In the later half of the last century great progress in alloy development for power applications was seen to improve thermal efficiency with increasing steam temperature. Meanwhile, many material-related troubles have been experienced due to rising temperature and uncertainty in the properties of fabricated metal. For further improvement in the thermal efficiency of fossil-fired power plants with ultra supercritical steam parameter conditions aiming at temperatures above 700°C, alloy development concepts and material issues with increasing steam temperature must be reviewed and discussed. In this paper new findings in the areas of alloy developments, creep failure in base metal and weldments, thermal fatigue failure and steam oxidation/hot corrosion are presented and discussed, as well as the economical aspect of material development, which is essential to realize unprecedented ultra supercritical steam conditions.

Prior to utilizing new advanced materials in coal power plants, a large number of experimental testing is required. Test procedures are needed in specialized high temperature laboratories with state of the art facilities and precise, accurate analytical equipment capable of performing tests at a variety of temperatures and environments. In this study, the results of a unique technique involving salt spray testing at high temperatures are presented. The Haynes 282 gamma – prime (γ’) strengthened alloy fabricated by means of three different manufacturing processes: HAYNES 282 WROUGHT alloy, Haynes 282-SINT alloy, and finally Haynes 282-CAST alloy have been tested. The materials have been exposed to a salt spray corrosion atmosphere using 1% NaCl - 1% Na 2 SO 4 . Post exposure investigations have included SEM, EDS and XRD examinations. The test using salt spray of 1% NaCl - 1% Na 2 SO 4 water solution at 550 °C for 500 hours indicted no influence on the corrosion products formation, where Cr 2 O 3 has been developed in all three alloys, whereas NiO has been found only in Haynes 282-CAST material. On the other hand, it has been found that the fabrication process of HAYNES 282 alloy strongly influences the corrosion products formation under the high temperature exposures.

Refractory metal, such as molybdenum and tungsten, are used in increasingly severe environments. Therefore, these materials are required to have superior mechanical properties, long life and excellent reliability. In this study, we developed newly Mo alloy dispersed Mo-base intermetallic compound for material forming such as hot extrusion die. This newly Mo alloy “MSB” exhibited superior mechanical properties compared with convention Mo alloy at temperatures below 1000℃. In addition, by adding a titanium alloy to the MSB, we have developed another alloy “T-MSB” that has high mechanical properties at higher temperatures. In the hot extrusion of brass, die made of these Mo alloys had 2.5 times longer tool life than the conventional Mo alloy die.

INCONEL alloy 740H has been specified for tube and pipe for the boiler and heat exchanger sections of AUSC and sCO 2 pilot plants currently designed or under construction. These systems also require fittings and complex formed components such as flanges, saddles, elbows, tees, wyes, reducers, valve parts, return bends, thin-wall cylinders and tube sheets. The initial evaluation of alloy 740H properties, leading to ASME Code Case 2702, was done on relatively small cross-section tube and plate. The production of fittings involves the use of a wide variety of hot or cold forming operations. These components may have complex geometric shapes and varying wall thickness. The utility industry supply chain for fittings is largely unfamiliar with the processing of age-hardened nickel-base alloys. Special Metals has begun to address this capability gap by conducting a series of trials in collaboration with selected fittings manufacturers. This paper describes recent experiences in first article manufacture of several components. The resulting microstructure and properties are compared to the published data for tubular products. It is concluded that it will be possible to manufacture most fittings with properties meeting ASME Code minima using commercial manufacturing equipment and methods providing process procedures appropriate for this class of alloy are followed. INCONEL and 740H are registered trademarks of Special Metals Corporation.

A new alloy design concept for creep- and corrosion-resistant, fully ferritic alloys was proposed for high-temperature structural applications in current/future fossil-fired power plants. The alloys, based on the Fe-30Cr-3Al (in weight percent) system with minor alloying additions of Nb, W, Si, Zr and/or Y, were designed for corrosion resistance though high Cr content, steam oxidation resistance through alumina-scale formation, and high-temperature creep performance through fine particle dispersion of Fe 2 (Nb,W)-type Laves phase in the BCC-Fe matrix. Theses alloys are targeted for use in harsh environments such as combustion and/or steam containing atmospheres at 700°C or greater. The alloys, consisting of Fe-30Cr-3Al-1Nb-6W with minor alloying additions, exhibited a successful combination of oxidation, corrosion, and creep resistances comparable or superior to those of commercially available heat resistant austenitic stainless steels. An optimized thermo-mechanical treatment combined with selected minor alloying additions resulted in a refined grain structure with high thermal stability even at 1200°C, which improved room-temperature ductility without sacrificing the creep performance. The mechanism of grain refinement in the alloy system is discussed.

