A 9% Cr steel containing cobalt and boron, X13CrMoCoVNbNB9-2-1, has been manufactured by electroslag remelting (ESR) to evaluate its performance and to compare its creep strength and microstructure to a forging made from electroslag hot-topping ingot. The evaluation results confirm that it is possible to produce rotor forgings with homogeneous composition and good properties by the ESR process. The results of creep rupture tests up to 5000 h indicate that the creep strength of the forging made from ESR ingot is similar to that of the forging produced by the electroslag hot-topping process. Martensitic lath microstructures with high density dislocations and the precipitations of M 23 C 6 , VX, NbX and M2X are observed after the quality heat treatments at the center portion of both forgings. There is no large difference in the martensitic lath widths, distributions, and sizes of those particles between both trial forgings.

Driven by the need to reduce CO 2 emissions through increased steam temperature and pressure in new power plants, research in Europe led to the development of enhanced high-chromium steels with improved creep resistance and service temperature stability. After years of development, Rotor E, a steel composition created during the COST programs (501, 522, and 536), has become a commercially available product. While traditionally forged and remelted using electroslag remelting (ESR), this paper demonstrates the successful production of large rotor components using a conventional process without ESR, achieved through tailored process control. This paper details Società delle Fucine's (SdF) current production of Rotor E using a conventional route based on ladle furnace and vacuum degassing, as well as the mechanical and creep behaviors of the resulting forged products. Additionally, SdF produced a prototype FB2 rotor using a conventional process. FB2, a 10% Cr steel containing cobalt and boron but lacking tungsten, emerged from the COST 522 program as the best candidate for scaling up from a laboratory experiment to a full-sized industrial component. Notably, the addition of boron effectively improved the microstructure's stability and consequently enhanced the creep resistance of these new, advanced martensitic steels. Finally, the paper will present updates on the long-term characterization program for the FB2 steel trial rotor.

A creep resistant martensitic steel, CPJ7, was developed with an operating temperature approaching 650°C. The design originated from computational modeling for phase stability and precipitate strengthening using fifteen constituent elements. Approximately twenty heats of CPJ7, each weighing ~7 kg, were vacuum induction melted. A computationally optimized heat treatment schedule was developed to homogenize the ingots prior to hot forging and rolling. Overall, wrought and cast versions of CPJ7 present superior creep properties when compared to wrought and cast versions of COST alloys for turbines and wrought and cast versions of P91/92 for boiler applications. For instance, the Larson Miller Parameter curve for CPJ7 at 650°C almost coincides with that of COST E at 620°C. The prolonged creep life was attributed to slowing down the process of the destabilization of the MX and M 23 C 6 precipitates at 650°C. The cast version of CPJ7 also revealed superior mechanical performance, well above commercially available cast 9% Cr martensitic steel or derivatives. The casting process employed slow cooling to simulate the conditions of a thick wall full-size steam turbine casing but utilized a separate homogenization step prior to final normalization and tempering. To advance the development of CPJ7 for commercial applications, a process was used to scale up the production of the alloy using vacuum induction melting (VIM) and electroslag remelting (ESR), and underlined the importance of melt processing control of minor and trace elements in these advanced alloys.

The US Advanced Ultra-Supercritical (A-USC) Consortium conducted an extensive program to evaluate available superalloys for use in rotors for steam turbines operating at a nominal temperature of 760 °C (1400 °F). Alloys such as 282, Waspaloy, 740H, 720Li, and 105 were tested in the form of bar supplied from the alloy producers. Ultimately, alloy 282 was down-selected for the turbine rotor based on its combination of creep strength, phase stability, ductility, and fatigue resistance. The next step in development was to produce a full-size rotor forging for testing. A team was established consisting of GE Power (project management and testing), Wyman-Gordon (forging and testing) and Special Metals (melting and billetizing) to pursue the work. A research license to melt the alloy was obtained from Haynes International. The first step of the development was to devise a triple melt (VIM-ESR-VAR) practice to produce 610 mm (24 inch) diameter ingot. Two ingots were made, the first to define the VAR remelting parameters and the second to make the test ingot utilizing optimum conditions. Careful attention was paid to ingot structure to ensure that no solidification segregation occurred. A unique homogenization practice for the alloy was developed by the US Department of Energy (DOE) and National Energy Technology Laboratory (NETL). Billetization was performed on an open die press with three upset and draw stages. This procedure produced an average grain size of ASTM 3. A closed die forging practice was developed based on compressive flow stress data developed by Wyman Gordon Houston for the consortium project. Multiple 18 kg forgings were produced to define the forging parameters that yielded the desired microstructure. The project culminated with a 2.19 metric ton (4830 lb), 1.22 m (48 inch) diameter crack-free pancake forging produced on Wyman Gordon’s 50,000 ton press in Grafton, MA. The forging process produced a disk with an average grain size of ASTM 8 or finer. Forging cut-up, microstructural characterization, and mechanical property testing was performed by GE Power. Fatigue and fracture toughness values of the disk forging exceeded those previously reported for commercially available rolled bar.

