1. Overview Thermal spraying technology is a new technology for strengthening, protecting, and restoring the dimensions of materials. It was a key technology promoted by the Chinese government during the Sixth, Seventh, and Eighth Five-Year Plans. This technology originated in China in the 1950s, and by the late 1970s, China had initially formed a complete system of equipment, materials, and process technologies, which were widely applied across various industries nationwide, making a significant contribution to China's national economic development. After more than 20 years of development, China's thermal spraying technology has made significant progress in equipment, materials, and process technologies. Currently, it can not only produce the world's most advanced thermal spraying equipment, such as explosive spraying, supersonic flame spraying, and high-power plasma spraying, but also supply users with a complete range of advanced thermal spraying materials with performance comparable to foreign products. Decades of experience have honed the technical team, making the application of this technology more mature and reliable. According to incomplete statistics, the promotion of thermal spraying technology in China alone yielded direct economic benefits of 3.5 billion yuan during the Eighth Five-Year Plan period. China's power plant boilers and industrial boilers are mainly coal-fired. Statistics show that in 1998, coal accounted for 75% of my country's electricity production. However, the quality of coal used for power generation is generally poor, with high ash and sulfur content. This makes the pulverized coal production and conveying systems, as well as the boiler's "four tubes" (water-cooled wall tubes, superheater tubes, reheater tubes, and economizer tubes), susceptible to wear and corrosion in coal-fired power plants and industrial boilers, leading to equipment damage and potential accidents. The surfaces of equipment with high-speed fluid flow, such as induced draft fans and cylinders, are also prone to failure due to wear and cavitation, resulting in significant losses. Many moving parts (such as various shafts) are also frequently worn out and rendered unusable. Furthermore, the power transmission and transformation facilities and various steel structures in power plants are often subjected to severe industrial atmospheric corrosion due to their exposed environment. All of these issues can be addressed through thermal spraying technology to protect and strengthen these structures, significantly extending their service life, or to repair them and restore their usability, thereby improving the production safety and economic efficiency of power plants. [b]2 Application of Thermal Spraying Technology in the "Four Tubes" of Power Plant Boilers[/b] The "four tubes" of boilers operate in harsh environments of high temperature, high pressure, and are subject to corrosion from flue gas and abrasive erosion. They are highly susceptible to high-temperature corrosion and wear, leading to thinning of the tube walls. Generally, this thinning rate is about 1 mm/year, and in severe cases, it can reach over 2 mm/year. The direct hazard of thinning the boiler's "four tubes" is leakage and tube rupture accidents. According to surveys, in my country's thermal power plants, the downtime for repairs caused by "four tube" leaks in 100MW and above units accounts for about 40% of the total unplanned downtime of the entire unit and over 70% of the unplanned downtime of the boiler equipment itself. Statistics from 1992 show that boiler accidents accounted for 56% of all power generation accidents that year. For example, the 300MW No. 1 boiler unit of the Hanchuan Power Plant, which began operation in 1990, experienced 11 tube rupture accidents in 1996 alone, resulting in direct economic losses of 20 million yuan. Therefore, high-temperature corrosion and wear of the boiler's four tubes not only seriously affect the safe operation of the boiler but also cause huge economic losses, becoming an urgent problem to be solved in boiler safety operation. Research on boiler four-tube protection technology began abroad in the early 1980s. After several years of research and experimentation, several major industrialized countries found what they considered to be the best protection methods. For example, TAFA in the United States used arc spraying of 45CT coating, Sweden used arc spraying of KANTHALM alloy, the UK Central Power Department used plasma spraying of Ni-Cr alloy powder, METCO in the United States used plasma spraying of METCO465 alloy powder, and Japan used supersonic flame spraying of Cr3C2-NiCr alloy powder, all achieving good results. Domestically, much research and experimentation has also been conducted on boiler four-tube protection technology. For example, the hot-dip