Heavy-duty gas turbines are key equipment in my country’s energy development strategy. Turbine blades working in high temperature and hot corrosion environments are the core hot end components of heavy-duty gas turbines. Compared with aircraft engine blades, advanced heavy-duty gas turbine turbine blades work for a long time in high temperature and hot corrosion environments, and the blades are larger in size. Therefore, special requirements are put forward for the high-temperature alloy materials used in the blades and their manufacturing processes. The hot corrosion resistant high-temperature alloy materials used for gas turbine blades have experienced a development process from polycrystalline to directional and single crystal. The alloys have several characteristics that are significantly different from aircraft engine blade materials in design, such as high Cr, high Ti, and Al ratio. The research and development of cast high-temperature alloys for gas turbines that take into account high strength, excellent hot corrosion resistance, and long-term organizational and performance stability is therefore more challenging. Directional solidification is one of the most critical technologies in the manufacturing process of large directional crystallization blades for heavy-duty gas turbines. This paper introduces the development of advanced directional solidification technology-high temperature gradient liquid metal cooling directional solidification technology, compares the effects of different directional solidification processes on the typical microstructure and mechanical properties of high-temperature alloys, and briefly introduces the recent progress in the development of large directional crystallization blades in my country.
Gas turbines include various types of aerospace jet engines, power generation and drive gas turbines, ship power gas turbines, and various micro gas turbines. At present, the gas-steam combined cycle consisting of a gas turbine and a steam turbine is the largest commercial power generation method with the highest heat-to-power conversion efficiency in human hands [1]. In recent years, about 36% of the annual increase in power generation capacity in the world has been provided by gas turbine combined cycle units [2], and the power and efficiency of gas turbines are still increasing.
At present, coal-fired power generation still occupies an absolute dominant position in my country’s thermal power generation industry, and gas turbine power generation is mainly used for peak load regulation, accounting for a very small proportion of total power generation. From an environmental protection perspective alone, coal-fired power generation pollution emissions (SO2, NOx) have become an important constraint on the sustainable development of the power industry. Based on the importance of gas turbines in the sustainable development of my country’s clean energy and its broad market prospects, heavy-duty gas turbines have been included in the research content of the clean and efficient development and utilization of coal, liquefaction and multi-generation, which are the priority development themes in the energy field in the National Medium- and Long-Term Science and Technology Development Plan. In the future, my country will vigorously develop the gas turbine industry, and it is expected that by 2020, a gas turbine power plant with an installed capacity of approximately 60,000 MW will be built [2-3].
The core technology of heavy-duty gas turbines is mainly in the hands of several foreign companies. After years of development, companies such as GE in the United States, Siemens in Germany, Alstom in France and MHI in Japan have formed a series of gas turbine products. Although China has introduced some manufacturing technologies of E-class and F-class gas turbines in recent years and has made great progress in the localization of heavy-duty gas turbines, several core technologies such as design technology, control technology, combustion technology and hot end component manufacturing technology of heavy-duty gas turbines have always been in the hands of the above-mentioned foreign companies. Obviously, in the face of my country’s huge market demand for gas turbines and the important position of gas turbines in my country’s future energy structure, the research and development and industrialization of core technologies for heavy-duty gas turbines are of great strategic significance to my country.
Characteristics of Heavy Gas Turbine Blades
The fuels used by heavy-duty gas turbines are mainly natural gas and fuel oil. In recent years, the integrated gasification combined cycle (IGCC) technology has also used coal gasification synthesis gas as fuel. The working environment and characteristics of heavy-duty gas turbines are completely different from those of aircraft engines, which also puts forward different requirements for the materials used in the hot end turbine blades of heavy-duty gas turbines and their manufacturing technology. Table 1 briefly compares the typical characteristics of the two blades [4]. Figure 1 shows two types of directional crystallization blades – GE’s F-class gas turbine high-pressure first-stage turbine blades and typical aircraft engine turbine blades [5]. First, the size and weight of heavy-duty gas turbine blades are much larger than aircraft engine blades. Compared with the typical aircraft engine blades with a length of 30 to 150 mm and a weight of 100 to 200 g, the length of heavy-duty gas turbine directional crystallization blades can reach 910 mm and the weight can reach 18 kg [6]. Second, the operating time and state of heavy-duty gas turbine blades are different from those of aircraft engine blades. Heavy-duty gas turbine blades mainly work in a steady state, with a blade overhaul cycle of 24,000 to 40,000 EOH (equivalent operating hours) and a total life of 60,000 to 80,000 EOH [1, 7]. The peak operating time of aircraft engine blades is short and the temperature is higher than that of gas turbine blades, but their cruising operating temperature is lower than the stable operating temperature of gas turbine blades; thirdly, compared with the clean fuel used in aircraft engines, heavy-duty gas turbines use a variety of fuels, which generally contain elements such as V and S that can cause thermal corrosion damage to high-temperature alloy materials. Therefore, heavy-duty gas turbine blades must use high-temperature alloy materials that are resistant to thermal corrosion while ensuring high strength. The high-temperature alloy materials used in aircraft engines usually have excellent oxidation resistance (Figure 2a), but they will be severely damaged in a short time in a thermal corrosion environment (Figure 2b). Similar to aircraft engine blades, turbine blades used in advanced gas turbines also have complex cooling structures [7-8], using directional (DS) or single crystal (SC) high-temperature alloys and thermal barrier coatings (TBC) on the surface. The usage of typical heavy-duty gas turbine blades and coatings is shown in Table 2[9].
