Cause Analysis and Solution of Stainless Steel Butterfly Valve Corrosion

Stainless steel butterfly valves show rust during use. After metallographic analysis, dyeing test face, heat treatment test face, SEM and other test analysis, the key factor of material corrosion was found because the carbides along the grain boundary in the material precipitated to form a chromium-depleted zone, which caused the stainless steel butterfly valve to rust.
The stainless steel butterfly valve made of CF8M has rust during use. After normal heat treatment of austenitic stainless steel, the microstructure should be austenite at room temperature, and the corrosion resistance is very good. In order to analyze the cause of corrosion of the butterfly valve, a sample was taken for analysis.
1 Test method Sampling for chemical composition analysis (to determine whether it meets the standard requirements), 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 The metallographic sample was cut from the butterfly valve with rust phenomenon. After grinding and polishing, it was etched with aqueous solution of ferric chloride and observed on the Neophot-32 metallographic mirror. The metallographic structure was composed of austenite. The body is composed of another precipitate. Theoretically, after austenitic stainless steel is subjected to normal heat treatment, a uniform austenite structure should be obtained. There is two kinds of judgments as to which organization of another precipitate appearing in the organization: one is σ phase and the other is carbide. The σ phase is different from the carbide formation conditions, but all have a common feature, which is the sensitivity of austenitic stainless steel to intergranular corrosion.
First, the variegated method was used to identify the sigma phase. Alkaline red blood salt aqueous solution (red blood salt 10g + potassium hydroxide 10g + water 100ml), after the sample is boiled in the reagent for 2~4min, the ferrite is yellow, the carbide is corroded, and the austenite is bright. The σ phase changes from brown to black. The sample cut from the butterfly valve was boiled in an alkaline red blood saline solution for 4 minutes by the above method, and observed under the microscope, the precipitate retained the original morphology, and no significant change was found. Therefore, it was decided to further test the face by heat treatment. 2.3 Heat Treatment Test Analysis The σ phase is an intermetallic compound with approximately the same ratio of iron to chromium atoms. Chemical composition, ferrite, cold deformation, and temperature change all affect the formation of σ phase to varying degrees. The dyeing method was used to observe the change of the precipitated phase under the microscope, so the heat treatment method was used to identify the sigma phase. According to the data, the σ phase is usually formed in the long-term aging at 500~800 °C. This is because aging at higher temperatures favors the diffusion of chromium. Heating the sigma phase at a high temperature will begin to dissolve, and the dissolution must be at least 920 °C. Heating at a stable temperature above the sigma phase can be eliminated. Although the time required to form the sigma phase is long, the elimination of the σ phase generally requires only a short period of heating. According to this theory, a heat treatment process was developed to see if the precipitated phase in the tissue could be eliminated. The sample cut from the butterfly valve was heated to 940 ° C, incubated for 30 min, and then observed on a Neophot-32 metallographic microscope. The precipitated phase in the heat-treated sample was not eliminated, and the original morphology was maintained, thereby confirming that the precipitated phase in the structure may not be the sigma phase.
2.3 SEM analysis Sometimes the sigma phase appearing in steel can not be distinguished by any dyeing method, and can be identified by SEM analysis. Since the σ phase is a compound of iron and chromium, the chromium content is 42% to 48%, and the composition of the unknown phase and its content are determined by EDS qualitative and quantitative analysis to determine the unknown phase.
The EDS analysis showed that the chromium content of the precipitate was 33.6%, which was significantly higher than the Cr content in the matrix of 16.3%, while the chromium content of the σ phase was 42% to 48%, thus denying the precipitation phase as the σ phase. As a result of comprehensive dyeing test and heat treatment test, it is considered that the precipitated phase in the stainless steel butterfly valve structure is not the sigma phase. The precipitated phase was observed by SEM to be a eutectic structure, which was a chromium-based carbide.
The material of the stainless steel butterfly valve is nickel-chromium austenitic stainless steel, which is generally used in a solid solution state. At room temperature, the microstructure is austenite, and austenitic stainless steel has good corrosion resistance in a wide range of corrosive media, especially in the atmosphere. The reasons for the corrosion of the stainless steel butterfly valve are as follows:
1 Combining the results of the above tests, it can be determined that the precipitation phase in the butterfly valve material structure is not the σ phase, so the rust phenomenon of the butterfly valve is not caused by the σ phase.
2 By SEM observation, it was confirmed that the precipitated phase in the structure of the butterfly valve is a chromium-based carbide, and this eutectic structure is distributed along the grain boundary. The results of EDS analysis show that the chromium content of the carbides distributed on the grain boundaries is significantly higher than that of the matrix. This carbide is of the M23C6 type. When the precipitation of carbides is not obtained by the diffusion of chromium, it is precipitated along the austenite grain boundaries in the form of chromium carbide, and a chromium-depleted region is formed around the carbides, so that the austenitic stainless steel grain boundaries are easily corroded. Therefore, the carbide precipitated along the grain boundary is the main cause of the corrosion of the butterfly valve.
3 After the solution treatment of the austenitic stainless steel, most of the carbides are dissolved in the austenite due to the high temperature heating, and the austenite is saturated with a large amount of carbon and chromium, and is fixed by the subsequent rapid cooling, so that the material is very Corrosion resistance of the quotient. Therefore, the heat treatment process should be strictly controlled. When the solution treatment is performed, the workpiece is heated to a high retreat, the carbide is sufficiently dissolved, and then rapidly cooled to obtain a uniform Aussie structure. After solution treatment, if slow cooling is used, chromium carbide will precipitate along the grain boundary during the cooling process, resulting in a decrease in corrosion resistance of the material.