Utilities worldwide are facing increased demand for additional electricity, reduced plant emissions and greater efficiency. Part of the solution is achieved by increasing boiler temperature, pressure and coal ash corrosion resistance of the materials of boiler construction. In this paper, a new nickel-base tube alloy, INCONEL alloy 740, meeting this challenge is characterized with emphasis on mechanical properties, coal ash and steam corrosion resistance as well as weldability. Microstructural stability as a function of temperature and time is addressed as well as some of the early methodology employed to arrive at the current chemical composition. Brief mention is made of certain current and future alloy characterization efforts and potential environmental benefits to be expected should the boiler technology utilizing INCONEL alloy 740 be adopted.

Despite the significant progress achieved in power generation technologies in the past two decades, finding effective solutions to further reduce emissions of harmful gases from thermal power plant still remains the major challenge for the power generation industry as well as alloy material developers. In the European material programmes COST 522 and COST 536, based on the existing 9-12%Cr creep resisting steels, an advanced 9%Cr-Mo martensitic alloy, C(F)B2 (GX13CrMoCoVNbNB9-2-1) alloy has been developed. By modification through alloying of boron and cobalt and together with other micro-adjustment of the composition, C(F)B2 alloys has showed very encouraging properties. The current paper summaries the development and evaluation of the matching filler metals for C(F)B2 grade. The design of the filler metal composition is discussed and comparison is made with the parent materials in respect to the alloy additions and microstructure. The mechanical properties of the weld metals at ambient temperature are examined. Creep properties of both undiluted weld metals and cross-weld joints are examined through stress rupture test and the data are evaluated and compared with those of the base alloy and other existing 9%Cr-Mo creep resisting steels.

New Fe-base ferritic alloys based on Fe-30Cr-3Al-Nb-Si (wt.%) were proposed with alloy design concepts and strategies targeted at improved performance of tensile and creep-rupture properties, environmental compatibilities, and weldability, compared to Grade 91/92 type ferritic-martensitic steels. The alloys were designed to incorporate corrosion and oxidation resistance from high Cr and Al additions and precipitate strengthening via second-phase intermetallic precipitates (Fe2Nb Laves phase), with guidance from computational thermodynamics. The effects of alloying additions, such as Nb, Zr, Mo, W, and Ti, on the properties were investigated. The alloys with more than 1 wt.% Nb addition showed improved tensile properties compared to Gr 91/92 steels in a temperature range from 600-800°C, and excellent steam oxidation at 800°C as well. Creep-rupture properties of the 2Nb-containing alloys at 700°C were comparable to Gr 92 steel. The alloy with a combined addition of Al and Nb exhibited improved ash-corrosion resistance at 700°C. Additions of W and Mo were found to refine the Laves phase particles, although they also promoted the coarsening of the particle size during aging. The Ti addition was found to reduce the precipitate denuded zone along the grain boundary and the precipitate coarsening kinetics.

To meet worldwide emission targets many Government policies either avoid the use of coal burning plant for future energy production, or restrict emissions per kilogram of coal consumed beyond the capability of most conventional plant. As a result this has accelerated current worldwide developments of steel and nickel alloys for coal-fired plant to operate at temperatures in excess of 625°C. Within the UK a modified 9%Cr steel has been developed which is based on the MarBN steel first proposed by Professor Fujio Abe of NIMS Japan, and has been designated IBN-1. The steel is modified by additions of, typically, 3% cobalt and tungsten with controlled additions of boron and nitrogen. While development of 9%Cr steels has continued since the last EPRI high temperature material conference in 2016 (Portugal), parallel developments in nickel alloy castings for even higher temperature and pressure applications have also continued. This paper summarises the latest developments in both of these material types.

