is essential to have high quality thermodynamic databases. Such databases can be used not only in phase equilibrium calculations but also as the critical input for further kinetic simulations. In the present paper, we present our work on the development of reliable thermodynamic databases for nickel-based superalloys and iron alloys. We first briefly describe the methodology of developing these databases and then discuss some specific examples using the databases. With the aid of these examples, the usefulness of thermodynamic databases in aiding the development of advanced materials is discussed.

A new nickel-base superalloy GH750 has been developed as boiler tube of advanced ultrasupercritical (A-USC) power plants at temperatures about and above 750°C in China. This paper researched the weld solidification of GH750 filler metal, microstructure development and property of GH750 welded joint by gas tungsten arc weld. Liquid fraction and liquid composition variation under non-equilibrium state were calculated by thermo-dynamic calculation. The weld microstructure and the composition in the dendrite core and interdendritic region were analyzed by SEM(EDX) in detail. The investigated results show that there is an obvious segregation of precipitation-strengthening elements during the weld solidification. Titanium and Niobium are the major segregation elements and segregates in the interdendritic region. It was found that the changing tendency of the elements’ segregation distribution during the solidification of GH750 deposit metal is agree with the thermodynamic calculation results. Till to 3,000hrs’ long exposure at 750°C and 800°C, in comparison with the region of dendrite core of solidification microstructure, not only the coarsening and the accumulation of γʹ particles are remarkable in the interdendritic region, but also the small quantity of the blocky and needle like η phases from. The preliminary experimental results indicate that the weakening effect of creep-rupture property of the welded joint is not serious compared with GH750 itself.

Creep strength enhanced ferritic steels like T/P 91 and T/P 92 are widely used for the fabrication of pressure vessel components in the petro-chemical and thermal power industry. Today, a new generation of 9-12% Cr CSEF steels like MARBN, Save12AD, G115 and Super VM12 are entering into the market. All CSEF steels require an accurate post-weld heat treatment after welding. This paper discusses the impact of chemical composition on Ac1 as well as the transformation behavior during post-weld heat treatment in a temperature range below and above Ac1. The Ac1 temperature of weld metals with variations in chemical composition has been determined and thermodynamic calculations has been carried out. Simulations of heat treatment cycles with variations in temperature have been carried out in a quenching dilatometer. The dilatation curves have been analyzed in order to detect any phase transformation during heating or holding at post weld heat treatment. Creep rupture tests have been carried out on P91 and Super VM12 type weld metals in order to investigate the effect of sub- and intercritical post weld heat treatment on creep rupture strength.

Metallurgical factors affecting the fusion boundary failure and damage mechanism of DMWs (Dissimilar Metal Welds) between the CSEF (Creep Strength Enhanced Ferritic) steels and austenitic steels were experimentally and theoretically investigated and discussed. Long-term exservice DMWs up to 123,000 hours were investigated; the precipitates near the fusion boundary were identified and quantitatively evaluated. Comparing with the other generic Ni-based weld material, MHPS original filler metal HIG370 (Ni bal.-16Cr-8Fe-2Nb-1Mo) showed superior suppression effect on fusion boundary damage of DMWs, which was verified by both of the microstructure observation and thermodynamic calculation. Based on the microstructure observation of crept specimen and ex-service samples of DMWs, temperature, time and stress dependence of fusion boundary damage of DMWs were clarified. Furthermore, fusion boundary damage morphology and mechanism due to precipitation and local constituent depletion was discussed and proposed from metallurgical viewpoints.

A new nickel-based superalloy, designated as GH750, was developed to meet the requirements of high temperature creep strength and corrosion resistance for superheater/reheater tube application of A-USC power plants at temperatures above 750°C. This paper introduces the design of chemical composition, the process performance of tube fabrication, microstructure and the properties of alloy GH750, including thermodynamic calculation, room temperature and high temperature tensile properties, stress rupture strength and thermal stability. The manufacturing performance of alloy GH750 is excellent and it is easy to forge, hot extrusion and cold rolling. The results of the property evaluation show that alloy GH750 exhibits high tensile strength and tensile ductility at room and high temperatures. The 760°C/100,000h creep rupture strength of this alloy is larger than 100MPa clearly. Microstructure observation indicates that the precipitates of GH750 consist of the precipitation strengthening phase γ’, carbides MC and M 23 C 6 and no harmful and brittle TCP phases were found in the specimens of GH750 after long term exposure at 700~850°C. It can be expected for this new nickel-based superalloy GH750 to be used as the candidate boiler tube materials of A-USC power plants in the future.

