Sufficient energy availability in combination with lowest environmental pollution is a basic necessity for a high living standard in each country. To guarantee power supply for future generations, improved technologies to achieve higher efficiency combined with reduced environmental impact are needed. This challenge is not only aimed to the power station manufacturers, but also to the producers of special steel forgings, who have to handle with more and more advanced materials and complex processes. Bohler Special Steel is a premium supplier of forged high quality components for the power generation industry. This paper reports about experiences in the fabrication of forged components for steam turbines for ultra-supercritical application - from basic properties up to ultrasonic detectability results. The materials used so far are the highly creep-resistant martensitic 9-10% Cr steel class for operating temperatures up to 625°C developed in the frame of the European Cost research program. Additionally our research activities on the latest generation of high temperature resistant steels for operating temperatures up to 650 degree Celsius – the boron containing 9% Cr martensitic steels (MARBN) - are discussed. In order to improve the creep behavior, MARBN steels with different heat treatments and microstructures were investigated using optical microscopy, SEM and EBSD. Furthermore, short term creep rupture tests at 650 degree Celsius were performed, followed by systematic microstructural investigations. As a result it can be concluded, that advanced microstructures can increase the time to rupture of the selected MARBN steels by more than 10 percent.

Within the pursuit of improved economic electricity production with reduced environmental pollution, the European research activities COST 501/522 aimed to develop advanced 9-12%Cr steels for highly stressed turbine components by increasing thermal efficiency through higher steam temperatures up to 600/625°C. One such modified Cr steel, a tungsten-alloyed 10%Cr steel, has been in industrial production for several years in steam and gas turbine applications. This paper firstly discusses experiences in manufacturing, non-destructive testing, and mechanical properties achieved in forgings of this COST grade E steel. Secondly, it reports on the manufacturing of a trial melt of a later 9%Cr steel containing cobalt and boron from COST development, describing its long-term creep behavior, microstructural features responsible for superior creep resistance, and test results including short-term properties, detectable flaw size, and initial creep results for a full-size trial rotor forging.

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.

Microstructural change of 10 % Cr steel trial forgings subjected to different heat treatment conditions which aim to improve the creep rupture strength and microstructural stability during creep was investigated. Creep rupture strength of the forging subjected to the quality heat treatment with the austenitizing temperature of 1090° C is higher than that of the forging solution treated at 1050°C, however, the difference of creep rupture strength is reduced in the long-term region around 40,000 h. Decrease in creep rupture ductility of the forging until 43,300 h is not observed. Progress of the martensite lath recovery in the forging solution-treated at 1090°C is slower than that in the forging austenitized at 1050°C. Higher temperature solution treatment suppresses the recovery of lath structures. Formations of Z-phase are found in the specimens creep-ruptured at 37,300 h in the forging solution-treated at 1050°C and at 43,400 h in the forging austenitized at 1090°C. Z-phase precipitation behavior in this steel is delayed in comparison with the boiler materials, regardless of austenitizing temperature.

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.

Demand of 9-12% chromium steel rotor forgings becomes higher from point of view of environmental protection in coal fired fossil power generations. Japan Casting & Forging Corporation (JCFC) has manufactured 9-12% Cr steel rotor forgings with JCFC's original techniques since 1991. Recently, type E steel developed by European COST program has been trial melted to meet the demand of such high Cr steel forgings in the world. Full size two forgings have been manufactured from approximately 70 ton ingot applying Electro Slag Hot Topping by JCFC (ESHT-J) process. One of the trial forgings has been austenitized at higher temperature in the quality heat treatment to improve long term creep strength. Their productivities and sufficient qualities have been ascertained.

In this work, two unique heats of 9Cr creep strength enhanced ferritic (CSEF) steels extracted from a retired superheat outlet header after 141,000 hours of service were evaluated. These two CSEF steels were a forging manufactured to SA-182 F91 (F91) reducer and a seamless pipe produced to SA-335 P91 (P91) pipe. Their creep deformation and fracture behavior were assessed using a lever arm creep frame integrated with in-situ high-temperature digital image correlation (DIC) system. Critical metallurgical and microstructure factors, including composition, service damage, grain matrix degradation, precipitates, and inclusions were quantitatively characterized to link the performance of the two service aged F91 and P91 CSEF steels. The creep test results show the F91 and P91 steels exhibit a large variation in creep strength and creep ductility. The F91 steel fractured at 572 hours while P91 steel fractured at 1,901 hours when subjected to a test condition of 650 °C and 100 MPa. The nominal creep strains at fracture were 12.5% (F91) and 14.5% (P91), respectively. The high-resolution DIC strain measurements reveal the local creep strain in F91 was about 50% while the local creep strain in P91 was >80%. The characterization results show that the F91 steel possessed pre-existing creep damage from its time in service, a higher fraction of inclusions, and a faster matrix grain coarsening rate. These features contribute to the observed reduction in performance for the F91 steel. The context for these findings, and the importance of metallurgical risk in an integrated life management approach will be emphasized.