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.

Advanced 700°C-class steam turbines demand austenitic alloys for superior creep strength and oxidation resistance beyond 650°C, exceeding the capabilities of conventional ferritic 12Cr steels. However, austenitic alloys come with a higher coefficient of thermal expansion (CTE) compared to 12Cr steels. To ensure reliability, operability, and performance, these advanced turbine alloys require low CTE properties. Additionally, for welded components, minimizing CTE mismatch between the new material and the welded 12Cr steel is crucial to manage residual stress. This research investigates the impact of alloying elements on CTE, high-temperature strength, phase stability, and manufacturability. As a result, a new material, “LTES700R,” was developed specifically for steam turbine rotors. LTES700R boasts a lower CTE than both 2.25Cr steel and conventional superalloys. Additionally, its room-temperature proof strength approaches that of advanced 12Cr steel rotor materials, while its creep rupture strength around 700°C significantly surpasses that of 12Cr steel due to the strengthening effect of gamma-prime phase precipitates. To assess the manufacturability and properties of LTES700R, a medium-sized forging was produced as a trial run for a turbine rotor. The vacuum arc remelting process was employed to minimize segregation risk, and a forging procedure was meticulously designed using finite element method simulations. This trial production resulted in a successfully manufactured rotor with satisfactory quality confirmed through destructive evaluation.

For safe operation of thick-walled components for Advanced Ultra Super Critical (A-USC) power plants, detailed knowledge of the creep crack initiation and growth behavior is essential. The high loading and high temperature conditions in an A-USC power plant require, in many cases, the employment of nickel base super alloys. Unfortunately, both manufacturing and nondestructive evaluation (NDE) of thick-walled components (> 50 mm) made of nickel base super alloys are quite challenging. In this paper, one candidate material for such applications, Alloy C-263, was tested for creep and creep crack behavior at 700 °C. Objective of the study was to determine a critical flaw size. In order to establish this size, the duration to achieve the 1%-strain limit at a given load is compared with the time to grow the initial flaw for Δa = 0.5 mm when the component was loaded with the same given load. It will be shown that manufacturing parameters, e. g. heat treatment procedures, have a significant influence on the creep crack initiation and growth behavior and thus on component life. Decoration of grain boundaries with precipitates, for instance caused by the manufacturing process, can reduce the creep crack resistance and thus increase the risk for premature component failure.

Inconel alloy 740 was initially developed to enable the design of coal-fired boilers capable of operating at 700°C steam temperature and high pressure. The alloy successfully met the European program's targets, including 100,000-hour rupture life at 750°C and 100 MPa stress, and less than 2 mm metal loss in 200,000 hours of superheater service. However, thick section fabrication revealed weldability challenges, specifically grain boundary microfissuring in the heat affected zone (HAZ) of the base metal. This paper describes the development of a modified variant with significantly improved resistance to HAZ microfissuring and enhanced thermal stability, while maintaining desirable properties. The formulation process is detailed, and properties of materials produced within the new composition range are presented. Additionally, the microstructural stability of the original and modified alloy compositions is compared, demonstrating the advancements achieved in this critical material for next-generation power plants.

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.