In an earlier paper, preliminary data for HAYNES 282 alloy was presented for potential advanced steam power plant applications. Since then, 282 alloy has continued to be evaluated for a variety of A-USC applications: superheater boiler tubing, large header piping, rotors, casings, etc. Per current practice the alloy achieves its strengthening by a two-step age hardening heat treatment. Given the difficulty of such a procedure, particularly for larger components in the power plant, interest has focused on the development of a single step age hardening treatment. While considerable work on 282 alloy is still going on by a number of investigators, during the preceding years a large amount of data was generated in characterizing the alloy at Haynes International. This paper will briefly review the behavior of 282 alloy in air and water vapor oxidation (10% H 2 O) at 760°C (1400°F), low cycle fatigue properties at 649°C to 871°C (1200°F to 1600°F) and long-term thermal stability at 649°C to 871°C (1200°F to 1600°F). Special focus of the paper will be mechanical behavior: tensile and creep; microstructural analysis, and weldability of 282 alloy as a result of single step age hardening heat treatment: 800°C (1475°F)/8hr/AC.

Large scale components of the conventional 600°C class steam turbine were made of the ferritic steel, but the steam turbine plants with main steam temperatures of 700°C or above (A-USC) using the Ni-base superalloys are now being developed in order to further improve the thermal efficiency. The weight of the turbine rotor for the A-USC exceeds 10ton. A lot of high strength superalloys for aircraft engines or industrial gas turbines have been developed up to now. But it is difficult to manufacture the large-scale parts for the steam turbine plants using these conventional high strength superalloys because of their poor manufacturability. To improve high temperature strength without losing manufacturability of the large scale components for the A-USC steam turbine plants, we developed Ni-base superalloy USC800(Ni-23Co-18Cr-8W-4Al-0.1C [mass %]) which has temperature capability of 800°C with high manufacturability achieved by controlling microstructure stability and segregation property. The 700°C class A-USC materials are the mainstream of current development, and trial production of 10 ton-class forged parts has been reported. However, there have been no reports on the development and trial manufacturing of the A-USC materials with temperature capability of 800°C. In this report, results of trial manufacturing and its microstructure of the developed superalloy which has both temperature capability 800°C and good manufacturability are presented. The trial manufacturing of the large forging, boiler tubes and turbine blades using developed material were successfully achieved. According to short term creep tests of the large forging and the tube approximate 100,000h creep strength of developed material was estimated to be 270MPa at 700 °C and 100MPa at 800°C.

Nickel-base alloys were exposed to flowing supercritical CO 2 (P = 20MPa) at temperatures of 700 to 1000°C for up to 1000 h. For comparison, 316L stainless steel was similarly exposed at 650°C. To simulate likely service conditions, tubular samples of each alloy were internally pressurised by flowing CO 2 , inducing hoop stresses up to 35 MPa in the tube walls. Materials tested were Haynes alloys 188, 230 and 282, plus HR120 and HR160. These alloys developed chromia scales and, to different extents, an internal oxidation zone. In addition, chromium-rich carbides precipitated within the alloys. Air aging experiments enabled a distinction between carburisation reactions and carbide precipitation as a result of alloy equilibration. The stainless steel was much less resistant to CO 2 attack, rapidly entering breakaway corrosion, developing an external iron-rich oxide scale and internal carburisation. Results are discussed with reference to alloy chromium diffusion and carbon permeation of oxide scales.

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.

Coatings are an essential part of the materials system to protect the turbine blades from oxidation and corrosive attack during service. Inter-diffusion of alloying elements between a turbine blade substrate and their coatings is a potential concern for coated turbine blades at ever increasing operating temperatures because this can cause the formation of undesirable Secondary Reaction Zones (SRZs), which may degrade the mechanical properties of coated Ni-based superalloys. Understanding the effects of each element on the SRZ formation is essential in order to understand both the mechanism and inter-diffusion behaviour between coatings and substrates. In this research, a number of simpler aluminized ternary Ni-Al-X (where X is Co, Cr, Re, Ru or Ta) alloys were investigated in order to elucidate the separate effects of each element on the microstructural evolution, especially at the coating/substrate interface. The aluminized ternary alloys developed distinctive diffusion zones, depending on the third alloy element, ‘X’. Specifically, it has been found that both Ni-Al-Re and Ni-Al-Ta alloys developed a continuous SRZ-like diffusion layer. This diffusion zone persisted in the Ni-Al-Re alloys after high temperature exposure, indicating that Re has a stronger effect on SRZ formation than Ta.