10CrMoWVNbN (X 12 CrMoWVNbN 10 1 1) steel trial forgings has been manufactured to clarify the effect of austenitizing temperature on the creep rupture strength and microstructure. From the results of creep rupture tests up to 30,000 hours, higher austenitizing temperature improves the rupture strength without large degradation of the rupture ductility. The microstructural investigations demonstrate that the prior austenite grain size and the precipitation behavior of fine M2X particles are presumed to contribute to the improvement of creep rupture strength.

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.

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.

There is an increased interest in miniature testing to determine material properties. The small punch test is one miniaturized test method that has received much interest and is now being applied to support the design and life assessment of components. This paper presents the results of a test program for a small punch creep test at 650°C of 316L stainless steel produced from additive manufacturing. A major finding is that the deflection rate curve versus time may have multiple minima as opposed to forged 316L with only one minimum. This is believed to be due to microcracking and has direct consequences on the determination of the creep properties that that are based on a single minimum value in the CEN Small Punch Standard. In the paper, aged and nonaged materials are compared, and small punch creep results are also compared with standard uniaxial creep tests. The multiple minima feature means that the approach to determine equivalent stress and strain rate from the minimum deflection rate needs to be modified. Some approaches for this are discussed in the paper. Under the assumption that the multiple minima represent cracking, it opens up opportunities to quantify reduced creep ductility by the small punch test.

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.

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.

In Europe and Japan, great efforts are currently being invested in the development of materials designed to increase the steam temperature in fossil power plants. In the steel segment, the COST program is concentrating on 10% Cr steels with the addition of boron with the aim of achieving a steam temperature of 650°C. With nickel-based materials, the goal is to achieve steam temperatures of 700°C and higher. Alloy 617 has proved to be a very promising candidate in this field and a modified version is currently being developed in Japan. Materials of this type are used in both the turbine and in parts of the boiler.

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.

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.

Sufficient available energy in combination with lowest environmental pollution is a basic necessity for a high standard of living in every country. In order to guarantee power supply for future generations it is necessary to use fossil fuels as efficient as possible. This fact calls for the need of power plants with improved technologies to achieve higher efficiency combined with reduced environmental impact. In order to realize this goal it is not only a challenge for power station manufacturers, but also for manufacturers of special steels and forgings, who have to produce improved components with more advanced materials and more complex manufacturing processes. This paper reports about experiences in the fabrication of forged components for gas and steam turbines followed by achievable mechanical properties and ultrasonic detectability results. The materials are the creep resistant martensitic Cr steels developed in the frame of the European Cost research programme. Whereas Boron containing 10% Cr steels are suitable for steam temperatures of 625°C and slightly higher, Ni-based alloys shall be used for temperatures of 700°C and above. One pilot rotor forging, representing a HP-rotor for welded construction, has been manufactured out of alloy Inconel 625 within the frame of the European Thermie project AD700.

Development activities initiated over a decade ago within the COST 522 program and continuing through the COST 536 Action have yielded significant progress in constructing a new generation of steam power plants capable of operating under advanced steam conditions. These innovative plants promise substantially improved thermal efficiency, with steam temperatures reaching up to 620°C (1150°F). Recent successful power plant orders in Europe and the United States stem from critical advancements, including the development, testing, and qualification of 10% Cr steels with enhanced long-term creep properties for high-temperature components such as turbine rotors and valve casings. Extensive in-house development efforts have focused on fabrication, weldability, mechanical integrity, and fracture mechanics evaluations of full-sized forged and cast components. These materials will be implemented in several new coal-fired power plants, notably the Hempstead plant in the USA, which will operate with live steam temperatures of 599°C (1111°F) and reheat steam temperatures of 607°C (1125°F). The improved creep properties enable the construction of casings with reduced wall thicknesses, offering greater thermal flexibility at lower component costs and facilitating welded turbine rotors for high-temperature applications without requiring cooling in the steam inlet region. Looking forward, further efficiency improvements are anticipated through the introduction of nickel alloys in steam turbine and boiler components, with the European AD700 project targeting reheat steam temperatures of 720°C (1328°F) and the US Department of Energy project aiming even higher at 760°C (1400°F). The AD700 project has already demonstrated the technical feasibility of such advanced steam power plants, with engineering tasks progressing toward the construction of a 550 MW demonstration plant, while DOE activities continue to address boiler concerns and focus on rotor welding, mechanical integrity, and steam oxidation resistance.

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.