Search Results for remelting

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.

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.

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.

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.

Steels with 9-12% chromium content are widely used in steam turbines operating above 550°C due to their improved creep properties. Saarschmiede has extensive experience manufacturing high chromium steels, such as the X12CrMoWVNbN10-11-1 steel designed through the European COST program for application up to 610°C (COST Rotor E). From this steel, Saarschmiede produces high-pressure rotor shafts and gas turbine discs. To meet ever-increasing steam temperatures, a modified steel type with elevated boron content was developed, and pilot rotors have been manufactured. For ingot manufacturing of high chromium steels, Saarschmiede utilizes the Electro-Slag-Remelting process, allowing ingots up to 165 tons. Optimized forging and heat treatment procedures ensure reproducible forging properties. All products undergo rigorous destructive and non-destructive testing.

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.

Martensitic 9Cr steels have been developed which are strengthened by boron in order to stabilize the microstructure and improve their long-term creep strength. Boron plays a key role in these steels by stabilising the martensitic laths by decreasing the coarsening rate of M 23 C 6 carbides, which act as pinning points in the microstructure. In this work two modified FB2 steel forgings are compared. Both forgings have similar compositions but one underwent an additional remelting process during manufacture. Creep tests showed that this additional processing step resulted in a significant increase in time to failure. In order to investigate the effect of the processing route on microstructural evolution during aging and creep, a range of advanced electron microscopy techniques have been used including ion beam induced secondary electron imaging and High Angle Annular Dark Field (HAADF) imaging in the Scanning Transmission Electron Microscope. These techniques have enabled the particle population characteristics of all the second phase particles (M 23 C 6 , Laves phase, BN and MX) to be quantified for materials from both forging processes. These quantitative data have enabled a better understanding of how the processing route affects the microstructural evolution of FB2 steels.

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.

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.

Nimonic 80A, a Ni-base superalloy mainly strengthened by Al and Ti to form γ'-Ni 3 (Al, Ti) precipitation in Ni-Cr solid solution strengthened austenite matrix, has been used in different industries for more than half century (especially for aero-engine application). In consideration of high strengths and corrosion resistance both Shanghai Turbine Company (STC) has adopted Nimonic 80A as bucket material for ultra-super-critical (USC) turbines with the steam parameters of 600°C, 25MPa. First series of two 1000MW USC steam turbines made by Shanghai Turbine Co. were already put in service on the end of 2006. Large amount of Nimonic 80A with different sizes are produced in Special Steel Branch of BAOSTEEL, Shanghai. Vacuum induction melting and Ar protected atmosphere electro-slag remelting (VIM+PESR) process has been selected for premium quality high strength Nimonic 80A. For higher mechanical properties the alloying element adjustment, optimization of hot deformation and heat treatment followed by detail structure characterization have been done in this paper. The Chinese premium quality high strength Nimonic 80A can fully fulfill the USC turbine bucket requirements.

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 global transition toward high-efficiency steam power plants demands increasingly advanced steel rotor forgings capable of operating at temperatures of 600°C and above. The European Cost program has been instrumental in developing creep-resistant 10%-chromium steels for these critical applications, with Steel Cost E emerging as a prominent material now widely utilized in steam turbine shafts and experiencing significant market growth. Saarschmiede has pioneered a robust, fail-safe manufacturing procedure for Cost E rotors, establishing a comprehensive database of mechanical properties and long-term performance data that enhances turbine design reliability. The company has expanded its manufacturing capabilities to include Cost F rotor forgings for high-pressure and intermediate-pressure turbines, with component weights reaching up to 44 tonnes. Investigating methods to further increase application temperatures, researchers within the Cost program discovered the potential benefits of boron additions to 10%-chromium steels. Leveraging this insight, Saarschmiede has produced full-size trial rotors to develop and refine production procedures, with these prototype components currently undergoing extensive testing to validate their performance and potential for advanced high-temperature applications.

The European Cost programmes have led to the development of improved creep resistant 9%-Cr-steels alloyed with boron, which are designed for turbine shafts subjected to steam temperatures up to 620°C. The production of forgings in steel Cost FB2 for application in power plants has commenced. Production experience and results are presented in the paper. Beyond that, Saarschmiede participates in projects targeting at steam temperatures above 700°C. In the frame of a Japanese development programme the worldwide largest trial shaft in a modified Alloy 617 Ni-Base material has been manufactured successfully from a 31 t- ESR ingot. Manufacturing route and results are presented. Contributing to the European NextGenPower project Saarschmiede has started activities to produce a large rotor forging in Alloy 263. Simulations of main manufacturing steps have been performed and a large trial forging has been produced from a triple melt ingot. First results are presented.

As conventional coal-fired power plants seek to reduce greenhouse gas emissions by increasing efficiency, the temperature limitations of traditional ferritic/martensitic steels used in high-temperature components present a significant challenge. With Advanced Ultra Supercritical (A-USC) power plants proposing steam temperatures of 760°C, attention has turned to nickel-based superalloys as potential replacements, since ferritic/martensitic steels cannot withstand such extreme conditions. However, the current absence of cast nickel-based superalloys combining high strength, creep-resistance, and weldability has led to the development of cast analogs of wrought nickel-based superalloys, including H263, H282, and N105. This paper examines the alloy design criteria, processing experiences, as-processed and heat-treated microstructures, and selected mechanical properties of these materials while also discussing their potential for full-scale development.

