The Haynes 282 Ni-based superalloy (57Ni-20Cr-10Co-8.5Mo-2.1Ti-1.5Al) is a very promising candidate for the fabrication by additive manufacturing of gas turbine components of complex geometries. Alloy 282 was fabricated by electron beam melting (EBM) and exposed to two different heat treatments, (a) solution anneal (SA) at 1135°C followed by the standard 2-step aging treatment (2h at 1010°C plus 8h at 788°C) and (b) SA followed by 4h 800°C. Large elongated grains were observed for the as-fabricated and annealed EBM 282 materials, with a γ′ (Ni 3 (Al,Ti)) average size of ~100 nm and 20 nm, respectively. The as-fabricated EBM 282 alloy exhibited good ductility at 20-900°C and tensile strength slightly lower than the tensile strength of wrought 282. Annealing the alloy resulted in a moderate increase of the alloy strength at 800 and 900°C but a decrease of the alloy ductility. The creep lifetime at 800°C, 200MPa of the as-fabricated and annealed EBM 282 specimens machined along the build direction was 2 times and 1.5 times superior to the expected lifetime for wrought 282, respectively. For creep specimens machined perpendicular to the build direction, the lifetimes were ~25% lower compared to the wrought alloy. These creep results are directly related to the strong grain texture of the EBM 282 alloy and the limited impact of the initial γ′ (Ni 3 (Al,Ti)) size on alloy 282 creep properties.

Electron beam melting (EBM) is one of the candidate manufacturing processes for TiAl alloys which have been considered as next generation high-temperature structural materials. The microstructure and mechanical properties of Ti-48Al-2Cr-2Nb (48-2-2) alloy bars fabricated using EBM were investigated, with a particular focus on the effect of processing parameters such as input energy density and building direction. We observed that the microstructure of the alloy bars fabricated using EBM depends strongly on the processing parameters used during the fabrication process of alloy. In particular, the alloy bars fabricated under appropriate processing parameters have a unique layered microstructure composed of duplex regions and equiaxed γ-grain regions (γ bands). Because of their fine microstructure and deformable soft γ bands, the alloy bars with the unique layered microstructure exhibit higher strength and higher ductility at room temperature (RT) than that of cast alloys. In addition, the alloy bars fabricated at an angle between the building direction and the loading axis of 45° show good fatigue properties at RT even without hot isostatic pressing treatment.

The current work presented a study of isothermal-oxidation behavior of the additive manufactured (AM) Alloy718 in air at 800°C. The oxidation behavior of Alloy718 specimens produced by selective laser melting (SLM) and electron beam melting (EBM) process were comparatively examined. No significant differences were observed in oxidation kinetics while different microstructures of the oxide scale were found. Coarse and columnar chromia grains developed on SLM specimens, whereas the chromia scale of EBM specimens consisted of extremely fine grains. Glow Discharge Optical Emission Spectrometry (GD-OES) analysis revealed that SLM specimens contain a higher content of Ti in chromia compared with EBM specimens. Process-induced supersaturation in SLM specimens might lead to a relatively high concentration of Ti in the chromia, which may affect the grain morphology of oxide scale in the SLM specimen.

Additive manufacturing (AM) is a process where, as the name suggests, material is added during production, in contrast to techniques such as machining, where material is removed. With metals, AM processes involve localised melting of a powder or wire in specific locations to produce a part, layer by layer. AM techniques have recently been applied to the repair of gas turbine blades. These components are often produced from nickel-based superalloys, a group of materials which possess excellent mechanical properties at high temperatures. However, although the microstructural and mechanical property evolution during the high temperature exposure of conventionally produced superalloy materials is reasonably well understood, the effects of prolonged high temperature exposure on AM material are less well known. This research is concerned with the microstructures of components produced using AM techniques and an examination of the effect of subsequent high temperature exposures. In particular, the paper will focus on the differences between cast and SLM IN939 as a function of heat treatment and subsequent ageing, including differences in grain structure and precipitate size, distribution and morphology, quantified using advanced electron microscopy techniques.

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.

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.

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.

Solar salts are used as an energy storage media and heat transfer fluid in power plants. The salts can cause significant corrosion to various steels that are in contact with the salt. Static corrosion tests performed with different steels show, that the corrosive attack by industrial grade salt melts is more severe than by defined grade salt melts and the sample corrosion is faster (i.e. the weight gain is larger) for higher temperatures. Slow strain rate (SSR) tests in salt are difficult to conduct due to the corrosive attack of the salt also on the test setup. The SSRT setup in salt could be realized and tests could be conducted successfully. No clear evidence for an accelerated failure of samples tested in salt compared to samples tested in air could be found on Alloy 347 Nb. Comparative low cycle fatigue (LCF) tests at air and in molten salt atmosphere were successfully performed and showed similar results on tubes out of Sanicro 25. No evidence of accelerated crack growth in molten salt could be found.

