Analysis of corrosion occurring during use of Programmable Switch Butterfly Valve

The Programmable Switch Butterfly Valve appears to be corroded during use. After metallographic structure analysis, dyeing test, heat treatment test, SEM and other experimental analysis, it was found that the key factor in material corrosion is that the carbides along the grain boundaries in the material precipitate to form a chromium-depleted area, which causes the non-Programmable Switch Butterfly Valve to rust, The Programmable Switch Butterfly Valve made of CF8M has rusted during use. After normal heat treatment, the structure of austenitic stainless steel should be austenite at room temperature and its corrosion resistance is very good. In order to analyze the cause of corrosion of the butterfly valve, samples were taken for analysis.

The material of the Programmable Switch Butterfly Valve is nickel-chromium austenitic stainless steel, which is generally used in a solid solution state. At room temperature, its structure is austenite. Austenitic stainless steel has good corrosion resistance in a wide range of corrosive media, especially in the atmosphere.
EDS analysis results show that the chromium content of this carbide distributed on the grain boundaries is significantly higher than that of the matrix. This carbide is type M23C6. As carbides precipitate and there is no diffusion of chromium, chromium carbide will precipitate along the austenite grain boundaries, forming a chromium-poor zone around the carbides, making the austenitic stainless steel grain boundaries susceptible to corrosion.
The carbides precipitated along the grain boundaries are the main cause of butterfly valve corrosion; austenitic stainless steel after solid solution treatment is saturated with a large amount of carbon and chromium because most of the carbides are dissolved when heated at high temperatures. And due to the subsequent rapid cooling, it is fixed, giving the material high corrosion resistance. Therefore, the heat treatment process should be strictly controlled. During solution treatment, the workpiece is heated to a high temperature to fully dissolve the carbides, and then cooled rapidly to obtain a uniform Austenitic structure. After solution treatment, if slow cooling is used, chromium carbide will precipitate along the grain boundaries during the cooling process, resulting in a reduction in the corrosion resistance of the material.