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1. The World's Biggest and Best Gas Turbine Can Power 400,000 Homes General Electric's 9HA is the most advanced and efficient gas turbine available today and though it may not be able to lift a 787 off the ground, this potent electricity producer can easily power a mid-size metropolis.

2. OPERATING CONDITIONS FOR TURBINE BLADES . In gas turbine industry, the blade of the high pressure turbine has received the highest attention of the researchers because the challenge it provides. The ability to run at increasingly high gas temperatures has resulted from a combination of material improvements and the development of more sophisticated arrangements for internal and external cooling; for instance, nowadays, high pressure turbine blades receive compressed air bled from the compressor and it is injected to the turbine blades though small holes drilled on them, with the purpose to establish a protection layer on the edge of the blades and guaranty that hot flue gases could not affect directly them .

3. MATERIALS USED IN GAS TURBINE BLADES. Modern gas turbines have the most advanced and sophisticated technology in all aspects; construction materials are not the exception due their extreme operating conditions. The most difficult and challenging point is the one located at the turbine inlet, because, there are several difficulties associated to it; like, extreme temperature (1400 O C –1500 O C), high pressure, high rotational speed, vibration,small circulation area, and so on. The aforementioned rush characteristics produces effects on the blades that are shown on the table. Oxidation Hot corrosion Interdiffusion Thermal Fatigue Aircraft Severe Moderate Severe Severe Power Generator Moderate Severe Moderate Light Marine Engines Moderate Severe Light Moderate

4. In order to overcome those barriers, gas turbine blades are made using advanced materials and modern alloys ( superalloys ) that contains up to ten significant alloying elements , but its microstructure is very simple; consisted of rectangular blocks of stone stacked in a regular array with narrow bands of cement to hold them together. This material (cement) has been changed because in the past,intermetallic form of titanium was used in it, but nowadays , it has been replaced by tantalum.This change gave improved high temperature strength,and also improved oxidation resistance. However , the biggest change has occurred in the nickel, where high levels of tungsten and rhenium are present. These elements are very effective in solution strengthening. hand , single crystal alloy, is able to run about 35°C hotter.

5. Temperature and pressure profile in gas turbine.

6. Increase of Turbine Efficiency Turbine components have been continually improved over decades, specifically with respect to temperature resistance. Initially the focus for improvement was as the blade material itself and its temperature capability. Large improvements have been achieved since the sixties by continually refining the casting methods, developing new Ni-base alloys, optimizing component shapes, component dimensions, grain structures and finally by applying special cooling methods to the components. This process continues today, but gains in temperature and effciency have reached the limits set by the laws of physics. Since the seventies, metal base vacuum coatings (e.g. MCrAlY ) have been applied to protect the Ni-base alloy component surface against corrosion by hot gas. The success of these coatings marked the starting point for the development of non-metallic coatings with thermal insulation properties. Today these coatings are an integral part of the design of all modern aircraft turbine engines.

7. U-500 This material was used as a first stage (the most demanding stage) material in the 1960s, and is now used in later, less demanding, stages. Rene 77 Rene N5 Rene N6 PWA1484 CMSX-4 CMSX-10 Inconel IN-738 - GE used IN-738 as a first stage blade material from 1971 until 1984, when it was replaced by GTD-111. It is now used as a second stage material. It was specifically designed for land-based turbines rather than aircraft gas turbines. GTD-111 Blades made from directionally solidified GTD-111 are being used in many GE Energy gas turbines in the first stage. Blades made from equiaxed GTD-111 are being used in later stages. EPM-102 (MX4 (GE), PWA 1497 (P&W)) is a single crystal super alloy jointly developed by NASA, GE Aviation, and Pratt & Whitney for the High Speed Civil Transport (HSCT). While the HSCT program was cancelled, the alloy is still being considered for use by GE and P&W. Nimonic 80a was used for the turbine blades on the Rolls-Royce Nene and de Havilland Ghost Nimonic 90 was used on the Bristol Proteus. Nimonic 105 was used on the Rolls-Royce Spey. Nimonic 263 was used in the combustion chambers of the Bristol Olympus used on the Concorde supersonic airliner. Note: This list is not inclusive of all alloys used in turbine blades List of turbine blade materials

