Current demands of the power generation market require components with improved materials properties. The focus is not only on the higher operation temperatures and pressures but also more frequent cycling to accommodate the energy produced from renewable sources. Following the successful developments of steels within the COST 501, 522 and 536 programmes, further advances are being researched. As nickel superalloys remain an expensive option for coal power plants, there is a significant drive for improvements of 9-12% Cr steels to meet new demands. The most promising of the potential candidates identified for 650°C application is MarBN steel (9Cr-3Co-3W-V-Nb). This paper reviews the current state of European developments on MarBN steel. Work on this alloy has been carried out for the last 5 years. Initial projects focused on development of the cast components. UK IMPACT and following INMAP projects successfully demonstrated manufacturing capabilities of large casting components. More recent collaborations aim to develop full-size boiler components and large rotor forgings as well as further examine the properties in the operating conditions (i.e. corrosion and oxidation resistance, creep-fatigue behaviour). Additionally significant focus is placed on modelling the behaviour of MarBN components, in terms of both microstructural changes and the resulting properties.

Nickel-based Alloy 617B (DIN 2.4673) and Alloy C-263 (DIN 2.4650) with high creep strength and good fabricability are promising material candidates for the design of next generation coal-fired “Advanced Ultra-Super-Critical A-USC” power plants with advanced steam properties and thus higher requirements on the material properties. Microstructural studies of the precipitation hardened alloy C-263 were performed with Electron Microscopy (TEM) with respect to their strengthening precipitates like carbides and intermetallic gamma prime. Specimens were subjected to different ageing treatments at elevated temperatures for different times. The microstructural results of the investigated nickel alloy C-263 are presented and discussed with respect to their correlation with required properties for A-USC, e.g. the mechanical properties, the creep resistance and the high temperature stability and compared to Alloy 617B. The manufacturing procedure for the prematernal and forgings as well as for thin walled tube components for A-USC power plants is presented.

G115 is a novel ferritic heat resistant steel developed by CISRI in the past decade. It is an impressive candidate material to make tubes, pipes, and forgings for advanced ultra super critical (A-USC) fossil fired power plants used for the temperature scope from 600°C to 650°C. The successful development of G115 extends the upper application temperature limitation of martensitic steel from 600°C to about 650°C. This breakthrough is imperative for the design and construction of 610°C to 650°C A-USC fossil fired power plants, from the viewpoint of the material availability and economics of coal fired power plant designs. This paper introduces the development history and progress of G115 steel. The strengthening mechanism of the novel martensitic steel is briefly discussed, and the optimized chemical composition and mechanical properties of G115 steel are described. The details of industrial trials of G115 tube and pipe at BaoSteel in the past years are reviewed, with the emphasis on the microstructure evolution during aging and creep testing. These tests clearly show that the microstructure of G115 steel is very stable up to the temperature of 650°C. Correspondingly, the comprehensive mechanical properties of G115 steel are very good. The creep rupture time is longer than 17000 hours at the stress of 120MPa and at the temperature of 650°C and 25000+ hours at the stress of 100MPa and at the temperature of 650°C, which is about 1.5 times higher than that of P92 steel. At the same time, the oxidation resistance of G115 steel is a little bit better than that of P92 steel. If G115 steel is selected to replace P92 pipes at the temperature scope from 600°C to 650°C, the total weight of the pipe can be reduced by more than 50% and the wall thickness of the pipe can be reduced up to about 55%. In addition, the upper application temperature limitation of G115 steel is about 30°C higher than that of P92 steel. Thus, G115 steel is a strong candidate material for the manufacturing of 600+°C advanced ultra-super-critical (A-USC) fossil fuel power plants in China and elsewhere.

Inconel 740H is one of the most promising candidate Ni-base superalloys for the main steam pipe of 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants. After processing and welding in manufacturing plant in solution-annealed state, large components was commonly suggested to have an extra aging treatment at 800 °C for 16 h, in order to obtain homogeneous γ′ precipitates. In this present work, creep tests and microstructure analyses were conducted on Inconel 740H pipe specimens under two different heat treatments to verify the necessity of aging process. Here we show that aging treatment has limited effect on the creep rupture life of Inconel 740H pipe. Both in grain interiors and along grain boundaries, crept specimens under two different heat treatments have the same precipitates. But the shape and distribution of γ′ in solution annealed sample is not as regular as the aged ones. Our results provide the underlying insight that aging treatment is not so necessary for the straight pipes if the on-site condition was hard to control. But for both groups of specimens, a small amount of h particles and some banded like M 23 C 6 were emerged during creep, which would be harmful to mechanical properties for the long run.

