Stainless steel butterfly valve rusts during use. Through metallographic structure analysis, dyeing test face, heat treatment test face, SEM and other test analysis, it is found that the key factor of material corrosion is the formation of chromium poor zone due to carbide precipitation along the grain boundary in the material, which results in stainless steel butterfly valve corrosion. The stainless steel butterfly valve made of CF8M was rusted during use. After normal heat treatment, the structure of austenitic stainless steel shall be austenite at room temperature, with good corrosion resistance. In order to analyze the corrosion reason of butterfly valve, samples shall be taken on it for analysis.
Stainless steel butterfly valve is made of nickel chromium austenitic stainless steel, which is generally used in the solution state. At room temperature, its structure is austenite. Austenitic stainless steel has good corrosion resistance in a wide range of corrosion media, especially in the atmosphere. Analysis on the cause of stainless steel butterfly valve corrosion: based on the results of the test, it can be determined that the precipitated phase in the material structure of the butterfly valve is not σ Therefore, the corrosion of butterfly valve is not caused by σ Phase induced; Through SEM observation, it is confirmed that the precipitated phase in the butterfly valve structure is mainly chromium carbide, and this eutectic structure is distributed along the grain boundary.
EDS analysis results show that the chromium content of the carbide distributed on the grain boundary is significantly higher than that of the matrix. This carbide is M23C6 type. With the precipitation of carbides and the lack of diffusion supplement of chromium, chromium carbide precipitates along the austenite grain boundary in the form of chromium carbide, forming a chromium poor zone around the carbide, thus the austenite stainless steel grain boundary is vulnerable to corrosion.
Carbide precipitated along the grain boundary is the main reason for corrosion of butterfly valve; The austenitic stainless steel after solution treatment has a large amount of carbon and chromium saturated in the austenite due to the dissolution of most carbides during high temperature heating, and is fixed due to the subsequent rapid cooling, so that the material has a very good corrosion resistance. Therefore, the heat treatment process shall be strictly controlled. During solution treatment, the workpiece shall be heated to high temperature annealing to fully dissolve the carbide, 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 cooling, thus reducing the corrosion resistance of the material.
