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Stainless steel butterfly valve rust phenomenon occurs during use. After metallographic analysis, dyeing test face, heat treatment test face, SEM and other test analysis, we found that the key factor of material corrosion is carbide precipitation along the grain boundary in the material to form a chromium-depleted zone, resulting in stainless steel butterfly valve corrosion. Keywords: butterfly valve; corrosion; carbide material CF8M stainless steel butterfly valve rust phenomenon occurs during use. Austenitic stainless steel after normal heat treatment, the organization should be austenitic at room temperature, corrosion resistance is very good. In order to analyze the cause of corrosion of the butterfly valve, samples were taken for analysis. A test method Sampling for chemical composition analysis (to determine compliance with the standards), metallographic examination, heat treatment process test and SEM analysis. 2 test results and analysis 2.1 Chemical composition Chemical composition analysis results and standard components. 2.2 Metallographic analysis From the rust phenomenon of the butterfly valve cut the metallographic sample, after grinding polishing, corrosion with aqueous ferric chloride, Neophot-32 metallographic microscope was observed and analyzed, the microstructure of austenitic Body with another precipitate composition. In theory, the normal austenitic stainless steel after heat treatment should be uniform austenite. Another precipitate appears in the organization what is tissue, there are two judgments: one is σ phase, the other is carbide. The conditions for the formation of sigma phase and carbides are different, but all have one common feature that is the sensitivity of austenitic stainless steel to intergranular corrosion. First, the use of variegated σ phase identification. Using alkaline solution of red blood salt (red blood salt 10g + potassium hydroxide 10g + water 100ml), the sample boiled in the reagent 2 ~ 4min, the ferrite yellow, carbides are corrosive, austenitic was bright color, σ phase from brown to black. Using the above method, the sample cut from the butterfly valve was boiled in alkaline aqueous solution of erythritol for 4 minutes, and observed under the microscope. The precipitates retained the original morphology and no obvious change was found. So decided to use heat treatment method to further test face analysis.
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