Abstract: Gas turbines are the main power devices of efficient energy conversion and clean utilization systems today and even in the future. Their advantages of safety, efficiency, and cleanliness have been valued by many countries. However, gas turbine turbines, like other turbines, are subject to fatigue. and creep problems. This paper summarizes the progress of gas turbine turbine blade materials in recent years, reviews the progress in solving problems such as fatigue creep of blade materials, and provides a preliminary outlook on related issues.
Keywords: gas turbine; turbine blade; fatigue; creep
Since its birth in the 1930s, gas turbines have developed rapidly. The thermal efficiency of the heavy-duty gas turbine combined cycle used for power generation has reached 60%, which is the most efficient large-scale commercial power generation method with the highest thermal cycle efficiency that humans have mastered. At present, most gas turbines use natural gas and fuel oil as fuel. According to statistics, global natural gas and oil power generation account for one-fifth of the total power generation. They are the third largest power generation method in the world after coal power and nuclear power. my country’s gas turbine power generation technology began in the 1950s, but due to the impact of technology and resources, the development speed is relatively slow. Entering the 21st century, my country has gradually attached importance to the research of gas turbine technology. The development of offshore natural gas resources, the “West-East Gas Transmission” project, and the introduction of foreign natural gas resources projects are in full swing… Based on completing the gas turbine design plan, our country has produced the first heavy-duty gas turbine with independent intellectual property rights. During the “Twelfth Five-Year Plan” period, my country conducted research on the three main parts of gas turbines (compressor, combustor, and turbine) as a long-term development plan, and completed several pilot projects on this basis. The “13th Five-Year Plan” will include several projects such as aero engines and gas turbines as major scientific and technological innovation 2030 projects. Compared with developed countries, my country’s gas turbine technology is still relatively backward, one of which is the lagging material and processing technology of hot-end components. Every time the inlet temperature of the gas turbine increases by 100°C, the output power increases by about 20% to 25%, saving 6% to 7% of fuel; however, as the temperature increases, problems such as fatigue creep of the blades arise. Blades are the core components of gas turbines. They are in the most complex working environment of temperature and pressure, and accidents often occur here. To develop advanced gas turbine blades, researchers from various countries have tried a variety of materials and processes, which have greatly improved the performance of gas turbine blades, but it is still difficult to meet engineering needs. Summarizing the progress of gas turbine turbine blade materials and processes and studying the fatigue creep mechanism of gas turbine turbine blades are of great significance for further research on gas turbine turbines.
1. Research progress of turbine blade materials
After decades of development and industrial needs, gas turbine parameters have continued to improve. The inlet temperature of some heavy-duty gas turbine turbines has reached 1700°C. The load on the turbine has greatly increased, resulting in a more harsh operating environment for turbine blades, prompting The rapid development of blade materials. Turbine blades are the main components of gas turbines that operate under harsh operating conditions as high-temperature components and are also components that are prone to accidents. Turbine blades are mainly affected by the following aspects during operation:
(1) The impact force and aerodynamic wear of high-speed gas on the blades.
(2) Strong vibration during high-speed rotation.
(3) Thermal stress caused by the gas turbine during starting, stopping, and changing working conditions.
(4) Oxidative corrosion of blades caused by high-temperature gas.
(5) The centrifugal force generated by the gas turbine during long-term operation.
After decades of development, the turbine inlet temperature has increased from 700°C in the 1940s to 1700°C today. Each step of improvement is inseparable from advanced design concepts, superior manufacturing technology, and the use of effective protective coatings. , not to mention the use of high-performance alloys used for casting high-pressure turbine blades. In the early days of using gas turbines, it was discovered that adding aluminum and titanium to the materials used to make turbine blades could enhance the material’s creep resistance, and adding elements such as tantalum and niobium could enhance the hardness of the material. Since then, the American GE Company used cobalt-based alloys instead of high-temperature forged alloys in the 1940s, creating a history of casting alloys for manufacturing turbine blades. In the 1950s, the invention of vacuum smelting technology not only reduced the impurity content in the alloy but also controlled the composition of each element in the alloy and made devices of different shapes. In the 1960s, the emergence of directional solidification technology further developed alloy materials and promoted the emergence of single-crystal alloys. Its excellent properties significantly increased the inlet temperature of gas turbines. In the 1980s, high-speed solidification technology was used in many aero engines, and this technology was also partially used in the manufacture of gas turbines that were several times larger than their size. The gas turbine market developed rapidly in the 1990s, and major gas turbine manufacturers gradually applied liquid metal directional solidification cooling technology in engineering. In the 21st century, major European gas turbine manufacturers have combined practical engineering applications with numerical simulations to further develop liquid metal directional solidification cooling technology.