To improve microstructure stability at temperature up to 700°C and avoid segregation of Nb during melting processes, we modified the chemical composition of conventional Ni-Fe base super alloy(Ni-36Fe-16Cr-3Nb-1.7Ti-0.3Al:Alloy706). It is known that Alloy706 is strengthened by γ'(Ni 3 Al) phase and γ”(Ni 3 Nb) phase. But these phases are unstable at high temperature and transform into Nb rich δ or η) phase after long-term exposure to elevated temperature. Therefore modified alloy contains lower Nb and higher Al than those of Alloy706, and it is mainly strengthened by γ’(Ni 3 Al) phase. In fact we could not find δ or η phase in the modified alloy after creep and aging at 700 °C. Tensile strengths of the modified alloy at temperature from room temperature to 700 °C were almost same as those of Alloy706. Yield strength of modified alloy at room temperature was slightly lower than that of Alloy706, but equivalent to that of Alloy706 at higher temperatures. Tensile and yield strengths of the modified alloy at temperature from room temperature to 700 °C were higher than those of Alloy706 after aging at 700 °C. In this paper, we discuss effects of Nb and Al on mechanical properties and microstructure at elevated temperature up to 700 °C, using mechanical testing, TEM observations and thermodynamics calculation results. And we show advantages of the microstructure stabilized Ni-Fe base superalloy(FENIX-700), which is a candidate material for 700 °C class USC steam turbine rotor.

In order to develop an Fe/Ni dissimilar-weld rotor structure for an Advanced Ultra Super Critical turbine, fundamental studies on the metallurgical properties of Fe/Ni welds are needed. In the work reported in this paper, we studied the microstructure evolution and creep rupture properties of Fe/Ni weld joints with different compositions. Investigation of thermally aged Fe/Ni diffusion couples revealed that Fe-based ferritic steel and Alloy 617 weld joints with a large difference in Cr content showed strong C diffusion at the weld interface. This decreased the creep rupture life of the weld joint, caused by coarsening of a martensitic structure near the interface. Analysis using Fe/Ni diffusion couples and thermodynamic calculations suggested that the driving force of C diffusion is the chemical potential gradient at the interface, and the difference in Cr content between Fe and Ni accelerates the C diffusion.

The development of materials technologies for piping and tubing in advanced ultrasupercritical (A-USC) power plants operating at steam temperatures above 700°C represents a critical engineering challenge. The 23Cr-45Ni-7W alloy (HR6W), originally developed in Japan as a high-strength tubing material for 650°C ultra-supercritical (USC) boilers, was systematically investigated to evaluate its potential for A-USC plant applications. Comparative research with γ-strengthened Alloy 617 revealed that the tungsten content is intimately correlated with Laves phase precipitation and plays a crucial role in controlling creep strength. Extensive creep rupture tests conducted at temperatures between 650-800°C for up to 60,000 hours demonstrated the alloy's long-term stability, with 105-hour extrapolated creep rupture strengths estimated at 88 MPa at 700°C and 64 MPa at 750°C. Microstructural observations after creep tests and aging confirmed the material's microstructural stability, which is closely linked to long-term creep strength and toughness. While Alloy 617 exhibited higher creep rupture strength at 700 and 750°C, the materials showed comparable performance at 800°C. Thermodynamic calculations and microstructural analysis revealed that the Laves phase in HR6W gradually decreases with increasing temperature, whereas the γ' phase in Alloy 617 rapidly diminishes and almost completely dissolves at 800°C, potentially causing an abrupt drop in creep strength above 750°C. After comprehensive evaluation of creep properties, microstructural stability, and other reported mechanical characteristics, including creep-fatigue resistance, HR6W emerges as a promising candidate for piping and tubing in A-USC power plants.