8. Compressor blading materials for gas turbines - Special steels A ll production blades for compressors are made from 12% chromium containing martensitic stainless steel grades 403 or 403 Cb ( Schilke , 2004). Corrosion of compressor blades can occur due to moisture containing salts and acids collecting on the blading. To prevent the corrosion, GE has developed patented aluminum slurry coatings for the compressor blades. The coatings are also meant to impart improved erosion resistance to the blades. During the 1980’s, GE introduced a new compressor blade material, GTD-450, a precipitation hardened martensitic stainless steel for its advanced and uprated machines ( Schilke , 2004). Without sacrificing stress corrosion resistance, GTD-450 offers increased tensile strength, high cycle fatigue strength and corrosion fatigue strength, compared to type 403. GTD-450 also possesses superior resistance to acidic salt environments to type 403, due to higher concentration of chromium and presence of molybdenum ( Schilke , 2004).

9. Titanium alloys used for compressor parts – chemical composition and maximum service temperature Compressor blade materials for land based gas turbines

10. Combustion hardware for gas turbines Driven by the increased firing temperatures of the gas turbines and the need for improved emission control, significant development efforts have been made to advance the combustion hardware, by way of adopting sophisticated materials and processes . The primary basis for the material changes that have been made is improvement of high temperature creep rupture strength without sacrificing the oxidation / corrosion resistance . Traditionally combustor components have been fabricated out of sheet nickel- basesuperalloys . Hastelloy X, a material with higher creep strength was used from 1960s to 1980s. Nimonic 263 was subsequently introduced and has still higher creep strength ( Schilke , 2004 ).As firing temperatures further increased in the newer gas turbine models, HA-188, a cobalt base superalloy has been recently adopted for some combustion system components for improved creep rupture strength ( Schilke , 2004). Coutsouradis et al. reviewed the applications of cobalt-base superalloys for combustor and other components in gas turbines ( Coutsouradis et al ., 1987). Nickel base superalloys 617 and 230 find wide application for combustor components

11. Combustor materials

12. Turbine Disk A286, an austenitic iron-base alloy has been used for years in aircraft engine applications ( Schilke , 2004). Superalloy 718 has been used for manufacture of discs in aircraft engines for Materials for more than 25 years ( Schilke , 2004). Both these alloys have been produced through the conventional ingot metallurgy route. Powder Metallurgy (PM) processing is being extensively used in production of superalloy components for gas turbines. PM processing is essentially used for Nickel-based superalloys . It is primarily used for production of high strength alloys used for disc manufacture such as IN100 or Rene95 which are difficult or impractical to forge by conventional methods. LC Astroloy , MERL 76, IN100, Rene95 and Rene88 DT are the PM superalloys where ingot metallurgy route for manufacture of turbine discs was replaced by the PM route.

13. The advantages of PM processing are listed in the following: Superalloys such as IN-100 or Rene95 difficult or impractical to forge by conventional methods . P/M processing provides a solution Improves homogeneity / minimizes segregation, particularly in complex Ni-base alloy systems Allows closer control of microstructure and better property uniformity within a part than what is possible in cast and ingot metallurgy wrought products. Finer grain size can be realized. Alloy development flexibility due to elimination of macro-segregation. Consolidated powder products are often super-plastic and amenable to isothermal forging , reducing force requirements for forging. It is a near net shape process; hence significantly less raw material input required and also reduced machining cost, than in case of conventional ingot metallurgy . Several engines manufactured by General Electric and Pratt and Whitney are using superalloy discs manufactured through PM route .

14. Turbine blades and vanes – Cast superalloyss Recognition of the material creep strength as an important consideration for the gas turbine engines , understanding generated between age hardening, creep and volume fraction and the steadily increasing operating-temperature requirements for the aircraft engines resulted in development of wrought alloys with increasing levels of aluminum plus titanium . Component forgeability problems led to this direction of development not going beyond a certain extent. The composition of the wrought alloys became restricted by the hot workability requirements. This situation led to the development of cast nickel-base alloys . Casting compositions can be tailored for good high temperature strength as there was no forgeability requirement.

15. Nozzle materials for gas turbines GE engines use FSX 414, a GE-patented cobalt base alloy for all stage 1 nozzles and some later stage nozzles. Cobalt base alloys possess superior strength at very high temperatures compared to nickel base superalloys – hence the choice of cobalt base superalloy . It has a two-three fold oxidation resistance compared to X40 and X45, also cobalt based superalloys used for nozzle applications. Use of FSX 414 over C40/C45 hence enables increased firing temperatures for a given oxidation life ( Schilke , 2004). Later stage nozzles must also possess adequate creep strength and GE developed a nickel base superalloy GTD222 for some stage 2 and stage 3 applications. The alloy has significantly higher creep strength compared to FSX414. N155, an iron-based superalloy , has good weldability and is used for later stage nozzles of some GE engines ( Schilke , 2004 ).