Although my country started late in this regard, departments such as the General Institute of Iron and Steel Research and the Institute of Metal Research of the Chinese Academy of Sciences have cooperated with gas turbine manufacturers to produce biaxially guided turbine blades and conduct related work on crystallized blade research. To summarize the development of gas turbine turbine blades, directional solidification technology, and single crystal alloy technology are the main research directions of various countries at present and even in the future. Mastering and conducting further research based on this technology is an effective way for my country to narrow the gap with developed countries in related fields.
2. Research progress on turbine blade fatigue problems
2.1. Fatigue crack initiation of turbine blades
The inclusion of non-metallic substances in turbine materials is an important cause of cracks. Therefore, steel produced by vacuum smelting technology has a higher fatigue limit than steel produced by high-temperature smelting technology. The refinement of metallographic particles can reduce material inhomogeneity, thereby improving the fatigue limit of metal materials. The strength of the material and its surface roughness also significantly affect the time of material crack initiation. The increase in the internal temperature of the gas turbine increases the thermal load on the turbine blades in the thermal cycle. To reduce the operating temperature of the blade, a hollow chamber structure is used in the blade manufacturing process. This blade can be cooled by air and steam. The increase in thermal fatigue promotes the initiation of cracks. Therefore, to increase the blade’s ability to withstand high loads, the blade needs good thermal fatigue strength. High-cycle fatigue caused by vibration is the main cause of turbine blade failure, and fatigue cracks generally first occur in areas of stress concentration. Chen Guoli mentioned that hot isostatic pressing technology can greatly eliminate defects in gas turbine parts, eliminate cavities caused by erosion, oxidation, thermal fatigue, and other reasons, and use the thermoplasticity of the material to close cracks.
2.2. Crack propagation of turbine blades
Peng Liqiang and others believe that normal stress, normal strain, and shear force on the critical surface are the main factors affecting fatigue life, and the issue of average blade stress cannot be ignored. Fatigue problems are divided into uniaxial fatigue and multi-axial fatigue. Uniaxial fatigue is a fatigue phenomenon caused by blades under unidirectional cyclic loading. The blades only bear unidirectional stress. Multi-axial fatigue is a fatigue phenomenon caused by blades being subjected to cyclic loads in multiple directions. The blades bear stress in multiple different directions, and the phenomenon of fatigue crack expansion also occurs. R. Sugiura et al. researched crack propagation and found that crack propagation is controlled by a cycle-dependent mechanism. The time scales of crack formation and propagation are at positions where the stress range is reduced by 10% and 25% respectively, and a life prediction based on linear cumulative damage theory was established. method. Zhang Yajie found in his research that the fatigue crack growth rate is positively correlated with the load level. The greater the load, the greater the crack growth rate; the crack growth rate is negatively correlated with the stress ratio. The greater the stress ratio, the smaller the crack growth rate; the crack growth rate with The frequency is positively correlated. The greater the frequency, the greater the crack expansion. The influence of fatigue factors on crack expansion is more significant. Wang Wenjie’s research shows that changing the microscopic geometry of the material and using shot peening and laser strengthening technology can slow down the crack initiation and expansion rate. Trying to avoid the turbine operating in the resonance area and designing the turbine size reasonably are effective measures to slow down the further expansion of cracks.
2.3. Evaluation method of fatigue life of turbine blades
German researcher Albert first studied the problem of uniaxial fatigue in 1829. After long-term research, a relatively mature theoretical system has been formed in this regard. The general slope method, Manson-Coffin method, and strain range division method are its main research methods.
(1) The universal slope method is an empirical formula established between the elastic and plastic parts and the uniaxial tensile properties.
(2) The Manson-Coffin method is the relationship between the number of reverse loads and the plastic strain caused by plastic strain when damage caused by fatigue occurs.
(3) The strain range division method is a research method for the two change mechanisms of flow and creep of inelastic materials at high temperatures.