aluminizing technology for water-cooled wall tubes developed in the 1970s and the Ni-W alloy spraying technology for water-cooled wall tubes developed in the 1980s have both achieved certain results. Since the 1990s, relatively ideal technical means have been gradually found, such as using electric arc spraying to coat high-chromium NiCr wire. More than 30 thermal power plants in northern my country have adopted thermal spraying technology to protect the "four tubes" of boilers, with a spraying area of ​​several thousand square meters. It is worth mentioning the 45CT alloy of TAFA Company in the United States. Using the electric arc spraying method, this alloy was sprayed on more than 1,200 m2 of the "four tubes" of more than 30 boilers around the world from July 1984 to September 1986. It was measured that the average abrasion loss of the coating was less than 0.025 mm/year. Therefore, this material has been recognized internationally and adopted by many countries. [b]3 Application of thermal spraying technology in exhaust fans and pulverized coal fans[/b] The discharge of fuel and dust and slag required by coal-fired boilers in thermal power plants is accomplished by various fans. Among the forced draft fans, induced draft fans, pulverized coal exhaust fans and primary air fans, the working environment of pulverized coal exhaust fans and induced draft fans is particularly harsh. The impeller is a key component of the fan, expelling dust and ash during high-speed rotation. The suspended dust and ash move at high speeds relative to the impeller blades, causing erosion and wear. The impeller's working environment also includes large amounts of flue gas and water vapor, which, combined with temperature and other factors, further corrode the impeller. In my country, power plants use coal with high sulfur content, resulting in high sulfur compound content in the flue gas, exacerbating the corrosion of the fan impeller. Consequently, the lifespan of induced draft fans in Chinese power plants is generally only 2000-3000 hours, and some even only a few hundred hours; while the lifespan of exhaust fans is generally 4000 hours. Rapid fan failure not only increases spare parts consumption and causes significant downtime losses, but also leads to frequent and intense vibrations due to dust particles entering the blade and airfoil cavities, causing fan damage accidents and directly impacting boiler safety. Currently, the main processes for strengthening fan impellers include welding, inlaying, oxy-acetylene flame spraying, arc spraying, plasma spraying, and oxy-acetylene flame melting. Welding involves a large heat input to the workpiece, and the problem of impeller deformation is difficult to overcome; the ceramic block embedding method is prone to local delamination, resulting in particularly rapid wear in that area and increased fan vibration; oxy-acetylene flame spraying coating has low bonding strength, and the coating has poor wear resistance due to insufficient density. Therefore, the above three methods are rarely used at present. Arc spraying, plasma spraying, and oxy-acetylene flame spraying are currently better methods for strengthening fan impellers. The bonding strength, density, and wear resistance of the first two are not as good as the latter, but in terms of heat input and the degree of impeller deformation, the first two are superior to the latter. After the impeller of the pulverizer at Hubei Qingshan Thermal Power Plant was treated with oxyacetylene flame spraying of FNi-15 and FNiWC-35, its service life increased from 0.5 years to over 2.5 years. During the overhaul of the Japanese-made induced draft fan at Hong Kong HOPEWELL Power Company's Shajiao B plant after 6 years of operation, oxyacetylene flame spraying of FNi-15B material was also used for reinforcement, with excellent results. Many power plants, including Hancheng Power Plant, Yangshupu Power Plant, and Fengshan Power Plant, have used oxyacetylene flame spraying for pre-protection of induced draft fan impellers, collectors, and regulators, increasing their service life by 3 to 5 times. Zhonghuai Power Company used arc spraying of chromium alloy to protect the impellers of the pulverizer and induced draft fan at Xuzhou Thermal Power Plant and Datun Power Plant, achieving a service life of over 3 years. [b]4 Application of Thermal Spraying Technology in Pulverized Coal Production and Conveying Systems[/b] Coal milling machinery and coal conveying machinery are subjected to direct wear from coal lumps and coal powder, which is a typical form of abrasive wear. For the protection of these easily worn parts, welding and thermal spraying are better methods. For example, welding is currently the most common method for coal mill rollers, but thermal spraying is more commonly used for roller