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.

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.

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

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

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.

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.

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.

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.

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.

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.

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.

The manufacture of large, complex components for ultra-supercritical and oxy-combustion applications will be extremely costly for industry over the next few decades as many of these components will be manufactured from expensive, high strength, nickel-based alloys casting and forgings. The current feasibility study investigates the use of an alternative manufacturing method, powder metallurgy and hot isostatic processing (PM/HIP), to produce high quality, and potentially less expensive components for power generation applications. Benefits of the process include manufacture of components to near-net shapes, precise chemistry control, a homogeneous microstructure, increased material utilization, good weldability, and improved inspectability.

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.

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.

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.

Improving power plant efficiency through supercritical steam pressures and very high steam temperatures up to 700°C and beyond is an effective approach to reducing fuel consumption and CO2 emissions. However, these extreme steam temperatures necessitate the use of nickel-base alloys in the high-pressure/intermediate-pressure turbine sections requiring very large component sections that cannot be met by steels. Saarschmiede, involved in manufacturing large components for the power generation industry and research programs on advanced 9-12% chromium steels, has extensive experience producing nickel and cobalt-base alloy forgings for applications like aircraft engines, aerospace, land-based gas turbines, and offshore. This paper reports on the manufacturing and testing of large-section forgings made from candidate nickel-base alloys like 617 and 625 for high-pressure/intermediate-pressure turbine components in power stations operating at 700°C and higher steam temperatures.

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.