Study On Structure And Property Of The Erosion-resistant Alloys At High Temperature

With the extensive use of Circulating Fluidized Bed Boiler(CFB) in our country, the phenomenon of heavy corrosion and wear of thermocouple-protection tube is more and more obvious and become a urgent problem that need to be solved. In this paper, the characteristic and working condition of CFB were studied, failure mechanism of thermocouple-protection tube used for CFB were analyzed, development and research present state of heat-resistant steel and CFB thermocouple-protection tube material were investigated. The effectiveness of alloying agents in high-temperature abrasion-resistant steel and impact of alloying agents to texture and performance were studied. On the base of these researches, several kinds of high-temperature abrasion-resistant alloy were designed. By medium-frequency induction smelting methods, uniform and compact high temperature abrasion-resistant steels were acquired. In addition, proper heat-treatments were proceeded. Alloy microstructures were assessed using modern analysis and testing technologies, such as scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). At last, High temperature resistance, hardness and high temperature erosion resistance were tested.The experimental results are below. The microstructure of designed materials at as-cast is austenite + a small amount of ferrite + primary carbide. The primary carbide is very coarse. It exists along grain boundary appearing reticular distribution. The microstructure at as-solution is similar to at as-cast, but tree-like texture become obscure. Precipitated phase along grain boundary is break chain. Partial primary carbide dissolve. The microstructure at as-aging is austenite + a small amount of ferrite + primary carbide + secondary carbide. Secondary carbide uniformly distributes and disperses in the grain. Close secondary carbide precipitates from austenite during aging process. The materials are totally oxidation resistant at 850℃and 950℃. By adding rear-earth elements in high temperature erosion resistant alloy, the performances of high temperature resistance, hardness and high temperature abrasion can be improved. At the same time, it can refine crystal grain, so to intensify mechanical property of materials. The erosion tests at elevated temperature show that test materials' erosion behavior is plastic. 9# material has the best erosion resistance in all test materials. 6# aging sample and 9# cast sample have better erosion resistance at high temperature. They can be used at high temperature for long-term. These materials can be used to solve the problem of CFB thermocouple-protection tube premature failure that caused by heavy high temperature erosion. The prototype castings of CFB thermocouple-protection tube with the high temperature erosion resistant alloy acquire good application in NanTun electric power plant Shandong HuaJu energy limited company.