Water erosion is one of the main causes of damage to the last-stage blades of steam turbines. In order to ensure the safe and stable operation of steam turbines, the factors affecting water erosion of the last-stage blades are summarized, including the microscopic secondary water droplet parameters and the changes in the macroscopic working capacity; the water erosion protection methods are compared, and the advantages of laser cladding technology in water erosion protection of the last-stage blades of steam turbines are pointed out. The application status of laser cladding technology in the field of water erosion protection is explained from the two aspects of coating material preparation and process parameters. It is pointed out that the research and preparation of water erosion-proof coatings have great development potential, and it is proposed that the bonding strength between the coating and the blade substrate and the maintenance of the coating of the cladding workpiece in the later stage are the issues that need to be focused on at present.
Water erosion often occurs in the last-stage blades of the low-pressure stage of steam turbines. This is because compared with the high- and medium-pressure parts of the steam turbine, the steam humidity in the low-pressure part is higher, the salt content in the steam is more, and the erosion effect on the blades is more significant. The water erosion of the last stage blade of the steam turbine mainly comes from the impact of large diameter secondary water droplets on the blade surface during the operation of the blade. Fatigue cracks will be generated under the continuous impact of large water droplets. In severe cases, honeycomb and serrated water erosion will occur [1], causing different degrees of damage to the blade, as shown in Figures 1 and 2.
Laser cladding technology is an advanced material strengthening technology. It uses a laser beam to release high-density energy to melt the substrate surface and self-fluxing alloy powder and then solidify it. According to the different performance advantages of the coating material, the wear resistance and corrosion resistance of the substrate are improved. Its application field has gradually expanded to the power industry in recent years. The parts treated by laser cladding technology have better bonding strength between the coating and the substrate. Compared with the surface brazed Stellite alloy sheet, the coating treated by laser cladding is denser, has higher bonding quality and is not easy to fall off. In addition, the laser cladding coating is not limited by the shape of the part and is suitable for substrates of any size and shape. Due to the different engineering requirements of the required protective parts, the requirements for coating performance are also different, and the laser cladding parameters used are also different.
This paper summarizes the factors affecting the erosion mechanism of the last-stage blades of steam turbines, reviews the development of erosion protection, and explains the current application status of laser cladding technology in the field of erosion protection from the aspects of laser parameters and coating material preparation. It puts forward the problems that need to be solved urgently and provides a reference direction for future erosion protection research.
1 Erosion mechanism
The last-stage blades of the low-pressure part of the steam turbine are in the wet steam zone, with a humidity ratio of 12% to 14%. Steam with high humidity will produce condensed water, thereby reducing the efficiency of the steam turbine unit. It can be seen from the literature [1] that for every 1% increase in the humidity of wet steam, the power generation efficiency will decrease by 1%. The complete mechanism of erosion phenomenon of the last-stage blades is shown in Figure 3.
There are many forms of water in the flow channel of the low-pressure part of the steam turbine: primary water droplets, secondary water droplets and accumulated water film. Gao Zhao used the research results of the erosion criterion to analyze the characteristics of wet steam two-phase flow and summarized the thermal and dynamic imbalances in the wet steam two-phase flow problem [1]. There is a thermodynamic imbalance in the wet steam two-phase flow problem of the last stage blade of the steam turbine. When the wet steam passes through the turbine flow channel, it will exceed the saturation line and reach the Wilson line, and water droplets will begin to be generated [2]. The thermodynamic imbalance process during the condensation of wet steam is shown in Figure 4.
As shown in Figure 4, the water droplets generated at this time are primary water droplets. The diameter of the primary water droplets is about 0.01 μm to 0.1 μm, and the volume is small. The number of primary water droplets accounts for about 90% of the liquid phase in the wet steam two-phase flow state. The influence of dynamic factors is very small and can be ignored. Therefore, the research on primary water droplets is of little significance to water erosion. When the primary water droplets pass through the stator flow channel, due to the difference in the mass of the water droplets, the positions attached to the surface of the stator are also different, and finally accumulate into a layer of liquid film on the surface of the stator, becoming an accumulated water film. Due to the speed difference between the liquid film and the flow channel steam, after the liquid film accumulates to a certain thickness, it will be torn apart by the high-speed steam flow, producing large water droplets of 20 μm to 200 μm [1], which are secondary water droplets. Although secondary water droplets only account for about 10% of the entire flow channel fluid liquid phase, due to their large mass, the inertia they generate is also large. Secondary water droplets will hit the leading edge of the moving blade, generating a large pressure in the contact part between the water droplets and the blade and inside. After the accumulation of time and times, cracks will eventually be generated after breaking through the material limit.