This paper briefly introduces the state-of-the-art of the research and development of candidate heat resistant materials used for the manufacturing of 700°C advanced ultra-super-critical (AUSC) fossil fuel power plants (PP) in China, especially, focus on the impressive progress in the past three years. The detailed advancements (technical exploration and industrial investigation) of candidate materials spectra for the boiler system of A-USC PP will be presented in the current paper, including novel ferritic heat resistant steels, advanced austenitic heat resistant steels, Fe- Ni-based alloys and Ni-based alloys, which serve and cover the steam temperature scope from 600°C to 720°C. Some newly available data associated with above materials will be released.

This paper addresses the limitations of P92 steel used in ultra-supercritical power plants, particularly ferrite formation in thick components and its impact on short- and long-term properties. A guideline for determining ferritic content in P92 steel is presented. Furthermore, a novel 12% Cr boiler steel grade, VM12-SHC, is introduced. This new material offers good creep properties and oxidation resistance, overcoming the limitations of P92 steel. Finally, the development of matching filler metals for welding P92 and VM12-SHC steels is presented, ensuring optimal weld compatibility and performance in power plant applications.

A European project (COST 522) aims to improve power plant efficiency by developing stronger steel for steam turbines. These turbines operate with extremely hot steam (up to 650°C) to maximize efficiency and minimize emissions. The project focuses on ferritic-martensitic steel, which is suitable for the thick components used in these high-temperature environments. Building on prior advancements, COST 522 explored new steel formulations and tested them thoroughly. This has resulted in steels capable of withstanding even hotter steam (610°C live steam and 630°C reheat steam), paving the way for the next generation of highly efficient power plants.

Achieving long-term stability of the tempered martensite is considered crucial for increasing the creep resistance of steels at elevated temperatures above 700°C. It is noted that at low stress levels, the creep deformation of the tempered martensite proceeds heterogeneously around prior austenite grain boundaries, as excess dislocations inside the grain are difficult to rearrange. This paper presents a new approach using carbon-free martensitic alloys strengthened by intermetallic compounds. An iron-nickel-cobalt martensite matrix with Laves phase as the precipitate is selected. The creep characteristics are discussed across a wide range of testing conditions, and the thermal cycle test behavior is examined to evaluate the potential of these alloys for future ultrasupercritical power plants operating in severe environments.

Three Ni-base wrought alloys with different hardening mechanisms (INCONEL 706, Waspaloy and INCONEL 617) were investigated as candidates for steam turbine rotor applications at temperatures up to 700 °C in respect to their microstructure and microstructural stability. The Nb containing alloy Inconel 706 had a complex microstructure with γ', γ" and η phases which are stable in long term service up to 620 °C. At higher temperatures significant particle coarsening and phase transformation were observed. Waspaloy is hardened by γ' particles and after ageing at 700 °C and higher, it tended to a stable microstructure. Inconel 617 is a solid solution hardened material additionally hardened by homogeneously distributed fine M 23 C 6 carbides. After long term ageing at temperatures of 650 °C to 750 °C the carbides tended to form carbide films along the grain boundaries and at 700 °C to 750 °C γ' precipitated as homogeneously distributed particles with low coarsening during long term service. In order to optimize the candidate alloys Inconel 706 and Waspaloy were modified to the new alloys DT 706 and DT 750. The aspects of modification and first experimental results are reported.

Creep deformation behavior of the T122 type steels with different matrix phases such as α’ (martensite) and α’+δ (martensite and delta-ferrite) at different stress levels has been studied comparing with those of the model steels with the initial microstructures consisting of the various combination of matrices such as ferrite (α), martensite (α’) and austenite (γ), and precipitates such as MX and M 23 C 6 . The heterogeneous creep deformation is found to be pronounced at lower stress level in the steel with a dual phase matrix of α’+δ, resulting in a complex sigmoidal nature in the creep rupture life. The creep deformation process of the steel with the dual phase matrix is similar to that of the model steel with the α phase matrix which exhibits a typical heterogeneous creep deformation and the early transition to the acceleration creep at a very small creep strain. Such a heterogeneous creep deformation is much pronounced along the interfaces between the soft δ ferrite and the hard martensite (α’) phases, and has a viscous nature in creep deformation which was first identified in P91 steel. It is concluded that the homogeneous microstructure is a key for achieving the long-term creep strength in the advanced ferritic steels at elevated temperatures over 600°C.

The U.S. Department of Energy (DOE) and the Ohio Coal Development Office (OCDO) have recently initiated a project aimed at identifying, evaluating, and qualifying the materials needed for the construction of the critical components of coal-fired boilers capable of operating at much higher efficiencies than current generation of supercritical plants. This increased efficiency is expected to be achieved principally through the use of ultrasupercritical steam conditions (USC). The project goal initially was to assess/develop materials technology that will enable achieving turbine throttle steam conditions of 760°C (1400°F)/35 MPa (5000 psi), although this goal for the main steam temperature had to be revised down to 732°C(1350°F), based on a preliminary assessment of material capabilities. The project is intended to build further upon the alloy development and evaluation programs that have been carried out in Europe and Japan. Those programs have identified ferritic steels capable of meeting the strength requirements of USC plants up to approximately 620°C (1150°F) and nickel-based alloys suitable up to 700°C (1300°F). In this project, the maximum temperature capabilities of these and other available high- temperature alloys are being assessed to provide a basis for materials selection and application under a range of conditions prevailing in the boiler. This paper provides a status report on the progress to date achieved in this project.