Research on multiaxial fatigue has progressed slowly. It was not until the emergence of electro-hydraulic servo fatigue testing machines in the 1950s that the research on multi-axis fatigue made significant progress. In this regard, the main research methods are the linear cumulative damage method and the nonlinear finite element method. The linear cumulative damage method proposed by Japanese professor Taira is to linearly superimpose the fatigue damage and creep damage of the material. Although the accuracy of this method is poor, the simple form has obvious advantages in situations where creep is dominant. In the 1990s, Palmgmn-Miner et al. used the linear cumulative damage method for the two processes of crack formation and crack propagation respectively, and proposed the bilinear fatigue cumulative damage method. Nonlinear finite element method. It is to give the generation law of creep in the computer program, add elastic deformation, plastic deformation, and creep iterate, and use the obtained total strain to judge the life of the blade. This method shows obvious advantages in large components with complex working environments.
3. Research progress on turbine blade creep
3.1. Damage mechanism of turbine blade creep
The operating temperature of gas turbine turbine blades is mainly determined by the creep strength of the material. The creep strength of materials can be enhanced through alloying processes. The content of each element in a metal determines the dissolution rate of the metal and the generation trend of harmful impurities. The creep of turbine blade metal materials refers to the slow and permanent deformation phenomenon that occurs when the turbine blades continue to operate at high temperatures under conditions that do not exceed the yield stress of the metal. Under the action of multi-directional stress and high temperature, gas turbine turbine blades will experience creep. Creep occurs mainly due to the slip of metal grain boundaries at high temperatures (more than two-thirds of the metal’s melting point). At temperatures in the creep range, load holding time and average stress determine the creep life of turbine blades. Deviation in orientation will cause significant changes in the mechanical properties of the blade. Crystals with better orientation have better creep resistance and fatigue resistance. Studying the mechanical properties and microscopic deformation mechanisms in different macroscopic directions is an important problem in accurately analyzing creep characteristics under complex conditions. Yue Zhufeng, Lu Zhenzhou, and others believe that single-crystal alloy creep is caused by material deterioration and cavity damage.
3.2. Prediction method of turbine blade creep life
Directional solidification technology and single crystal alloy technology have become the main processes for making gas turbine turbine blades. Both technologies have been widely used in the production of advanced industrial gas turbines to reduce the occurrence of creep. But even in single-crystal materials where grain boundaries are not obvious, under certain pressure and temperature, the material will creep for a long enough time. The main methods for studying turbine blade creep include the parameter method, θ mapping method, and creep energy equation method.
(1) The idea of the parameter method is to increase the test temperature and predict the creep phenomenon of the material through the obtained data in a short time. Liu Hui, Xu Guoping, and others discovered through research that there is a linear relationship between parameters and stress, and then derived two formulas for predicting creep.
(2) The insinuation method is based on experiments, considering the first and third stages as the main processes of creep, and predicts the remaining life of the material based on the material creep time measured experimentally. The advantage of this method is that after a certain number of tests, the creep characteristics of a certain material under other temperatures and pressures can be predicted, thus reducing the number of experiments.
(3) The creep energy equation method uses the least squares method and combines experimental data points to fit the data. It can be used to more accurately predict the long-term creep life of the determined parameters. It can predict a wide range of temperature and pressure and is suitable for Different grades of creep-resistant ferritic steel.
4 Summary and outlook
(1) The turbine blades of gas turbines work under high temperature, high pressure, and other effects. Although creep research and fatigue research can predict the characteristics of materials in practice, research under multiple couplings needs to be improved.
(2) Explore the effects of different element contents on the mechanical properties of turbine blades, study the failure causes of ductile dimple fatigue, slow down the occurrence of large dimples, dimple connections, and cracks, and accurately predict the life of the material. Prediction is a future research direction for studying turbine blades at the microscopic scale.
(3) The research on gas turbine turbine blades cannot be completed through long-term experiments. Therefore, the finite element method has been widely used in practice. The research on many materials under the action of multiple couplings can also be done by establishing models. desired result. It is worth noting that in many finite element method models, the establishment of boundary conditions is sometimes not accurate enough, causing the results of numerical simulations to be inconsistent with reality. Numerical simulation should be combined with necessary tests, which will be the development direction of studying the fatigue creep problem of gas turbine turbine blades.