A new nickel-base superalloy, Inconel alloy 740, is being developed for ultra-supercritical (USC) boiler applications operating above 750°C, designed to meet critical requirements for long-term high-temperature stress rupture strength (100 MPa for 10 5 hours) and corrosion resistance (2 mm/2 × 10 5 hours). Experimental investigations revealed key structural changes at elevated temperatures, including γ coarsening, γ' to η transformation, and G phase formation. To enhance strengthening effects and structural stability, researchers conducted a systematic optimization process based on thermodynamic calculations, implementing small adjustments to several alloying elements and designing modified alloy compositions. Comprehensive testing examined the long-term structural stability of these modifications, with investigations conducted up to 5,000 hours at 750 and 800°C, and 1,000 hours at 850°C. Mechanical property and oxidation resistance tests compared the modified alloys with the original Inconel alloy 740, yielding preliminary results that demonstrate minimal modifications can improve stress rupture strength while maintaining corrosion resistance. Microstructural examinations further confirmed the enhanced thermal stability of the modified alloy, positioning Inconel alloy 740 as a promising candidate for USC boiler applications at 750°C or higher temperatures.

In this paper we tried to model the creep-strength degradation of selected advanced creep resistant steels which occurs under operating conditions. In order to accelerate some microstructure changes and thus to simulate degradation processes in long-term service, isothermal ageing at 650°C for 10 000 h was applied to P91, P92 and P23 steels in their as- received states. The tensile creep tests were performed at temperature 600°C in argon atmosphere on all steels both in the as-received state and after isothermal ageing, in an effort to obtain a more complete description of the role of microstructure stability in high temperature creep of these steels. Creep tests were followed by microstructure investigations by means of transmission and scanning electron microscopy and by the thermodynamic calculations. The applicability of the creep tests was verified by the theoretical modelling of the phase equilibrium at different temperatures. It is suggested that under restricted oxidation due to argon atmosphere microstructure instability is the main detrimental process in the long-term degradation of the creep rupture strength of these steels.

The voestalpine foundry group, operating at locations in Linz and Traisen, Austria, specializes in heavy steel casting components ranging from 1 to 200 tons for power generation, oil and gas, chemical processing, and offshore applications. Their manufacturing expertise encompasses high-alloyed martensitic 9-12% Cr-steels and nickel-based Alloy 625, particularly for ultra-supercritical (USC) and advanced USC power generation systems operating at temperatures from 600°C to over 700°C. The production of these complex, thick-walled components relies on advanced thermodynamic calculation and simulation for all thermal processes, from material development through final casting. The foundries’ comprehensive capabilities include specialized melting, molding, heat treatment, non-destructive testing, and fabrication welding, with particular emphasis on joining dissimilar cast, forged, and rolled materials. Looking toward future innovations, the group is exploring additive manufacturing for mold production and robotic welding systems to enhance shaping and surface finishing capabilities.

A combination of creep tests, ex-service blade samples, thermodynamic equilibrium calculations, combined thermodynamic and kinetic calculations, image analysis, chemical composition mapping and heat treatments have been conducted on PWA1483 to determine if microstructural rejuvenation can be achieved when taking the presence of oxidation coatings into account as part of a blade refurbishment strategy. The work has shown that the γ′ morphology changes during creep testing, and that through subsequent heat treatments the γ′ microstructure can be altered to achieve a similar γ′ size and distribution to the original creep test starting condition. Thermodynamic equilibrium calculations have been shown to be helpful in determining the optimum temperatures to be used for the refurbishment heat treatments. The interaction of oxidation resistant coatings with the alloy substrate and refurbishment process have been explored with both experimental measurements and coupled thermodynamic and kinetic calculations. The predictive nature of the coupled thermodynamic and kinetic calculations was evaluated against an ex-service blade sample which had undergone refurbishment and further ageing. In general there was good agreement between the experimental observations and model predictions, and the modelling indicated that there were limited differences expected as a result of two different refurbishment methodologies. However, on closer inspection, there were some discrepancies occurring near the interface location between the coating and the base alloy. This comparison with experimental data provided an opportunity to refine the compositional predictions as a result of both processing methodologies and longer term exposure. The improved model has also been used to consider multiple processing cycles on a sample, and to evaluate the coating degradation between component service intervals and the consequences of rejuvenation of the blade with repeated engine exposure. The results from the experimental work and modelling studies potentially offer an assessment tool when considering a component for refurbishment.