Early supercritical units such as American Electric Power (AEP) Philo U6, the world’s first supercritical power plant, and Eddystone U1 successfully operated at ultrasupercritical (USC) levels. However due to the unavailability of metals that could tolerate these extreme temperatures, operation at these levels could not be sustained and units were operated for many years at reduced steam (supercritical) conditions. Today, recently developed creep strength enhanced ferritic (CSEF) steels, advanced austenitic stainless steels, and nickel based alloys are used in the components of the steam generator, turbine and piping systems that are exposed to high temperature steam. These materials can perform under these prolonged high temperature operating conditions, rendering USC no longer a goal, but a practical design basis. This paper identifies the engineering challenges associated with designing, constructing and operating the first USC unit in the United States, AEP’s John W. Turk, Jr. Power Plant (AEP Turk), including fabrication and installation requirements of CSEF alloys, fabrication and operating requirements for stainless steels, and life management of high temperature components

By utilizing computational thermodynamics in a Design of Experiments approach, it was possible to design and manufacture nickel-base superalloys that are strengthened by the eta phase (Ni3Ti), and that contain no gamma prime (Ni3Al,Ti). The compositions are similar to NIMONIC 263, and should be cost-effective, and have more stable microstructures. By varying the aging temperature, the precipitates took on either cellular or Widmanstätten morphologies. The Widmanstätten-based microstructure is thermally stable at high temperatures, and was found to have superior ductility, so development efforts were focused on that microstructure. High temperature tensile test and creep test results indicated that the performance of the new alloys was competitive with NIMONIC 263. SEM and TEM microscopy were utilized to determine the deformation mechanisms during creep.

This overview paper contains a part of structure stability study on advanced austenitic heat-resisting steels (TP347H, Super304H and HR3C) and Ni-base superalloys (Nimonic80A, Waspaloy and Inconel740/740H) for 600-700°C A-USC fossil power plant application from a long-term joint project among companies, research institutes and university in China. The long time structure stability of these advanced austenitic steel TP347H, Super304H, HR3C in the temperature range of 650-700 °C and Ni-base superalloys Nimonic80A, Waspaloy and Inconel740/740H in the temperature range of 600-800°C till 10,000h have been detailed studied in this paper.

To improve efficiency and flexibility and reduce CO 2 emissions, advanced ultra super critical (AUSC) power plants are under development, worldwide. Material development and its selection are critical to the success of these efforts. In several research and development programs / projects the selection of materials is based on stress rupture, oxidation and corrosion tests. Without doubt, these criteria are important. To improve the operational flexibility of modern power plants the fatigue properties are of increased importance. Furthermore, for a safe operation and integrity issues the knowledge about the crack behavior is essential. Crack initiation and crack growth may be caused by natural flaws or cracks induced by component operation. In order to develop new materials, properties like tensile strength and creep strength are an important part of qualification and subsequent approval by notified bodies. Consequently short term properties as well as time-temperature dependent properties are generated and taken into considerations. In the case of high strength γ'-strengthening nickel-base alloys investigating the creep crack behavior is also strongly recommended. This article shows results of currently investigated nickel-based alloys for newly developed headers, pipes and other high temperature boiler applications and their critical creep crack propagation behavior.

A new 9%Cr steel with high boron levels (boron steel) has been developed by optimization studies on steels and alloys that are applicable to advanced ultra-super critical power plants operated at steam conditions of 700°C and 30 MPa and above. The composition and heat treatment condition of boron steel was optimized by the initial hardness, tensile strength, yield strength, and Charpy impact values on the basis of the fundamental investigation with the stability of the long-term creep strength. Creep testing of boron steel was conducted at temperatures between 600 and 700°C. The creep rupture strength at 625°C and 105 h is estimated to be 122 MPa for the present 9% Cr steel with high boron by Larson-Miller parameter method. Furthermore, physical properties as a function of temperature, metallurgical properties, tensile properties, and toughness were examined to evaluate the applicability of the steel for a 625°C USC power plant boiler. It was also confirmed that the steel has good workability for such an application by the flaring and flattening tests with tube specimens having an outer diameter of approximately 55 mm.