16. Nozzle materials

17. CRITERIA FOR MATERIAL SELECTION For the material selection, the key points are the first stages turbine disks and blades where the stresses and temperatures are the highest . Table summarizes the thermal and mechanical loads for the turbine disks and blades. The materials for these components should ensure a safe operation for at least 60000 hours. Therefore, a ground rule for the material selection of the most critical parts was proposed as following

18. MATERIALS FOR TURBINE BLADES Materials selection and testing for HTGR turbine blades has been extensively studied. Essentially 2 types of metallic materials were investigated: - Nickel-base cast superalloys . - Molybdenum-base grades.

19. Ni-base cast superalloys In several past R&D programs, selected alloys were ranked by their high temperature strength , castability and cobalt content. Most promising alloys for the blades are therefore: - 713LC [6,7,8,10,11,12,13] - M21 [7,10,12] - MAR-M 004 [7,13,12 ]

20. Alloy 713 LC is a cast nickel base precipitation hardened alloy that combines superior castability and creep resistance. Alloy 713LC has the advantage of wide industrial experience in conventional gas turbine (turbine housings, case, stator). Alloy 713 LC neither contains Co or Ta and should therefore not present any contamination problems, hi HTGR environment , alloy 713LC can be susceptible to carburisation and sulfidation problems, and coatings have to be envisaged. Alloy 713LC is well suited for the turbine blades required specifications , except for the first row of blades where cooling would be necessary to achieve the required lifetime . Directionally Solidified (DS) or Single Crystal (SC) blades would solve this problem, but alloys commonly used for DS or SC blades contain about 10% cobalt ( DS Mar M 247, SC Rene N4 ). Alloy 713LC was used for the fabrication of the turbine blades in the HHV project [11 ].During this project held in Germany, a full-scale test turbine driven by HTGR-type helium was built. During the short high temperature lifetime experienced (—325 hours at 850°C), no material problems appeared with the working blades. Alloy M21 is a low chromium nickel base alloy that combines precipitation hardening and solid solution hardening (—10% tungsten addition). It was selected for its superior corrosion resistance in impure helium.Alloy MM004 has been developed from alloy 713LC and is claimed to have a better toughness because of Hafnium addition

21. Molybdenum-base alloys The molybdenum-base alloy TZM (Mo-0.5Ti-0.08Zr) has not been used in industrial gas turbines because of its poor oxidation resistance in air. However, R&D efforts performed in German HTGR programs have demonstrated a promising application for this alloy for helium turbine blades. As shown in figure 2, Mo-TZM exhibits a completely different creep resistance behaviour as compared to the nickel-base alloys, mainly because of its high melting point (2607°C) [15]. Almost flat creep curves make a 100 OOOh life time appear possible with an uncooled blade, as shown in figure 3 .

22. INTRODUCTION Advanced GE materials are paving the way for dramatic improvements in gas turbines — improvements that are setting new records in giving customers the most fuel-efficient power generation systems available. Combined-cycle efficiencies as high as 60% are now achievable because of increased firing temperature coupled with more efficient component and system designs . Ongoing GE developments now promise that the coming decade will witness continued growth of gas turbines with higher firing temperatures, pressures and outputs .

23. Bucket Materials The stage 1 bucket must withstand the most severe combination of temperature, stress and environment ; it is generally the limiting component in the machine. Since 1950, turbine bucket material temperature capability has advanced approximately 850°F/472°C , approximately 20°F/10°C per year . The importance of this increase can be appreciated by noting that an increase of 100°F/56°C in turbine firing temperature can provide a corresponding increase of 8% to 13 % in output and 2% to 4% improvement in simple-cycle efficiency. Advances in alloys and processing , while expensive and time-consuming, provide significant incentives through increased power density and improved efficiency. Figure shows the trend of firing temperature and bucket alloy capability. The composition of the new and conventional alloys discussed is shown in Table. The increases in bucket alloy temperature capability accounted for the majority of the firing temperature increase until the 1970s, when air cooling was introduced , which decoupled firing temperature from bucket metal temperature. Also, as the metal temperatures approached the 1600°F/870°C range, hot corrosion of buckets became more life-limiting than strength until the introduction of protective coatings.