The powder metallurgy (P/M) process has been applied to a high strength turbine disk alloy. It is known that P/M alloys show characteristic microstructures such as prior powder boundaries (PPB) compared to microstructures of conventional cast and wrought (CW) alloys. High temperature tensile tests were conducted on CW and P/M processed alloy720Li in order to reveal the effect of temperature and strain rate on deformation behavior and to demonstrate the effect of microstructure derived from P/M process on deformability. The fracture mode of the P/M material changed from grain interior fracture to fracture around large PPB with an increment of strain rate. In addition, samples ruptured at higher temperature showed grain boundary fracture regardless of strain rate. On the other hand, the CW material showed good deformability with chisel point fracture in the entire temperature and strain rate condition range. In the P/M material, melting of grain boundaries occurred at super solvus temperature conditions. Large PPB acts as nucleation site of voids at higher strain rate conditions. Precipitation strengthening by γ’ phase degrades deformability at sub solvus temperature conditions. However, deformability near the solvus temperature and low strain rate condition in as HIPed P/M material increased with fine grain size distribution in spite of the presence of large grains resulting from PPB.

The A286 is one of the earliest superalloys developed. It has been used for manufacturing different components of turbo machineries because of its balanced high temperature properties. These components include shafts, discs, spacers, blades and fasteners. This paper reviews some of the issues and recent field experiences related to metallurgy, fabrication, in-service evaluation and failure of some of these components. The fabrication aspects including the effects of alloy melting processes, forging parameters and different types of heat treatments on the processed parts are discussed. The importance of these factors on the microstructure and properties of A286 are highlighted. The effects of service exposure on some of these parts are also discussed. In service evaluation involves checking for service induced damage or changes in microstructures and properties so that the suitability of the part for continued service can be determined. The failure analysis section of the paper briefly discusses failures of two expander wheels and an expander disc made out of A286 to pinpoint some of the salient features of damage accumulation and fracture during service.

A European project (COST 522) aims to improve power plant efficiency by developing stronger steel for steam turbines. These turbines operate with extremely hot steam (up to 650°C) to maximize efficiency and minimize emissions. The project focuses on ferritic-martensitic steel, which is suitable for the thick components used in these high-temperature environments. Building on prior advancements, COST 522 explored new steel formulations and tested them thoroughly. This has resulted in steels capable of withstanding even hotter steam (610°C live steam and 630°C reheat steam), paving the way for the next generation of highly efficient power plants.

Austenitic and super-austenitic stainless steels are a critical component of the spectrum of high temperature materials. With respect to power generation, alloys such as Super 304H and NF709 span a gap of capability between ferritic and martensitic high chromium steels and nickel-based alloys in boiler tube applications for both conventionally fired boilers and heat-recovery steam generators (HRSG). This research explores a wrought version of a cast austenitic stainless steel, CF8C-Plus or HG10MNN, which offers promise in creep strength at relatively low cost. Various manufacturing techniques have been employed to explore the impact of wrought processing on nano-scale microstructure and ultimately performance, especially in high temperature creep. Transmission electron microscopy has been used to quantify and characterize the creep-strengthening particles examining the relationship between traditional melting and extrusion as compared to powder metallurgy.

The presented work summarizes the results of more than 10 years of research at TU Graz and TU Chemnitz and partners on a martensitic boron and nitrogen stabilized 9Cr3W3Co (MARBN) steel grade. The design philosophy of MARBN steels is presented and critical issues regarding boron and nitrogen balance are discussed. Microstructural characterization of two different laboratory heats, is presented and efforts in European projects towards an upscaling of melts are presented. Base material creep testing data at 650 °C up to 50.000 hours is presented and assessed to commercial alloys such as ASTM grades P91 and P92. An increase of creep rupture stress of more than +20% was recorded. Oxidation tests in steam at 650°C revealed an anomalous response of the material. Several specimens exhibited excellent oxidation resistance commonly only seen for grades of higher chromium content. The anomalous oxidation behaviour is identified and discussed, although the causes remain yet unclear. Results of manufacturing, characterization and testing of different MARBN welds, including gas-tungsten-arc-, gas-metal-arc-, friction stir and electron beam welds reveal a microstructure memory effect in the heat affected zone, so that no uniform fine-grained zone is present. The behaviour of crosswelds during long-term creep testing at 650 °C up to more than 32.000 hours is assessed and the susceptibility to Type IV cracking is discussed. The manuscript summarizes research of more than 10 years, presents current research activities on MARBN and describes open questions for an alloy identified as a promising martensitic steel grade for elevated temperature components.

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.

To improve microstructure stability at temperature up to 700°C and avoid segregation of Nb during melting processes, we modified the chemical composition of conventional Ni-Fe base super alloy(Ni-36Fe-16Cr-3Nb-1.7Ti-0.3Al:Alloy706). It is known that Alloy706 is strengthened by γ'(Ni 3 Al) phase and γ”(Ni 3 Nb) phase. But these phases are unstable at high temperature and transform into Nb rich δ or η) phase after long-term exposure to elevated temperature. Therefore modified alloy contains lower Nb and higher Al than those of Alloy706, and it is mainly strengthened by γ’(Ni 3 Al) phase. In fact we could not find δ or η phase in the modified alloy after creep and aging at 700 °C. Tensile strengths of the modified alloy at temperature from room temperature to 700 °C were almost same as those of Alloy706. Yield strength of modified alloy at room temperature was slightly lower than that of Alloy706, but equivalent to that of Alloy706 at higher temperatures. Tensile and yield strengths of the modified alloy at temperature from room temperature to 700 °C were higher than those of Alloy706 after aging at 700 °C. In this paper, we discuss effects of Nb and Al on mechanical properties and microstructure at elevated temperature up to 700 °C, using mechanical testing, TEM observations and thermodynamics calculation results. And we show advantages of the microstructure stabilized Ni-Fe base superalloy(FENIX-700), which is a candidate material for 700 °C class USC steam turbine rotor.