The need to reduce carbon dioxide emissions of new fossil power plants is one of the biggest challenges of mankind in the next decades. In this context increasing net efficiency is the most important aspect which has led to the development of not only new steels for potential plant operation up to 650°C, but also to forged nickel alloys for 700°C and maybe 750°C. For steam temperatures of 700°C Alloy 617 and variants like TOS1x have been already intensively investigated, and manufacturability of large rotor parts was demonstrated. For operation temperatures of 750°C, only the use of γ‘ age-hardenable nickel base alloys is possible. Alloy 263 is one of the most promising alloys for manufacturing large forged components. For this material grade Saarschmiede has produced successfully a large rotor forging for the first time. Considering the complexity in manufacturing large nickel base alloy forgings, the implementation of simulation tools for calculation and optimization of production parameters becomes especially important. Numerical simulation methods are essential to predict material behavior and to optimize material quality-related manufacturing steps. In reference to mechanical properties, microstructure, uniformity of chemical composition FEM computer simulations for the key manufacturing processes re-melting, forging and heat treatment are in application. This paper will present the current status of production of very large prototype nickel base alloy rotor forgings for 700°C and 750°C A-USC power plants. Test results of an Alloy 617 large full scale turbine rotor component recently with improved properties produced will be highlighted. Experiences and results in applying numeric simulation models to ingot manufacturing and forging will also be reported.

The A-USC technology is still under development due to limited number of materials complying with the requirements of high creep strength and high performance in highly aggressive corrosion environments. Development of power plant in much higher temperatures than A-USC is currently impossible due to the materials limitation. Currently, nickel-based superalloys besides advanced austenitic steels are the viable candidates for some of the A-USC components in the boiler, turbine, and piping systems due to higher strength and improved corrosion resistance than standard ferritic or austenitic stainless steels. The paper, presents the study performed at 800 °C for 3000 hours on 3 advanced austenitic steels; 309S, 310S and HR3C with higher than 20 Cr wt% content and 4 Ni-based alloys including: two solid-solution strengthened alloys (Haynes 230), 617 alloy and two (γ’) gamma - prime strengthened materials (263 alloy and Haynes 282). The high temperature oxidation tests were performed in water to steam close loop system, the samples were investigated analytically prior and after exposures using Scanning Electron Microscopy (SEM) coupled with Energy Dispersive Spectrometry (EDS), and X-Ray Diffractometer (XRD). Mass change data have been examined every 250 hours.

Sanicro 25 material is approved for use in pressure vessels and boilers according ASME code case 2752, 2753 and VdTUV blatt 555. It shows higher creep rupture strength than any other austenitic stainless steels available today. It is a material for superheater and reheaters, enabling higher steam parameters of up to about 650 °C steam (ie about max 700 °C metal) without the need for expensive nickel based alloys. The aim of the present study is the investigation of the steam oxidation resistance of the Sanicro 25. The long term test was conducted in the temperature range 600 -750 °C up to 20 000 hours. The morphology of the oxide scale and the microstructure of the bulk material were investigated. In addition, the effect of surface finish and pressure on the steam oxidation were also studied.

Following the successful completion of a 15-year effort to develop and test materials that would allow advanced ultra-supercritical (A-USC) coal-fired power plants to be operated at steam temperatures up to 760°C, a United States-based consortium has been working on a project (AUSC ComTest) to help achieve technical readiness to allow the construction of a commercial scale A-USC demonstration power plant. Among the goals of the ComTest project are to validate that components made from the advanced alloys can be designed and fabricated to perform under A-USC conditions, to accelerate the development of a U.S.-based supply chain for key A-USC components, and to decrease the uncertainty for cost estimates of future commercial-scale A-USC power plants. This project is intended to bring A-USC technology to the commercial scale demonstration level of readiness by completing the manufacturing R&D of A-USC components by fabricating commercial scale nickel-based alloy components and sub-assemblies that would be needed in a coal fired power plant of approximately 800 megawatts (MWe) generation capacity operating at a steam temperature of 760°C (1400°F) and steam pressure of at least 238 bar (3500 psia).The A-USC ComTest project scope includes fabrication of full scale superheater / reheater components and subassemblies (including tubes and headers), furnace membrane walls, steam turbine forged rotor, steam turbine nozzle carrier casting, and high temperature steam transfer piping. Materials of construction include Inconel 740H and Haynes 282 alloys for the high temperature sections. The project team will also conduct testing and seek to obtain ASME Code Stamp approval for nickel-based alloy pressure relief valve designs that would be used in A-USC power plants up to approximately 800 MWe size. The U.S. consortium, principally funded by the U.S. Department of Energy and the Ohio Coal Development Office under a prime contract with the Energy Industries of Ohio, with co-funding from the power industry participants, General Electric, and the Electric Power Research Institute, has completed the detailed engineering phase of the A-USC ComTest project, and is currently engaged in the procurement and fabrication phase of the work. This paper will outline the motivation for the effort, summarize work completed to date, and detail future plans for the remainder of the A-USC ComTest project.