In the early stage of water erosion development, due to the repeated impact of secondary water droplets on the front end of the steam inlet side of the moving blade, fatigue cracks will occur on the surface of the blade material, and will continue to expand with the increase of erosion time. At this time, the water erosion speed is relatively fast. Under the combined action of the normal stress and shear stress of the water droplet impact, the blade surface material will fall off and pits will be generated on the surface of the blade material. The pits will accumulate some water during the operation of the blade, thereby reducing the erosion of the blade by water droplets and slowing down the water erosion speed. Figure 5 is a macroscopic morphology of fatigue cracks caused by water erosion on a blade that has been running for a long time. In the figure, the cracks are linearly distributed in the water erosion area. The initiation of cracks poses a huge challenge to the operating efficiency and safety of the steam turbine.
The study of secondary water droplets is the key to water erosion research. Yang Jiandao [2] took secondary water droplets as the research object and conducted CFD simulation on the impact of secondary water droplets on moving blades under different working conditions. The results showed that the top of the last-stage blade (1/11 of the blade height) was most severely impacted by water droplets, and the water erosion was more serious under small volume flow (50% THA). The fatigue curve of 2Cr13 as the material of the last-stage blade of the steam turbine in a corrosive environment is shown in Figure 6 [2].
It can be seen from Figure 6 that when the last-stage blade is in the transition zone, that is, in the intermittent 3% NaCl solution environment at room temperature, the vibration resistance of the blade material is the lowest, and the blade is most susceptible to damage. Liu Yunfeng et al. [3] studied the water erosion under variable working conditions from the perspective of relative volume flow. When the relative volume flow is small, the secondary water droplets will impact the root of the moving blade and cause more serious water erosion.
Since renewable energy generation is greatly affected by uncontrollable factors (water flow, wind power), the steam turbines of thermal power units need to actively carry out deep peak regulation. Deep peak regulation meets the technical requirements for both nuclear power and thermal power, but nuclear power accounts for a small proportion of the total power generation and has higher requirements for operational safety, so it is generally not involved in peak regulation. However, when nuclear power is performing peak regulation, small volume flow will also lead to increased water erosion of the last-stage blades.
Compared with nuclear power, deep peak regulation has a greater impact on steam turbines of thermal power units. When the steam turbines of thermal power units are changing their peak regulation mode, they face huge heat consumption and economic losses generated in the process from single cycle to combined cycle [4]. In response to the serious water erosion problem under deep peak regulation, Hao Zhenzhen et al. [5] proposed a series of solutions such as adding monitoring points, spraying key areas of blades, and interfering flow fields to improve the economic safety of low-load operation. Wu Xin et al. [6] used finite element simulation technology to establish a single-channel five-stage Ye Shan model, changed the operating conditions, and simulated from the acceptance (THA) condition to 1.5%THA. The results showed that the blast heating state appeared at 14%THA, and high temperature appeared in some areas at 3%THA. Deep peak regulation will change the operating conditions of the steam turbine, and the operating environment of the last-stage blade part of the low-pressure cylinder will also change, which will bring great safety risks and blade erosion risks to the operation of the last-stage blade.
The minimum load of a thermal power plant is directly determined by the peak regulation capacity of the thermal power unit. Water erosion will be more serious when operating at a low load. When the thermal power unit is deeply peaked, the last-stage blade part of the steam turbine will change from the wet steam zone to the transition zone, and the uncertainty and danger of operation will be greatly increased, and the damage to the blade will also increase. Compared with the wet steam zone, the flow rate of the fluid in the transition zone drops sharply, the flow channel cannot be filled with steam, and even backflow will occur [7]. Therefore, when the thermal power unit is deeply peaking, higher requirements are placed on the water erosion resistance of the last-stage blades of the turbine.
In summary, the water erosion of the last-stage blades of the turbine is mainly caused by the secondary water droplets generated by the tearing of the water film on the surface of the stationary blades. When the relative volume flow rate is reduced during deep peaking, the water erosion caused by the secondary water droplets on the moving blades will be more serious, and the safety of the blades will be lower. Without changing the peaking requirements, strengthening the protection of the last-stage blades and increasing the relative volume flow rate in the flow channel are both effective methods to delay water erosion.
