Stainless steel butterfly pneumatic valve corrosion cause analysis and solution < / div >

abstract: stainless steel butterfly corrosion phenomena appeared in the process of pneumatic valve in use. After dyeing try face, heat treatment, microstructure analysis, face, such as SEM test analysis, found the key factors for material corrosion because of materials along the grain boundary carbide precipitation form poor chrome area, causing no stainless steel butterfly pneumatic valve rust.
material for stainless steel butterfly CF8M corrosion phenomena appeared in the process of pneumatic valve in use. Austenitic stainless steel after normal heat treatment, the organization shall be the austenite at room temperature, corrosion resistant performance is very good. In order to analyze the corrosion cause of butterfly pneumatic valve, on which the sample is analyzed.

1 test method sampling for chemical analysis ( Determine whether accord with a standard to ask) Heat treatment process, microstructure examination and test and SEM analysis.

2 experimental results and analysis 2.
1 chemical composition of chemical composition analysis and standard components.
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2 metallographic analysis from a corrosion phenomenon, in the pneumatic butterfly valve for the metallographic sample, after grinding and polishing, corrosion with ferric chloride aqueous solution, the Neophot - 32 show badge mirror on metallographic observation analysis, the microstructure of austenite and another kind of precipitates. In theory after normal heat treatment of austenitic stainless steel, austenitic organization should be uniform. In the organization of the other exudates what organization, there are two kinds of judgment: one is the sigma phase, the other is a carbide. Sigma facies of the formation of carbides, but has a common characteristic, that is caused by austenitic stainless steel on the sensitivity of intergranular corrosion.
first adopted the sigma phase identification of noise method. Using alkaline red blood brine solution ( Salt 10 g + red blood potassium hydroxide 10 g + water 100 ml) , sample after boil 2 ~ 4 min in the reagent, ferrite yellow, carbide corrosion, austenite in bright light, sigma phase changed from brown to black. With the above method will be cut out from the pneumatic valve butterfly specimens in alkaline red blood boiling in saline solution after 4 min, the emblem the microscopic observation, separation material to keep the original topography, found no apparent change. Therefore decided to adopt the method of heat treatment face test analysis further. 2. 3 test analysis
sigma phase heat treatment is a kind of chromium iron atoms roughly equal proportion of intermetallic compounds. Chemical composition, ferrite to varying degrees, cold deformation, temperature influence on the sigma phase formation. Staining test, under the microscope precipitated phase change is not obvious, therefore, the heat treatment method was adopted to identify the sigma phase. Relevant information, sigma phase is usually formed in 500 ~ 800 ℃ aging for a long time. This is because the high temperature aging is conducive to the spread of chromium. High temperature heating sigma phase will begin to dissolve, dissolve to complete at least above 920 ℃. Above the sigma phase stable temperature heating can eliminate. Although time needed for formation of sigma phase is very long, but heating can eliminate sigma in general as long as a short time. According to this theory, make the heat treatment process, watch the precipitated phase can eliminate. Will be cut out from the butterfly pneumatic valve sample heated to 940 ℃, the heat preservation for 30 min, then in Neophot - 32 analysis on the metallographic microscope observation. After heat treatment of the precipitated phase in a sample of unsolved, and keep the original topography, which proves that the organization of the precipitated phase may not be sigma phase.
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3 SEM analysis in ShiGang sigma phase, using any dyeing methods are unable to discern the color, can use SEM analysis method to identify. Because sigma phase known as iron and chromium compounds, chromium content is 42% ~ 48%, through the EDS quantitative and qualitative analysis to measure the unknown phase composition element and its content, and to determine the unknown.
EDS analysis results show that the deposition of chromium content for 33. 16 6%, significantly higher than the matrix of the Cr content. The chromium content of 3%, while the sigma phase is 42% ~ 48%, thus denying precipitated phase of sigma phase. Comprehensive dyeing test face, heat treatment, test results, think precipitated phase in the stainless steel butterfly pneumatic valve group is not sigma phase. Through the SEM observation of precipitated phase for a eutectic organization, mainly on chromium carbide.
the material of the stainless steel butterfly pneumatic valve for austenitic stainless steel, nickel chromium this kind of material is generally used in solid solution state. At room temperature, its organization for austenitic austenitic stainless steel in a wide range of corrosive medium, especially the atmosphere with good corrosion resistance. Pneumatic valve the analysis of the causes of corrosion of stainless steel butterfly is as follows:
1) integrated the results of the experiments, can determine the butterfly pneumatic valve material not precipitate phase in the sigma phase, so the adjustment the corrosion phenomenon of the pneumatic valve is not caused by the sigma phase.
(2) through SEM observation, confirm butterfly pneumatic valve in the organization of chromium carbides precipitate phase, the eutectic distribution along the grain boundary. EDS analysis results show that this kind of distribution on the grain boundary carbides chromium content is significantly higher than the substrate. This kind of carbide M23C6 type. With the precipitation of carbide, and lack of chromium diffusion added, in the form of chromium carbide along the austenite grain boundary precipitation, form poor chrome area around carbide, austenitic stainless steel grain boundaries are easy to corrode. So precipitate along grain boundary carbide is the main cause of butterfly pneumatic valve rust.
(3) after solid solution treatment of austenitic stainless steel, because most of the carbide was dissolved in high temperature heating, austenitic saturation in a large amount of carbon and chromium, and fixed by the subsequent rapid cooling, the corrosion resistance of material has a business. So should strictly control the heat treatment process, the solid solution treatment when the workpiece heated to a high return, the carbide dissolved in full, and then rapidly cooling, to get uniform austenite organization. After solid solution treatment, if use the slow cooling, the cooling process of chromium carbide along the grain boundary precipitates, which results in the decrease of the corrosion resistance of materials.