Detailed knowledge of the creep and creep crack behavior is essential for a safe operation of thick-walled components in thermal power plants. High mechanical loads and temperatures of more than 700 °C often require the application of nickel-based alloys, e.g. alloy C-263. Unfortunately, manufacturing and non-destructive evaluation (NDE) of thick-walled components (> 50 mm) made of nickel-based alloys are quite challenging. Tolerable critical flaw sizes, experimentally validated for long service durations, play an important role in the quality assurance of such components. It is commonly accepted that manufacturing parameters, e.g. heat treatment procedures, have a significant influence on creep ductility and time-dependent crack behavior. By means of adjusting the process parameters, the ductility and the creep life of notched specimen can be significantly improved in the case of alloy C-263. Essential root cause is the decoration of grain boundaries with carbides which drastically influences creep crack initiation and growth. This results in significant differences for allowable critical flaw sizes and thus, the potential use of the candidate material. On a first generation of alloy C-263 “G1”, a dense population of carbides on the grain boundaries was found, which resulted in an inadmissible creep crack behavior. The resulting critical flaw sizes were only a few tenths of a millimeter. On a second generation “G2”, the grain boundary occupation was positively influenced, so that a satisfactory creep crack behavior could be found. The critical flaw sizes are in the order of one millimeter or more. A critical or impermissible material behavior under creep conditions can be demonstrated by testing smooth and notched round specimens. For example, the first generation “G1” notched round specimens fails earlier than the smooth round specimens, indicating notch sensitivity. On the second generation “G2”, however, a notch insensitivity was found. The critical defect sizes can be determined by a method that takes into account a simultaneous examination of the crack tip situation and the ligament situation.

Hitachi and Hitachi Metals have developed low thermal expansion Ni-base superalloy, Ni-20Cr-10Mo-1.2Al-1.6Ti alloy (USC141) for use as A-USC steam turbine material. The approximate 10 5 h creep rupture strength at 740° C is 100MPa, so USC141 can be expected to apply for blades and bolts. Now we have been studying to get better creep properties by microstructure controlling such as grain size or grain boundary morphology. In addition, the segregation test of USC141 shows good Freckle tendencies, it means that it would be easy to make a large ingot which could be used as rotors or pipes. From these calculation results, we have been tried to make an 850mmϕ ESR ingot of USC141.

A significant research and development effort is underway to support the qualification of Alloy 709 as a Class A construction material in the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, Section III, Division 5, High Temperature Reactors. This initiative includes a comprehensive Alloy 709 code qualification plan aimed at generating extensive material testing data crucial for compiling the code case data package. The data package is essential in establishing material-specific design parameters for Alloy 709 to be used as Section III, Division 5 Class A construction material for fast reactors, molten salt reactors and gas-cooled reactors. An ASME Section III, Division 5 material code case requires the evaluation of mechanical properties from a minimum of three commercial heats, covering anticipated compositional ranges. A key part of the data package involves fatigue and creep-fatigue testing at elevated temperatures, needed for developing the fatigue design curves and the damage envelope of the creep-fatigue interaction diagram (D-diagram). This paper summarizes the strain-controlled fatigue testing on three commercial heats of Alloy 709 at 760 and 816°C with strain ranges between 0.25% and 3%. The fatigue failure data are used to generate a preliminary fatigue design curve. Additionally, the creep-fatigue testing results at 816°C with tensile hold times of 10, 30, and 60 minutes are presented in support of developing the D-diagram for Alloy 709.

The aim of this work was to reveal the effects of trace elements on the creep properties of nickel-iron base superalloys, which are the candidate material for the large components of the advanced-ultrasupercritical (A-USC) power generation plants. High temperature tensile and creep properties of forged samples with seven different compositions were examined. No significant differences were observed in the creep rate versus time curves of the samples, of which contents of magnesium, zirconium, manganese and sulfur were varied. In contrast, the curves of phosphorus-added samples showed very small minimum creep rates compared to the other samples. The creep rupture lives of phosphorus-added samples were obviously longer than those of the other samples. Microstructure observation in the vicinity of grain boundaries of phosphorus-added samples after aging heat treatment revealed that there were fine precipitates consisting of phosphorus and niobium at the grain boundaries. The significant suppression of the creep deformation of phosphorus-added sample may be attributed to the grain boundary strengthening caused by the fine grain boundary precipitates.