conveyors and conveyor plates because welding can easily cause deformation. For example, the conveyor plates of Qinling Power Plant have increased their service life by 5 to 7 times by using thermal spraying. [b]5 Application of thermal spraying technology on shaft workpieces in power plants[/b] Shaft workpieces in power plants are generally scrapped due to excessive wear at the journal. The bearing oil level of the turbine main shaft and the bearing bush of the generator main shaft are prone to groove wear due to vibration and oil supply problems. Using thermal spraying to repair shafts that exceed the tolerance can not only restore their performance, but also make the service life of the sprayed parts 3 to 5 times longer than that of new parts due to the high wear resistance of the sprayed coating [3], thereby enabling the power plant to obtain considerable safety and economic benefits. The main shaft of the 500t/h coal mill at Jilin Thermal Power Plant, 3.5m long and 300mm in diameter, experienced severe wear between the mill's partition and the main shaft after five years of operation, causing intense vibrations and forcing a shutdown. The plant repaired this shaft, valued at 120,000 yuan, using thermal spraying for less than 10,000 yuan, and achieved three times less annual wear than the original new shaft. At Wuhan Steel & Electricity Co., Ltd.'s thermal power plant, two water pump shafts had unilateral wear depths exceeding 0.5mm at their bearing positions. These shafts, 4m long with two bearing positions (φ200×150mm), would have required significant cost and a long manufacturing cycle for replacement, failing to meet maintenance time requirements. Oxy-acetylene flame wire spraying quickly repaired both shafts, and their performance after installation is excellent. [b]6 Application of Thermal Spraying Technology in Other Equipment and Facilities of Thermal Power Plants[/b] Steam leakage often occurs in the cylinder body and cylinder head of power plants due to deformation. Once steam leakage occurs, the joint will be eroded and damaged more quickly, making the turbine unable to operate normally. Most power plants at home and abroad currently use thermal spraying or electroplating technology to restore the cylinder sealing surface. Among them, thermal spraying is an effective method for restoring the cylinder sealing surface. Many outdoor steel structural components of power plants, such as outdoor pipelines and power transmission and transformation facilities, are exposed to the industrial atmosphere for a long time, exposed to sun and rain. Although traditional paint protection and hot-dip galvanizing have lower initial investment, the protection cycle is short (especially paint protection), and the coating maintenance and renewal are frequent. From the perspective of long-term protection cost, it is not as good as long-lasting thermal spraying zinc and aluminum coatings. The protection cycle of thermal spraying zinc coating can reach 30 years, and the protection period of thermal spraying aluminum coating can reach 50 years. Since the 1920s and 1930s, foreign countries have been using thermal spraying technology for long-term corrosion protection of outdoor steel structural components. For example, a steel structure purification unit at the Granwich natural gas chemical plant in the United States, with a 0.13mm thick aluminum coating applied by thermal spraying, remained intact after 30 years of use. Numerous engineering applications show that the investment ratio of thermally sprayed zinc and aluminum protective coatings to heavy-duty anti-corrosion coatings and conventional anti-corrosion coatings is 3.1:1.9:1.2, but the lifespan ratio is 10:4:2.9. Calculated over a 20-year period, the total investment ratio is 5:7:10. Therefore, the most ideal solution for protecting outdoor boiler structural components is thermally sprayed long-lasting zinc and aluminum coatings. [b]7 Conclusion[/b] Wear and corrosion are the main causes of equipment failure in thermal power plants, and thermal spraying technology is a highly effective new technology for surface repair, protection, and strengthening against wear and corrosion. With the further promotion and application of thermal spraying technology in the thermal power industry, it will undoubtedly bring increasingly greater safety, economic, and social benefits to the thermal power generation industry. References: 1. Ding Zhangxiong et al. Research on corrosion and wear properties of thermal spray coatings for boiler pipes. Proceedings of the 15th National Conference on Thermal Spraying Technology, 2001. 2. Zhang Hengxiang. Application and development trend of surface pre-protection technology for "four tubes" of power plant boilers. Proceedings of the National Conference on Thermal Spraying Technology, 1982.