The phenomenon of stainless steel corrosion occurred during the use of butterfly valve. Through metallographic structure analysis, dyeing test, heat treatment test and SEM, it is found that the key factor of material corrosion is chromium poor area formed by carbide precipitation along grain boundary, which results in corrosion of stainless steel butterfly valve. The stainless steel butterfly valve made of CF8M is rusted during use. After normal heat treatment, the microstructure of austenitic stainless steel should be austenite at room temperature, and its corrosion resistance is very good. In order to analyze the rust reason of butterfly valve, samples were taken on it for analysis.
Stainless steel butterfly valve material for nickel chromium austenitic stainless steel, this material is generally used in the solid solution state. At room temperature, the microstructure is austenite. Austenitic stainless steel has good corrosion resistance in a wide range of corrosive media, especially in the atmosphere. Cause analysis of stainless steel butterfly valve corrosion: according to the comprehensive test results, it can be determined that the precipitated phase in butterfly valve material structure is not σ phase, so the corrosion phenomenon of butterfly valve is not caused by σ phase; through SEM observation, it is confirmed that the precipitated phase in butterfly valve structure is mainly chromium carbide, which is distributed along the grain boundary.
The results of EDS analysis show that the chromium content of carbides distributed on the grain boundary is obviously higher than that of the matrix. This carbide is M23C6 type. When the chromium carbide is diffused along the austenite grain boundary, it is easy to form chromium carbide along the austenite grain boundary.
The carbide precipitated along the grain boundary is the main cause of butterfly valve corrosion; after solid solution treatment, most carbides are dissolved during high temperature heating, and a large amount of carbon and chromium are saturated in austenite, and then fixed by rapid cooling, so that the material has good corrosion resistance. Therefore, the heat treatment process should be strictly controlled. During solution treatment, the workpiece should be heated to high temperature to fully dissolve carbides, and then rapidly cooled to obtain uniform austenite structure. After solution treatment, if slow cooling is adopted, chromium carbide will precipitate along the grain boundary during the cooling process, resulting in the decrease of corrosion resistance.