Abstract: stainless steel butterfly valve is rusted 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 that the carbides along the grain boundary in the material precipitate to form chromium poor area, resulting in the corrosion of stainless steel butterfly valve.
Key words: butterfly valve; rust; carbide
The stainless steel butterfly valve made of CF8M is rusted during use. After normal heat treatment, austenitic stainless steel shall be austenitic at room temperature with good corrosion resistance. In order to analyze the corrosion cause of butterfly valve, samples were taken on it for analysis.
1 test method
Take samples for chemical composition analysis (judge whether it meets the standard requirements), metallographic structure inspection, heat treatment process test and SEM analysis.
2 test results and analysis
2.1 chemical composition
See Table 1 for chemical composition analysis results and standard composition.
Table 1 chemical composition analysis results /%
Composition C Si Mn p s Cr Ni Mo
CF8M 0.08 1.5 1.5 0.04 0.04 18~21 9~12 2~3
Butterfly valve 0.10 0.60 0.61 0.024 0.009 18.05 9.71 1.45
2.2 metallographic analysis
The metallographic sample was cut from the rusted butterfly valve. After grinding and polishing, it was corroded with ferric chloride aqueous solution. It was observed and analyzed on the neophot-32 metallographic microscope. The metallographic structure was composed of austenite and another precipitate. Theoretically, austenitic stainless steel should obtain uniform austenite structure after normal heat treatment. There are two kinds of judgment on what the other precipitate in the organization is: one is σ The other is carbide. σ The formation conditions of phase and carbide are different, but they all have a common feature, which is the sensitivity of austenitic stainless steel to intergranular corrosion.
Firstly, the variegation method is used σ Phase identification. Alkaline red blood saline solution (10g red blood Salt + 10g potassium hydroxide + 100ml water) is used. After the sample is boiled in the reagent for 2 ~ 4 minutes, the ferrite is yellow, the carbide is corroded, and the austenite is bright, σ The phase changes from brown to black. After the sample cut from the butterfly valve was boiled in alkaline red blood saline solution for 4 min by the above method, the precipitate maintained its original shape and no obvious change was found under the microscope. Therefore, it is decided to use the method of heat treatment for further face analysis.
2.3 analysis of heat treatment test
σ Phase is a kind of intermetallic compound with roughly equal proportion of iron and chromium atoms. Chemical composition, ferrite, cold deformation and temperature change are all affected to varying degrees σ Phase formation. The change of precipitates was not obvious under the microscope, so the method of heat treatment was used to identify σ Phase. Relevant information, σ The phase is usually formed during long-term aging at 500 ~ 800 ℃. This is because aging at higher temperature is conducive to the diffusion of chromium. Reheat at high temperature σ The phase will begin to dissolve and at least above 920 ℃ after dissolution. Above σ The stable temperature heating of the phase can eliminate it. Form σ Although the phase takes a long time, it can be eliminated σ Generally, it can be heated in a short time. According to this theory, the heat treatment process is formulated to observe whether the precipitates in the microstructure can be eliminated. Heat the sample cut from the butterfly valve to 940 ℃ for 30 min, and then observe and analyze it on the neophot-32 metallographic microscope. The precipitates in the sample after heat treatment are not eliminated and maintain the original shape, which proves that the precipitates in the structure may not be σ Phase.
2.4 SEM analysis
Sometimes it appears in steel σ Phase, it can not be distinguished by any dyeing method, and can be identified by SEM analysis method. Because known σ The phase is a compound of iron and chromium, and the chromium content is 42% ~ 48%. The constituent elements and content of the unknown phase are measured by EDS qualitative and quantitative analysis, so as to determine the unknown phase.
The results of micro zone quantitative analysis of matrix and precipitated phase are shown in Table 2.
Table 2 EDS quantitative analysis results /%
Composition Fe Cr Ni Mo Si Mn
Matrix 70.463 16.365 10.211 1.239 0.466 1.257
Precipitates 56.908 33.629 3.681 4.835 0.040 0.907
The results of EDS analysis show that the chromium content of precipitates is 33.6%, which is significantly higher than that of Cr in matrix by 16.3% σ The chromium content of the phase is 42% ~ 48%, so it is denied that the precipitated phase is σ Phase. Based on the results of dyeing test and heat treatment test, it is considered that the precipitated phase in the structure of stainless steel butterfly valve is not σ Phase. SEM observation shows that the precipitate is a eutectic structure, which is mainly chromium carbide.
The material of stainless steel butterfly valve is nickel chromium austenitic stainless steel, which is generally used in 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. The causes of stainless steel butterfly valve corrosion are as follows:
① Based on the above test results, it can be determined that the precipitated phase in the butterfly valve material is not σ Therefore, the corrosion of butterfly valve is not caused by σ Caused by phase.
② Through SEM observation, it is confirmed that the precipitate phase in the butterfly valve is chromium based carbide, and this eutectic structure is distributed along the grain boundary. The results of EDS analysis show that the chromium content of carbide distributed on the grain boundary is significantly higher than that of the matrix. This carbide is m < sub > 23 < / sub > C < sub > 6 < / sub >. With the precipitation of carbides and without the 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 carbides, so that the austenite stainless steel grain boundary is easy to be corroded. Therefore, the carbide precipitated along the grain boundary is the main reason for the corrosion of butterfly valve.
③ Austenitic stainless steel after solid solution treatment, because most carbides are dissolved during high temperature heating, a large amount of carbon and chromium are saturated in austenite and fixed due to subsequent rapid cooling, so that the material has good corrosion resistance. Therefore, the heat treatment process should be strictly controlled. During solution treatment, the workpiece should be heated to high annealing to fully dissolve the carbides, and then cooled rapidly to obtain uniform austenite structure. After solution treatment, if slow cooling is adopted, chromium carbide will precipitate along the grain boundary during the cooling process, resulting in the reduction of corrosion resistance of the material.