24. Firing temperature trend and bucket material capability

25. High-Temperature Alloys

26. Future Buckets With the introduction of DS GTD-111, a commercial reality , development efforts are now focusing on single-crystal processing and advanced DS alloy development. Single-crystal airfoils offer the potential to further improve component high-temperature material strength and, by control of crystal orientation , can provide an optimum balance of properties. In single-crystal material, all grain boundaries are eliminated from the material structure and a single crystal with controlled orientation is produced in an airfoil shape. By eliminating all grain boundaries and the associated grain boundary strengthening additives, a substantial increase in the melting point of the alloy can be achieved , thus providing a corresponding increase in high-temperature strength. The transverse creep and fatigue strength is increased, compared to equiaxed or DS structures . GE Aircraft Engines has been applying single-crystal bucket technology for more than 10 years in flight engines. The advantage of single- crystal alloys compared to equiaxed and DS alloys in low-cycle fatigue (LCF) is shown in Figure . GE is currently evaluating and Rainbow rotor testing some of these single-crystal alloys for application in our next generation gas turbines . The continuing and projected temperature capability improvements in bucket material capabilities are illustrated in Figure . Together with improved coatings, these new bucket materials will provide continued growth capability for GE gas turbines in the years to come.

27. Nozzle Materials Stage 1 nozzles (GE terminology for the stationary vanes in the turbine) are subjected to the hottest gas temperatures in the turbine , but to lower mechanical stresses than the buckets . Their function is to direct the hot gases toward the buckets and they must, therefore , be able to withstand high temperatures and provide minimal gas turning losses. The nozzles are required to have excellent high-temperature oxidation and corrosion resistance , high resistance to thermal fatigue, relatively good weldability for ease of manufacture and repair , and good castability . Latter-stage nozzles must also possess adequate creep strength to support themselves and the attached diaphragms from the external casing.

28. FSX-414 Nozzles The current alloy used for all production stage 1 nozzles and some latter-stage nozzles is FSX- 414 , a GE-patented cobalt base alloy. Cobaltbase alloys generally possess superior strength at very high temperatures, compared to nickelbased alloys. This alloy is a derivative of X-40 and X-45, both of which were also developed by GE and first used in the 1960s. FSX-414 contains less carbon than X-40 to enhance weldability , and more chromium to improve oxidation / corrosion resistance. Long-life tests in a simulated gas turbine combustion chamber have demonstrated a two- to three-fold increase in oxidation resistance compared to X-40 and X-45 . This improvement permitted an increase in the firing temperatures of approximately 100°F/56°C for equivalent nozzle oxidation life.

29. GTD-222 Nozzles The latter-stage, nickel-based nozzle alloy , GTD-222 , was developed in response to the need for improved creep strength in some stage 2 and stage 3 nozzles. It offers an improvement of more than 150°F/66°C in creep strength compared to FSX-414, and is weld-repairable. An important additional benefit derived from this alloy is enhanced low-temperature hot corrosion resistance . By tailoring the alloy to provide an optimum combination of creep strength and weldability , a unique GE-patented nickel-base alloy was created to satisfy the demands of advanced and uprated GE gas turbines . This alloy is vacuum investment cast and has exhibited good producibility . Rainbow nozzle segments were fabricated from GTD-222 and have shown excellent performance after more than 40,000 hours of service. This nozzle alloy is now being used in the 6FA, 7FA, 9FA 9E , 9EC and 6B machines.

30. N-155 Nozzles N-155, also referred to as Multimet , is an ironbased alloy chemically similar to the S-590 used in early bucket applications. It is more readily available , possesses better weldability than S- 590 and is used in the latter-stage nozzles of the MS3000 and MS5000 series of turbines .

31. Future Nozzle Materials and Coatings FSX-414 nozzle material has been extremely successful since the 1960s. However, because of the continuous increase in turbine operating temperatures , developmental programs have been initiated to bring advanced nozzle alloys into commercial production. The first of these programs resulted in the introduction of GTD- 222 for latter-stage nozzles. In the stage 2 nozzle application , GTD-222 is coated with an aluminide coating to provide added oxidation resistance to this component. Another program is directed toward the evaluation and modification of currently used Aircraft Engine alloys with improved high-temperature strength and high temperature oxidation resistance .

