Material Stainless Steel butterfly valve CF8M corroded 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 and 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. 2.3 Heat Treatment Test Analysis σ phase is a kind of iron-chromium atomic ratio roughly equal intermetallic compounds. Chemical composition, ferrite, cold deformation, temperature changes have varying degrees on the formation of sigma influence. Using staining test, under the microscope to observe the precipitation phase change is not obvious, so the use of heat treatment method to identify σ phase. For information, sigma phase is usually formed in the long-term aging at 500 ~ 800 ℃. This is because aging at higher temperatures favors the diffusion of chromium. Then high temperature heating σ phase will begin to dissolve, dissolved at least over at 920 ℃. It is eliminated by heating at a steady temperature above the sigma phase. The time required to form the sigma phase is long, but the elimination of the sigma phase usually takes only a short time to heat. According to this theory, a heat treatment process was developed to see if the precipitates in the tissue could be eliminated. The specimen cut from the butterfly valve was heated to 940 ° C for 30 minutes and then observed on a Neophot-32 metallurgical microscope. The precipitated phase in the heat-treated sample was not eliminated and the original morphology was maintained, which proves that the precipitated phase in the microstructure may not be σ phase. 2.4SEM analysis sometimes appear in the steel sigma phase, using any staining method are unable to distinguish the color, SEM analysis can be used to identify. Because it is known that the σ phase is a compound of iron and chromium, the chromium content is 42% to 48%. The unknown phase components and their contents are determined by qualitative and quantitative EDS analysis to determine the unknown phase. EDS analysis showed that the chromium content of precipitates was 33.6%, which was significantly higher than 16.3% of the Cr content in the matrix, while the chromium content in the σ phase was 42% ~ 48%, thus denying the precipitate phase as σ phase. Comprehensive staining test face, the result of heat treatment test, that the stainless steel butterfly valve tissue is not precipitated phase σ phase. SEM analysis of precipitated phase is a eutectic structure, based on chromium carbide. Stainless steel butterfly valve material is nickel-chromium austenitic stainless steel, this material is generally used in solution state. At room temperature, its structure is austenitic, austenitic stainless steel in a wide range of corrosive media, especially in the atmosphere has good corrosion resistance. The reason for the corrosion of the stainless steel butterfly valve is as follows: (1) Based on the results of the above tests, it can be judged that the precipitated phase in the butterfly valve material is not σ phase, so the corrosion of the butterfly valve is not caused by σ phase. ② SEM observation confirmed that the precipitated phase of the butterfly valve is chromium-based carbide, which is distributed along the grain boundary. EDS analysis results show that the distribution of chromium in the grain boundary carbide was significantly higher than the matrix. This carbide is M23C6 type. With the precipitation of carbides, but also without the proliferation of chromium added to the form of chromium carbide along the austenite grain boundary precipitation, the formation of chromium-depleted areas around the carbide, austenitic stainless steel grain boundaries easily corroded. Therefore, precipitation along the grain boundary carbide is the main reason for the butterfly valve corrosion. ③ after solution treatment of austenitic stainless steel, due to the high temperature heating most of the carbides are dissolved, austenitic saturated with a large amount of carbon and chromium, and because of the subsequent rapid cooling and fixed, the material is very Business corrosion resistance. Therefore, the heat treatment process should be strictly controlled, the workpiece is heated to high back solution treatment, the carbide fully dissolved, and then quickly cooled to obtain a homogeneous austenitic tissue. After solution treatment, with slow cooling, chromium carbide will precipitate along the grain boundary during cooling, resulting in a decrease of corrosion resistance of the material.
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