Constricted steam oxidation resistance and finite microstructural stability limits the use of 9 - 12 wt.-% chromium ferritic-martensitic steels to steam temperatures of about 620 °C. Newly developed 12 wt.-% Cr steels are prone to Z-phase precipitation, which occurs at the expense of the strengthening precipitates, and therefore suffer an accelerated decline in strength during longterm operation. While the concept of ferritic-martensitic chromium steels thus seems to hit technological limitations, further improvement in steam power plant efficiency necessitates a further increase of steam pressure and temperature. Furthermore increasing integration of intermitting renewable energy technologies in electrical power generation poses a great challenge for supply security, which has to be ensured on the basis of conventional power plant processes. Besides improved efficiency for resource preservation, load flexibility, thermal cycling capability and downtime corrosion resistance will play key roles in the design of tailored materials for future energy technology. Under these preconditions a paradigm shift in alloy development towards improvement of cyclic steam oxidation and downtime corrosion resistance in combination with adequate creep and thermomechanical fatigue strength seems to be mandatory. The steam oxidation, mechanical and thermomechanical properties of fully ferritic 18 - 24 wt.-% chromium model alloys, strengthened by the precipitation of intermetallic (Fe,Cr,Si)2(Nb,W) Laves phase particles, indicate the potential of this type of alloys as candidate materials for application in highly efficient and highly flexible future supercritical steam power plants.

Preface for the 2004 Advances in Materials Technology for Fossil Power Plants conference.

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.

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.

Enhancement of the steam conditions is one of the most effective measures to achieve the goal of higher thermal efficiency. 700°C class A-USC (Advanced Ultra Super Critical Steam Conditions) power plant is one of the remarkable technologies to achieve the goal and reduce CO 2 emissions from fossil fuel power plants. Toshiba has been working on the A-USC development project with subsidy from METI (Ministry of Economy, Trade and Industry) and NEDO (New Energy and Industrial Technology Development Organization). In this project, A-USC power plants with steam parameters of 35MPa 700/720/720°C were considered. To date, various materials have been developed and tested to verify their characteristics for use in A-USC power plants. And some of these materials are being investigated as to their suitability for use in long term. Together with members of the project, we carried out the boiler component test using a commercially-operating boiler. We manufactured a small-scale turbine casing made of nickel-based alloy, and supplied it for the test. In addition, we manufactured a turbine rotor for turbine rotation tests, and carried out the test at 700°C and rotating speed of 3,600rpm conditions. In this paper, we show the results of the A-USC steam turbine development obtained by the project.

COST FB2 steel alloyed with boron is currently the best available martensitic 9% Cr steel for turbine shafts subjected to steam temperatures up to 620°C and meanwhile introduced into production for application in commercial power plants. Currently several development programs are running to develop materials for further increase of application temperature up to 650°C. For realization of a 650ºC power plant not only creep strength, but also resistance against steam oxidation must be improved by increase of Cr content up to 11-12%. In the past all attempts to develop stable creep resistant martensitic 11-12% Cr steels for 650°C failed due to breakdown in long-term creep strength. Therefore new alloy concepts have been developed by replacing the fine nitride strengthening particles by controlled and accelerated precipitation of the more stable Z phase. Therefore the European project “Z-Ultra” was launched for further development and manufacture of this new alloy type. Saarschmiede participates in this project and contributed by manufacturing trial melts, boiler tubes and a large scale turbine rotor forging. Production experience and test results are presented. In order to exceed the temperature limit of 650°C, only nickel base alloys can be used. One of the most promising candidate alloys for rotor forgings subjected to steam temperatures of 700°C is Alloy 617, which was already intensively investigated. For still higher temperatures in the range of 750°C only γ‘-precipitation hardened nickel base alloys, such as Alloy 263, can be applied. Therefore the “NextGenPower” project was launched and aimed at manufacture and demonstration of parts from Ni-based alloys for application in steam power plants at 750°C. One of the main goals was to develop turbine rotor materials and to demonstrate manufacturability of forgings for full scale turbine rotor parts. Contributing to this project, Saarschmiede has produced for the first time a large rotor forging in the Ni base Alloy 263. Numeric simulations of ingot manufacture, forging and heat treatment have been performed and a large trial rotor forging in Alloy 263 with a diameter of 1000 mm was successfully produced from a triple melt ingot. Experiences in manufacture and test results are presented.