32. Combustion Hardware The combustion system is a multiple-chamber assembly composed of three basic parts: the fuel injection system, the cylindrical combustion liner and the transition piece. Driven by the ever-increasing firing temperatures of the gas turbines and the need for improved emissions control , significant development efforts are being made to advance the combustion hardware of heavy-duty gas turbines. The primary basis for the material changes that have occurred has been increased high temperature creep rupture strength. These material changes had to be done while maintaining satisfactory oxidation/corrosion resistance. An indication of the strength improvement is shown in Figure , which compares the creep rupture strength of the three material classes now in use . Nimonic 263, the most recently introduced alloy , is some 250°F/140°C stronger than the original AISI 309 stainless steel. Hastelloy -X , which was used in the 1960s through the early 1980s , is intermediate in strength between the two

33. TURBINE WHEEL ALLOYS This nickel-based, precipitation-hardened alloy is the newest to be used in turbine wheel application . It is the 7FA, 9FA, 6FA and 9EC turbine wheel and spacer alloy, and it offers a very significant increase in stress rupture and tensile yield strength compared to the other wheel alloys . ( See Figures. ) This alloy is similar to Alloy 718, an alloy that has been used for wheels in aircraft turbines for more than 20 years . Alloy 706 contains somewhat lower concentrations of alloying elements than Alloy 718 , and is there-fore possible to produce in the very large ingot sizes needed for the large 7FA and 9FA wheel and spacer forgings. ( See Figure 27. ) Alloy 706 Nickel-Base Alloy

34. Other Rotor Components All of the other rotor parts are individually forged . This includes compressor wheels , spacers , distance pieces and stub shafts. All are made from quenched and tempered low-alloy steels (Cr–Mo–V or Ni–Cr–Mo–V) with the material and heat treatment optimized for the specific part. The intent is to achieve the best balance of strength, toughness/ductility, processing and non-destructive evaluation capability , particularly when it is recognized that some of these parts may be exposed to operating temperatures as low as -60°F/-51°C . All parts are sonic and magnetic particle tested . Many last-stage compressor wheels are spun in a manner analogous to turbine wheels as a means of proof testing and imparting bore residual stresses. This last-stage compressor steel is probably the next most critical rotor component after the turbine wheels.

35. Casings For all models except the F-technology machines , the entire "tube" surrounding the gas turbine rotor is composed of a series of cast iron castings bolted together end-to-end . The castings (inlet and compressor) at the forward end of the machines are made of gray iron , while those at the aft end (discharge and turbine shell) are generally made of ductile iron or, in some, steel castings or fabrications . The excellent castability and machinability offered by cast iron makes it the obvious choice for these somewhat complex parts that have close tolerances. Cast iron is less prone to hot tears and shrinkage problems than cast steel . Experience has also shown it to provide a higher degree of dimensional stability during shop processing . Although stress is important in determining which of the two types of cast iron ( gray or ductile ) is used in the castings, operating temperature is of prime importance.

36. Gray iron is generally limited to applications where temperatures do not exceed 450°F/239°C, ductile iron to applications no greater than 650°F/343°C. In the case of gray iron, GE uses a type that has a minimum tensile strength of 30 ksi (2.1 kg/cm2 x 10-3), similar to ASTMA48, Class 30. Ductile iron, on the other hand, is a ferritic type [60 ksi (4.2 kg/cm2 x 10–3) TS, 40 ksi (2.8 kg/cm2 x 10–3) YS, 18% E1], similar to ASTM-A395. The 7FA and 9F machines utilize ductile iron for the inlet and compressor casing and a fabricated CrMo steel combustion wrapper and turbine shell. More recently, cast 2 1/4 Cr - 1Mo steel is being introduced into the F-technology machines for the combustion wrapper and turbine shells.

37. Future Materials Advances in ductile iron have been made in laboratory trial castings that will enable this material to be extended to higher temperature applications . These trial heats have shown the capability to extend the useful temperature of this material by 100°F/56°C. This development program is now in the Rainbow field trial phase and will most likely find application in advanced and uprated GE gas turbines . Additional Sand Castings . In addition to the casings, several other large omponents , such as bearing housings, inner barrels , upport rings and diaphragms in the stator section of the urbine , are produced from sand castings. Cast iron is again used where possible; however, where higher temperature or planned welding is encountered, steel is employed. For example, Cr-Mo- V has been used for support rings where temperatures reach 1000°F/538°C, and carbon steel has been used for bearing housings requiring weld fabrication .