25Cr-20Ni-Nb-N (Tp310HCbN) steel is a promising austenitic steel for applications in superheater tubes in coal fired thermal power plants due to the high creep strength and oxidation resistance. In this work, the microstructural evolution of this material during heat treatment and thermal ageing has been investigated. The investigations were carried out by Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Energy Dispersive Spectroscopy (EDS). Besides, equilibrium and Scheil calculations were carried out using the thermodynamic software MatCalc to analyse the stable phases and the solidification process, respectively. Precipitation calculations during solution annealing and subsequent ageing at 650 and 750°C were performed to predict the phase fraction and precipitates radius up to 10.000h ageing time. SEM and TEM investigations of aged specimens revealed the presence of six different precipitates: M 23 C 6 , Cr 2 N, sigma, Z-phase, eta-phase (Cr 3 Ni 2 Si(C,N)) and Nb(C,N). These precipitates were predicted and confirmed by MatCalc simulations. The calculated phase fraction and mean radius show good agreement with experimental data. Finally, simulations of different Cr-, C- and N-content in Tp310HCbN were performed.

The precipitation behavior of various phases in austenitic heat-resistant model steels, including the Fe 2 Nb Laves phase (C14 structure) on grain boundaries (GB) and grain interiors (GI), and the Ni 3 Nb metastable γ“ phase and stable δ phase on GI, was investigated through experimental study at different temperatures and thermokinetic calculation. The steel samples were prepared by arc melting followed by 65% cold rolling. Subsequently, the samples were solution treated within the γ single-phase region to control the grain size to approximately 150 μm. Aging of the solution-treated samples was carried out at temperatures ranging from 973 K to 1473 K for up to 3600 hours. Microstructural observations were conducted using FE-SEM, and the chemical compositions of the γ matrix and precipitates of Laves and δ phases were analyzed using EPMA. The precipitation modeling was performed using MatCalc software, utilizing a thermodynamic database constructed by our research group to calculate the chemical potential of each phase. Classical nucleation theory was applied for nucleation, while the SFFK model was employed for the growth and coarsening stages. Distinct phases were defined for grain boundary and grain interior Laves phase, with all precipitates assumed to have spherical morphology in the calculations. The precipitation start time was defined as the time when the precipitate fraction reached 1%. Experimental results indicated that above 973 K, Laves phase nucleation primarily occurred on grain boundaries before extending into the grain interior, with the nose temperature located around 1273 K. To replicate the experimentally determined Time-Temperature-Precipitation (TTP) diagram, interaction parameters among elements were adjusted. Additionally, by introducing lower interfacial energy between the γ matrix and Laves phase, the TTP diagram was successfully reproduced via calculation, suggesting relative stability at the interface.

A new Ni-base superalloy has been developed for Advanced Ultra Super Critical (A-USC) power plants operating above 750°C, targeting reduced CO 2 emissions through improved efficiency. While existing research focuses on 700°C-class materials, this study presents a novel alloy design for higher-temperature applications. Using the CALPHAD method, a prototype alloy (Ni-23Co-18Cr-8W-4Al-0.1C) was developed by eliminating Ti, Nb, and Ta to improve hot-workability while maintaining strength. The resulting alloy demonstrates twice the creep strength of Nimonic 263, with an estimated 10 5 h steam turbine creep resistance temperature of 780°C, marking a significant advancement in A-USC material capabilities.

For VM12-SHC 11-12 wt. % Cr steel, there have been no systematic investigations to define the regions or characterise the microstructures within the heat-affected zone (HAZ) of weldments. In similar steels, these regions relate to the Ac 1 and Ac 3 transformation temperatures and can affect weldment performance. In this study, controlled thermal cycles were applied to VM12-SHC parent metal using a dilatometer and the Ac 1 and Ac 3 temperatures were measured for various heating rates. The Ae 1 and Ae 3 temperatures were also calculated by thermodynamic equilibrium modeling. Through dilatometry, thermal cycles were then applied to simulate the microstructures of the classically defined HAZ regions. The microstructural properties of each simulated material were investigated using advanced electron microscopy techniques and micro-hardness testing. It was found that the simulated HAZ regions could be classified as; (1) the completely transformed (CT) region, with complete dissolution of pre-existing precipitates and complete reaustenitisation; (2) the partially transformed (PT) region, exhibiting co-existing original martensite with nucleating austenite microstructures with partial dissolution of precipitates; and (3) the over tempered (OT) region, with no phase transformation but precipitate coarsening and decreased hardness.