The trial production of FENIX-700 turbine rotors for advanced-ultra super critical (A-USC) power generation was conducted, and their microstructure, tensile, impact, and creep properties were evaluated. Two 10-ton class trial forgings were successfully produced through double melting of VIM and ESR and free forging with a 14,000 ton hydraulic press. For examining the effect of the forging condition on the microstructure of the rotors, we adopted lower finish temperatures and an increased forging ratio on the last forging for the second trial. The grains of the second trial forging were refined by changing the forging condition. In particular, the grain size of the center of the rotor was remarkably decreased from the grain size number 0.5 to 2.8. Grain refinement improved the permeability of the ultrasonic wave in the ultrasonic inspection test, resulting in decreasing the minimum detectable flaw size (MDFS). The ductility and toughness were also improved by grain refinement. Although the grain size was decreased, the time to rupture in the creep test at 700 °C was comparable to the previous results of FENIX-700, and the estimated 105 h rupture stress at 700 °C was sufficiently higher than 100 MPa. However, it was clarified that the particles of gamma-prime in the center of the rotor had been coarsened due to the mass effect. The slight decrease of 0.2% proof stress and shortening of creep rupture time at 700 °C were attributed to the coarse gamma-prime particles. The results of the present trial expressly demonstrated that it is possible to manufacture 10-ton class A-USC turbine rotors of FENIX-700 with excellent mechanical properties and good permeability of the ultrasonic wave.

For last half century the development of creep strength enhanced ferritic steels has been continued and presently ASME grades 91, 92 and 122 extremely stronger than conventional low alloy steels have extensively been used worldwide in high efficient power plants. However the use of these creep strength enhanced 9-12%Cr steels is limited to around 630°C or 650°C at maximum in terms of high temperature strength and oxidation resistance. Consequently the appearance of ferritic steels standing up to higher temperature of around 700°C to substitute of high strength austenitic steels is strongly desired. Under the state, the addition of high nitrogen to ferritic steels is attracting considerable attention because of improving high temperature strength and oxidation resistance of them. This work was done to evaluate the oxidation resistance of high nitrogen steels and to investigate the effect nitrogen and microstructure on oxidation resistance using 9-15%Cr steels with about 0.3% nitrogen manufactured by means of Pressurized Electro- Slag Remelting (PESR) method in comparison with ASME grades 91 and 122. As a result, high nitrogen ferritic steels showed excellent oxidation resistance comparing with nitrogen-free steels and ASME grades 91 and 122. The oxidation resistance of 9%Cr ferritic steels depends on the nitrogen content in the each steel. That is, the weight gain decreases with an increase in nitrogen content. Moreover, the oxide scale of high nitrogen steel contained a high concentration of Cr. It is conjectured that, in high temperature oxidation, nitrogen plays a key role in promoting the formation of the oxide scale which has high concentration of Cr, inhibiting oxidation from proceeding. And also it was found that the oxidation resistance of the high nitrogen steels does not depend greatly on Cr content but on their microstructure. The oxidation resistance of high nitrogen ferritic heat-resistant steels increased as the fraction of martensite structure increased. These results indicate for high nitrogen steels Cr diffusion along grain boundaries is further promoted resulting in the formation of protective oxide scale having high Cr concentration. Furthermore as new findings it was confirmed that the Cr diffusion in substrate of steels to form Cr concentrated oxide scale on the metal surface is accelerated by nitrogen while suppressed by carbon in matrix of steel.

The effect of grain size after solution treatment on the mechanical properties of FENIX-700, including its cooling rate, was investigated. In addition, the dependance of precipitation observed at grain boundaries on the heat treatment conditions was also discussed on the basis of the results of microstructure observations. It was confirmed that the tensile ductility, the creep rupture ductility, and the absorbed energy decreased as the grain size increased. The creep rupture strength, in contrast, increased remarkably as the grain size increased. The tensile strength increased as the cooling rate increased. Experimental results showed that satisfactory mechanical properties would be obtained for a grain size of ASTM G.S.No. 1.0-3.0.

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.

The mechanical behavior of a cast form of an advanced austenitic stainless steel, CF8C-Plus, is compared with that of its wrought equivalent in terms of both tensile and creep-rupture properties and estimated allowable stress values for pressurized service at temperatures up to about 850°C. A traditional Larson-Miller parametric model is used to analyze the creep-rupture data and to predict long-term lifetimes for comparison of the two alloy types. The cast CF8C-Plus exhibited lower yield and tensile strengths, but higher creep strength compared to its wrought counterpart. Two welding methods, shielded-metal-arc welding (SMAW) and gas-metal-arc welding, met the weld qualification acceptance criteria in ASME BPVC Section IX for the cast CF8C-Plus. However, for the wrought CF8C-Plus, while SMAW and gas-tungsten-arc welding passed the tensile acceptance criteria, they failed the side bend tests due to lack of fusion or weld metal discontinuities.