2 Water erosion protection methods and comparison
Based on the water erosion mechanism and water erosion characteristics, a water erosion protection method is formulated. The operating conditions of the low-pressure part of the turbine, the relative volume flow rate during operation, and the surface material properties of the last-stage blades of the turbine are all factors that affect the speed of water erosion of the last-stage blades. The corresponding water erosion protection methods can also be divided into active methods and passive methods [8].
The positive method of water erosion protection starts from the quality of wet steam flowing through the last stage blades. Common treatment methods include grooving the cylinder wall of the last stage stationary blade to remove moisture, selecting a reasonable relative volume flow rate operation mode, shortening the start-up and shutdown time of the unit, and reducing the number of start-up and shutdown times of the unit. The negative method is to use surface modification technology to enhance the water erosion resistance of the blade material and optimize the size and shape of the moving blades. Common protection measures include electric spark strengthening technology, quenching, electron beam strengthening, embedding Stellite alloy sheets and laser cladding. Han An et al. [9] selected the working conditions of the last stage blade of the low-pressure cylinder of the turbine under small volume flow for research. The back pressure surface of the last stage moving blade was designed to be in the shape of the raised head of the humpback whale. The results showed that the bionic design improved the torque of the last stage moving blade. Reference [10] measured the mass loss of blades under different treatment methods within a certain period of time. The results showed that when the impact angle was 90°, the water erosion resistance was from strong to weak in the order of brazing tungsten-cobalt alloy, laser cladding, laser solid solution strengthening and matrix, as shown in Figure 7. Although tungsten-cobalt alloy has good hardness and erosion resistance, it is very expensive. In addition, the method of brazing alloy sheets is greatly restricted by the shape of the blade and is easy to fall off.
In recent years, new technologies for passive erosion protection have also emerged. Reference [11] tested the erosion resistance of the aluminized diffusion coating under water drop impact by changing the thickness of the coating. The results showed that the erosion resistance of the large-particle alumina diffusion coating was not as good as that of the metal composite coating containing small-particle alumina, and the thickness of the diffusion layer did not affect the erosion resistance. Reference [12] combined the shot peening method with laser shock peening (LSP) to explore the erosion resistance of titanium alloy materials. The results showed that different sample shapes had different effects on the erosion resistance. For airfoil samples, the erosion resistance was also different at different impact velocities.
Among the methods of water erosion protection for the last stage of steam turbines widely used in thermal power units, brazing of Stellite alloy sheets is an earlier method, but this method has been gradually replaced due to the shape limitations and bonding strength problems of the alloy sheets during the brazing process; electric spark strengthening technology and quenching technology can only be used for blade design, not for blade repair and restoration; laser cladding technology has become one of the commonly used technologies for water erosion protection due to its unique advantages and feasibility. The materials treated by laser cladding technology have the following advantages [13]:
1) The heat affected zone is small, the solidification speed after cladding is fast, and the cladding layer formed has a dense structure [14];
2) The energy utilization rate of the laser beam is high, and the thermal impact on the substrate is small;
3) The coating preparation methods are diverse, and cladding coatings with different properties can be prepared according to different needs [15].
3 Laser cladding technology
Laser cladding technology is to use laser to quickly melt, combine and solidify the substrate surface and self-fluxing alloy powder when the substrate surface performance is insufficient, and then form a dense, fine-grained cladding layer on the substrate surface [16-17]. The equipment composition of laser cladding technology is shown in Figure 8. At present, most laser cladding equipment has achieved intelligence and automation [18].
According to the different powder feeding methods, laser cladding processes are divided into preset and synchronous types. The main difference between the two is the different discharge time and the different cooperation methods with the laser. The synchronous powder feeding method has a higher degree of automation and is less affected by the environment, while the preset powder feeding method has a higher utilization rate [19]. After determining the cladding parameters and cladding plan, the coaxial powder feeding laser cladding process will automatically clad the substrate under the protection of Ar, as shown in Figure 9 [20].
The coating quality of the laser cladding process is determined by factors such as cladding parameters and alloy powder materials, such as laser power, multi-pass cladding overlap rate, feed speed and preheating temperature of the substrate. Yang Xing et al. [21] used 6 different laser powers to prepare FeCoNiCrMo high entropy alloy cladding layers. The macroscopic morphology of the coating is shown in Figure 10. The powers a, b, c, d, e, and f correspond to 1.0 kW, 1.2 kW, 1.4 kW, 1.6 kW, 1.8 kW, and 2.0 kW, respectively.
As shown in Figure 10, when the laser power increases to 1.8 kW, pits begin to appear on the surface of the cladding coating, and the coating quality begins to decline.