A comprehensive fireside corrosion study was undertaken to better understand the corrosion mechanisms operating on the superheaters and lower furnace walls of advanced coal- fired utility boilers. The study intended to evaluate the fireside conditions generated from burning eight U.S. coals individually in a pilot-scale combustion facility. These coals consisted of a wide range of compositions that are of interest to the utility industry. The combustion facility was capable of producing the realistic conditions of staged combustion existing in coal-fired utility boilers. During each of the combustion tests, gas and deposit samples were collected and analyzed via in-furnace probing at selected locations corresponding to the waterwalls and superheaters. Testing of five of the eight coal groups has been completed to date. Results of these online measurements helped reveal the dynamic nature of the combustion environments produced in coal-fired boilers. Coexistence of reducing and oxidizing species in the gas phase was evident in both combustion zones, indicating that thermodynamic equilibrium of the overall combustion gases was generally unattainable. However, the amount of sulfur released from coal to form sulfur-bearing gaseous species in both the reducing and oxidizing zones was in a linear relationship with the amount of the total sulfur in coal, independent of the original sulfur forms. Such a linear relationship was also observed for the measured HCl gas relative to the coal chlorine content. However, the release of sulfur from coal to the gas phase appeared to be slightly faster and more complete than that of chlorine in the combustion zone, while both sulfur and chlorine were completely released and reacted to form respective gaseous species in the oxidizing zone. The information of sulfur and chlorine release processes in coal combustion generated from this study is considered new to the industry and provides valuable insight to the understanding of fireside corrosion mechanisms.

A global movement is pushing for improved efficiency in power plants to reduce fossil fuel consumption and CO 2 emissions. While raising operating temperatures and pressures can enhance thermal efficiency, it necessitates materials with exceptional high-temperature performance. Currently, steels used in power plants operating up to 600°C achieve efficiencies of 38-40%. Advanced Ultra Supercritical (A-USC) designs aim for a significant leap, targeting steam temperatures of 700°C and pressures of 35 MPa with a lifespan exceeding 100,000 hours. Ni-based superalloys are leading candidates for these extreme conditions due to their superior strength and creep resistance. Haynes 282, a gamma prime (γ′) precipitation-strengthened alloy, is a promising candidate for A-USC turbine engines, exhibiting excellent creep properties and thermal stability. This research investigates the microstructural evolution in large, sand-cast components of Haynes 282. Microstructure, referring to the arrangement of grains and phases within the material, significantly impacts its properties. The research examines the alloy in its as-cast condition and after various pre-service heat treatments, aiming to fully identify and quantify the microstructural changes. These findings are then compared with predictions from thermodynamic equilibrium calculations using a dedicated Ni alloy database. The research reveals that variations in heat treatment conditions can significantly affect the microstructure development in Haynes 282, potentially impacting its mechanical properties.

To develop 10-ton class forgings with adequate long-term strength and without segregation defects for A-USC steam turbine rotors, researchers modified the chemical composition of Alloy 706 to improve its microstructure stability and segregation properties. The modified Alloy, named FENIX-700, is a γ' phase strengthened alloy without a γ" phase, and its microstructure stability is superior to Alloy 706 at 700°C, as demonstrated by short-term aging tests and phase stability calculations using the CALPHAD method. A trial disk 1-ton class forging of FENIX-700 was manufactured from a double-melted ingot, with tensile and creep strength of the forging equivalent to that of 10-kg class forgings, indicating a successful trial. Long-duration creep tests were performed using 10-kg class forgings, revealing an approximate 105-hour creep strength at 700°C higher than 100 MPa. Manufacturability tests showed that FENIX-700 performs better than Alloy 706, as evidenced by segregation tests using a horizontal directional solidification furnace and hot workability tests. Microstructure observation and tensile tests on 10,000-hour aged specimens (at temperatures of 650, 700, and 750°C) revealed degradation of tensile strength and yield stress due to coarsening of the γ' phase, but also showed enhanced ductility through aging. The microstructure stability of FENIX-700 at 700°C was confirmed as excellent through microstructure observation of the 10,000-hour aged sample and supporting thermodynamic considerations.