Advanced 700C class steam turbines require austenitic alloys instead of conventional ferritic heat-resistant steels which have poor creep strength and oxidation resistance above 650C. Austenitic alloys, however, possess a higher thermal expansion coefficient than ferritic 12Cr steels. Therefore, Ni-based superalloys were tailored to reduce their coefficients to the level of 12Cr steels. Regression analysis of commercial superalloys proves that Ti, Mo and Al decrease the coefficient quantitatively in this order, while Cr, used to secure oxidation resistance, increases it so significantly that Cr should be limited to 12wt%. The newly designed Ni-18Mo-12Cr-l.lTi-0.9Al alloy is strengthened by gamma-prime [Ni 3 (Al,Ti)] and also Laves [Ni 2 (Mo,Cr)] phase precipitates. It bears an RT/700C mean thermal expansion coefficient equivalent to that of 12Cr steels and far lower than that of low-alloyed heat resistant steels. Its creep rupture life at 700C and steam oxidation resistance are equivalent to those of a current turbine alloy, Refractaloy 26, and its tensile strength at RT to 700C surpasses that of Refractaloy 26. The new alloy was trial produced using the VIM-ESR melting process and one ton ingots were successfully forged into round bars for bolts without any defects. The bolts were tested in an actual steam turbine for one year. Dye penetrant tests detected no damage. The developed alloy will be suitable for 700C class USC power plants.

Steel castings of creep-resistant steels are critical components in the high and intermediate pressure turbine sections of fossil fuel-fired power plants. As plant efficiencies improve and emission standards tighten, steam parameters become more stringent, necessitating constant enhancement of material creep resistance. Steel foundries alone cannot conduct necessary material development at an appropriate scale, so all power plant component suppliers cooperate to define optimal chemical compositions, perform test melts, creep tests, microstructure investigations, and test pilot components, such as through the COST program developing new 9-12%Cr cast steel grades. This paper illustrates a steel foundry's role in COST, describing the transfer of these new cast steel grades from research into commercial production of heavy cast components, outlining incurred problems, process development cycles, comparisons with low-alloy steels, welding tests, base material/weld investigations, heat treatment optimization, and casting of pilot components/weldability test plates to verify castability of larger parts and make necessary adjustments. Parallel to ongoing COST creep tests, the steel grades were introduced into commercial large component production, involving solutions to process-related issues, with over 180 components successfully manufactured to date, while further COST program developments present ongoing challenges.

Investigations on welded joints made from a modified parent material and welding consumables are described. Tubes and pipes with typical dimensions have been welded using different welding processes and consumables (GTAW, SAW, SMAW, modified filler metals). The influence of melting loss and chemical composition of the consumables on the weld performance was studied. Short-term tensile and long-term creep tests on cross weld specimens were carried out in order to evaluate strength. The results obtained so far show that the properties of the welded joints are rather optimistic, it could be assumed that the modified Alloy 617 and the welding consumables used will meet the requirements for use in a plant operated at ultra critical steam conditions with live steam temperatures up to 720°C and pressure up to 300 bar. This allows for first practical applications in test loops of plants. These applications including the Welding Procedure Qualifications are described.

The measurement of damage from high temperature solid particle erosion (HTSPE) can be a lengthy process within the laboratory with many lab-based systems requiring sequential heat and cooling of the test piece to enable mass and/or scar volume measurements to be made ex situ. Over the last few years a new lab-based system has been in development at the National Physical Laboratory which has the ability to measure the mass and volume change of eroded samples in situ without the need to cool the sample. Results have previously been shown demonstrating the in situ mass measurement, more recently the in situ volume measurement capability has been added and used to evaluate the erosion performance of additively manufactured materials. Selective laser melting (SLM) is an advanced manufacturing method which is growing in popularity and application. It offers the ability to manufacture low volume complex parts and has been used in rapid prototyping. As the technique has developed there is increasing interest to take advantage of the ability to manufacture complex parts in one piece, which in some case can be more cost and time effective than traditional manufacturing routes. For all the benefits of SLM there are some constraints on the process, these include porosity and defects in the materials such as ‘kissing bonds’, surface roughness, trapped powder and microstructural variation. These features of the processing route may have implications for component performance such as strength, fatigue resistance wear and erosion. To investigate this further SLM IN718 has been used to evaluate factors such as surface roughness, microstructure and morphology on the erosion performance as measured in situ and compared with conventional produced wrought IN718 material.

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.