Figure 11 shows the residual stress under different laser powers. When the laser power is greater than 1.6 kW, the residual stress increases significantly, which indicates that there is a corresponding relationship between the residual stress of the coating and the change in surface morphology. If the laser power is too low, it will affect the bonding strength between the coating and the substrate; if the laser power is too high, the coating quality will be reduced. Therefore, in the actual cladding process, the laser power should be selected at a moderate value.
Li Hongyu et al. [22] clad iron-based alloy powder on the surface of cast steel by changing the preheating temperature. The results show that the stress and temperature change trends in the two directions are the same. At 200 °C, the stress value in the X direction is smaller than the stress value in the Y direction. The stress changes in the X and Y directions at different preheating temperatures are shown in Figure 12.
The study of the influence of laser cladding parameters on coating performance is of great significance to the application and development of laser cladding. The applicable working conditions of the coating determine the application prospects of laser cladding technology. However, compared with laser parameters, the material properties of the coating are more important for laser cladding. Among the common cladding materials, self-fluxing alloy powders are mainly made of cobalt, nickel, and iron. Self-fluxing properties are achieved by adding specific elements (boron, silicon, etc.). Among them, iron-based self-fluxing powder has the advantages of low cost and good mechanical properties, and is often used in low-temperature machinery that is prone to wear and deformation [23]; cobalt-based self-fluxing powder has good mechanical properties at high temperatures and is often used in aviation, electric power and other industries, but the price of cobalt-based alloys is relatively high and the cost performance is relatively low; compared with cobalt-based, nickel-based self-fluxing powder is cheaper and has better ductility, and is the most widely used laser cladding coating material.
4 Application of laser cladding technology in anti-water erosion of turbine last-stage blades
In the early 1990s, research on the application of laser cladding to surface strengthening of turbine last-stage blades appeared in China. Zhu Beidi et al. [24] proposed to use laser cladding surface modification technology to replace the anti-water erosion method of brazed Stellite alloy sheets. Using cobalt-based alloy powder, the laser parameters were determined and the metallographic structure of the coating after cladding was observed, and the feasibility of the application of this technology was preliminarily determined.
Different alloy powders used in cladding produce different coating properties. Li Cong et al. [25] clad Zr coating on the SP-700 titanium alloy turbine blade substrate and found that the coating performance was optimal when the powder feeder speed was 4 r/min and the scanning speed was 6 mm/s; Liu Jianming [26] used a homogeneous repair method to clad 17-4PH self-fluxing alloy powder on 17-4PH stainless steel blades, proving that the mechanical properties and material properties of the repair layer were better than those of the substrate. The above studies used different self-fluxing alloy powders to analyze the changes in material properties and mechanical properties of the cladding coating in terms of erosion resistance, hardness, wear resistance, etc. The results all show that the laser cladding process has broad development prospects and application space in the field of erosion of the last-stage blades of steam turbines.
At present, there are many studies on the use of Stellite6 alloy powder to solve the erosion protection problem of the last-stage blades of steam turbines. Zhao Wenyu [27] selected turbine blades with a substrate material of 2Cr12MoV for cladding, and measured the wear resistance, erosion resistance and fatigue limit of the surface of the cladding material; Ren Chao et al. [28] clad Stellite6 alloy on the surface of 17-4PH stainless steel substrate and analyzed the advantages and performance of the cladding layer; Sun Yue [29] studied the changes in residual stress and microstructure properties during blade repair under different magnetic field environments.
In the process of using laser cladding to achieve erosion protection of the last-stage blades, the selection and determination of laser parameters are the key to obtaining a coating with good mechanical properties. According to the research results of literature [30], when the overlap rate reaches 38%, the quality of the cladding layer begins to decline; when the scanning speed reaches 25%, the height is uneven.
5 Conclusion
At present, the application of laser cladding technology to resist water erosion on the last-stage blades of steam turbines is at a critical stage from research to application. More and more coating materials have been proven to have good water erosion resistance, but most of the research is still in the stage of numerical simulation and experimental analysis, lacking practical experience. In addition, the last-stage blades of steam turbines are consumable and easily damaged devices. Although some coatings have good performance, the cost performance is also a key factor hindering their application [31].
In addition to considering laser parameters, the initiation and expansion of cracks after cladding is also an urgent problem to be solved in the field of laser cladding technology. The regular changes in crack initiation mainly depend on the impact resistance of the cladding coating and the bonding strength between the cladding coating and the substrate. It is also related to the properties and mechanical properties of the cladding coating and the substrate materials. These factors are the key research directions in the future to meet the higher water erosion protection requirements in engineering practice.