Approximately 75% of the worldwide energy supply is based on fossil energy but the discussions on CO 2 emission require improvements of the conventional power technologies and also an increase of renewable energy resources. Over the past 40 years, enormous efforts, especially in the development of new materials, were made to establish the technology for the ultra-supercritical power plants, which are the standard of today’s power generation. For decades voestalpine Boehler Special Steel has been a full package supplier of customized high quality special steels and forgings with close relationships to plant manufacturers to provide products ahead of their time. This paper reports on improvements and research activities of the currently best available martensitic 9% Cr steel FB2 and the latest generation, the so-called MarBN steels, raising the operating temperatures of the 9% Cr steel class from 620 °C to 650 °C. Increasing the operating temperature requires adaptations in processes and manufacturing methods to adjust optimized microstructures with improved toughness properties and increased creep rupture strength at the same time. The microstructure of two Boron containing 9% Cr steels, FB2-2 and NPM1, developed within the framework of COST / KMM-VIN, have been investigated comparatively after different heat treatments and discussed after creep rupture tests at 650°C. The results show a dependency of the creep rupture strength on the stability of precipitates and the creep rupture time of both steels was increased by more than 30 % without negatively affecting the creep rupture strain and impact values.

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.

Alloy 718, widely used for its high-temperature performance in various applications, is being investigated for use in advanced power plants. Driven by the need for efficiency improvements, these plants demand higher temperatures and pressures, putting significant stress on critical components like boiler tubes and turbines. With existing steels and alloys struggling at such high temperatures, researchers are exploring alternatives. New generation plants target steam turbine inlet temperatures of 720°C and pressures of 350MPa, necessitating superalloys for high- and intermediate-pressure rotor sections. The Thermie Advanced project explored the potential of 718 for these applications. A trial rotor disk, forged using advanced processes, underwent a novel heat treatment to enhance microstructural stability and improve creep behavior. Ongoing creep tests exceeding 100,000 hours suggest a potential 50°C increase in the operational limit compared to standard 718. This 12-year research effort holds promise for utilizing 718 in forged components of advanced ultra-supercritical power plant steam turbines, potentially operating up to 700°C.

The development of new ferritic-martensitic steels for rotor applications was a primary focus of the joint research projects COST 501 and COST 522. During COST 501, multiple trial compositions of 9-10% chromium steels underwent comprehensive testing, with the COST 522 project ultimately selecting the most promising candidate, FB2, a 10% Cr steel containing cobalt and boron additions, notably without tungsten. Società delle Fucine (SdF) successfully produced an FB2 prototype rotor using a conventional manufacturing process involving ladle furnace and vacuum degassing techniques. A comprehensive creep test program was initiated to characterize the full-size component's properties, with results demonstrating consistency with laboratory material performance in both creep resistance and ductility. The extensive testing, which exceeded 30,000 hours, aimed to achieve a 15-20 MPa improvement over Grade 92, targeting 100,000 creep hours at 600°C. Complementing the mechanical testing, a parallel microstructural investigation program was launched to evaluate structural evolution and gain deeper insights into boron's role as a creep-strengthening element in advanced ferritic-martensitic steels.

Research conducted under European COST programs has demonstrated the beneficial role of boron in enhancing the microstructural stability and creep performance of new martensitic steels. The FB2 steel (a 10%Cr steel containing Co and B, without W) emerged as the most promising candidate and was successfully scaled up to a full industrial rotor component by Società delle Fucine. Extensive creep testing, now reaching 50,000 hours, indicates an improvement of 15-20 MPa over Grade 92 at 600°C for 100,000 hours. STEM and X-ray analysis of long-term aged specimens confirmed that boron significantly enhances precipitate stability compared to Grade 91 and 92 steels, validating its role as a creep-strengthening element and stabilizer of carbides and martensitic structure.

It is required to reduce the lifetime cost of turbine blades. To achieve the cost reduction, a refining and recycling method of scrapped turbine blades is proposed. For the establishment of the method, desulfurization mechanism of Ni-base superalloy by solid CaO was studied. 500 g of superalloy containing sulfur was heated in a vacuum induction furnace and kept at 1600 °C. A CaO rod was inserted into the molten alloy and held for 600 s. After the experiment, sulfur content in the alloy decreased from 200 ppm to 54 ppm. On the surface of the CaO rod after the experiment, only Ca, O, Al, and S were found by EPMA analysis. Especially, Al and S were distributed at the surface and grain boundaries of the rod. By powder XRD analysis, CaO, CaS and 3CaO･Al 2 O 3 were identified as constituent phases on the rod. The desulfurization mechanism of superalloy at 1600 °C is supposed to be three steps: (1) Al and S in the alloy react with CaO to generate CaS and Al 2 O 3 , respectively. (2) Al 2 O 3 melts with CaO as liquid slag. (3) CaS is captured by the slag, therefore, sulfur is removed from the alloy.