Figure 3 briefly summarizes the development history of GE’s gas turbine hot end blade materials, cooling methods, and operating temperatures [10]. It can be seen that before the 1970s, the increase in gas turbine combustion temperature was entirely dependent on the temperature bearing capacity of the high-temperature alloy blade material itself. The subsequent development of cooling technology has led to a continuous increase in the combustion temperature of gas turbines, and the operating temperature of the material has also increased to above 850°C. This not only requires the material to have excellent high-temperature mechanical properties, but also puts forward more stringent requirements on the material’s thermal corrosion resistance. Since the 1980s, gas turbines have begun to use directional crystallization blades with complex cooling structures, which has significantly improved the cooling efficiency and significantly increased the combustion temperature of the gas turbine.
High temperature alloy materials used in heavy-duty gas turbine blades
Similar to the development history of high-temperature alloy materials used in aircraft engine turbine blades, the heat-corrosion-resistant high-temperature alloy materials used in foreign gas turbine blades have also experienced a development from traditional equiaxed grain (CC) casting alloys to oriented columnar grains and single crystal alloys (Figure 4[11]).
The high-temperature alloys for gas turbine blades must take into account the material’s thermal corrosion resistance, high-temperature strength, organizational stability, and casting process performance. Table 3 lists the main components of typical foreign high-temperature alloys that resist thermal corrosion. It can be seen that the composition of high-temperature alloys for gas turbines has the following characteristics:
- ①The Cr content in the alloy is generally greater than 12% (mass fraction, the same below). A higher Cr content can ensure that a basically continuous Cr2O3 protective film can be formed on the surface of the alloy in a thermal corrosion environment. However, Cr is a formation element of the harmful phase-TCP phase in high-temperature alloys. As a solid solution strengthening element, its strengthening effect in nickel-based high-temperature alloys is not as good as W, Mo, Ta, etc. Therefore, the Cr content in high-temperature alloys that resist thermal corrosion must be controlled at a reasonable level to avoid affecting the alloy’s organizational stability and mechanical properties;
- ②The alloy develops from polycrystalline to directional and single crystal, and the content of Ta in the alloy gradually increases. With the increase in the operating temperature of gas turbine blades, the requirements for the high-temperature strength of high-temperature alloys that resist thermal corrosion continue to increase. As an important strengthening element, the content of Ta in high-temperature alloys that resist thermal corrosion also gradually increases. In addition to effectively improving the high-temperature strength of the alloy, due to the segregation of Ta between dendrites, it can also reduce the tendency of defects such as freckles in castings by adjusting the density of the interdendritic liquid in the mushy zone during directional solidification [16-17]. It is generally believed that Ta also has a certain improvement on the oxidation resistance of high-temperature alloys [18-21]. Recent studies have also found that Ta has limited contribution to improving the oxidation resistance of alloys, but can significantly improve the thermal corrosion resistance of alloys [22-23];
- ③The Mo content in the alloy is low. Mo is an effective solid solution strengthening element in high-temperature alloys, but because it easily causes acidic melting reaction in a hot corrosion environment, resulting in severe hot corrosion [12, 24], the Mo content is generally low in hot corrosion resistant high-temperature alloys for gas turbines;
- ④The total amount of Ti and Al in the alloy is basically maintained at 7% to 8%, but the strengthening effect of Ti in nickel-based high-temperature alloys is not as good as W, Mo, Ta, etc. Therefore, the Cr content in hot corrosion resistant high-temperature alloys must be controlled at a reasonable amount, generally higher than the Al content. Ti and Al are the main γ’ phase forming elements. The total amount of the two elements is maintained at 7% to 8% to fully guarantee the volume fraction of the γ’ phase in the alloy, thereby ensuring the precipitation strengthening effect of the alloy. Since Ti may react with S to form stable solid sulfides, it delays the formation of metal-metal sulfide liquid eutectic, thereby delaying the progress of hot corrosion reaction [13] and improving the hot corrosion resistance of the alloy. Therefore, the Ti content in hot corrosion resistant alloys is generally high. However, the increase in Ti content in the alloy will significantly increase the hot cracking tendency of the directional alloy [25-26], so the Ti/Al ratio in the alloy must be reasonably controlled;
- ⑤Hot corrosion resistant high temperature alloys generally do not contain precious metal elements such as Re and Ru. Re and Ru are important strengthening elements in advanced directional and single crystal high temperature alloys for aero engines, which can significantly improve the high temperature strength of the alloy. With the continuous improvement of the temperature bearing capacity of single crystal high temperature alloys, the content of Re and Ru elements in the alloy is also increasing. The content of the two elements in the fourth generation single crystal alloy has reached about 6% and 3%, respectively. However, the two elements are expensive and scarce. Considering the manufacturing cost of gas turbines, the blade materials used in large-scale commercial heavy-duty gas turbines and the high-strength, heat-resistant, and corrosion-resistant single-crystal high-temperature alloys being developed abroad do not contain Re and Ru. Among the G/H-class gas turbines being marketed, only GE in the United States uses the second-generation single-crystal high-temperature alloy containing 3% Re;
- ⑥Single-crystal alloys generally contain trace amounts of C, B, Hf and other elements. Due to the large size of the blades of heavy-duty gas turbines, defects such as small-angle grain boundaries are more likely to occur during directional solidification. Therefore, in recent years, the traditional grain boundary strengthening elements C and B have been re-added to heat-resistant corrosion-resistant single-crystal alloys to improve the alloy’s tolerance to defects such as small-angle grain boundaries [21, 27]. For example, when the small-angle grain boundary exceeds 10°, the transverse stress resistance of the PWA1483 single crystal alloy will be significantly reduced. However, in alloys containing trace amounts of C, B, and Hf, even if the grain boundary angle is as high as 25°, the stress resistance of the alloy is still not significantly reduced (the stress axis of the transverse stress resistance sample is in the <100> direction) [27].
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