The reason for the corrosion of stainless steel butterfly valve has found a solution for this company.

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 Neophot-32 metallographic mirror. The metallographic structure was composed of austenite and another Kind of precipitate composition. 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 the austenitic stainless steel to intergranular corrosion.

First, the variegation method was used to identify the σ phase. Using alkaline red blood salt aqueous solution (red blood salt 10g + potassium hydroxide 10g + water 100ml), the sample is boiled in the reagent for 2~4 minutes, the ferrite is yellow, the carbide is corroded, and the austenite is present. Bright color, 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 in which the ratio of iron to chromium atoms is approximately equal. 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 σ phase. According to the data, the σ phase is usually formed during long-term aging at 500~800°C. This is because aging at higher temperatures favors the diffusion of chromium. Heating the σ 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 σ phase can eliminate it. Although the time required to form the σ 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 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 demonstrating that the precipitated phase in the structure may not be the sigma phase.

2.4 SEM analysis

Sometimes the σ phase appearing in steel cannot be discerned 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 results of quantitative analysis of the microdomains of the matrix and the precipitated phase are shown in Table 2.

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 by 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 σ 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 Through SEM observation, it is 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. EDS analysis showed that the chromium content of the carbides distributed on the grain boundaries was significantly higher than that of the matrix. This carbide is of the M ₂₃ C ₆ 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.

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