Cause analysis and solution of stainless steel butterfly val

Butterfly valves get their name from the winglike configuration of the butterfly plate, which is installed in the direction of the diameter of the pipe. In the cylindrical channel of the butterfly valve body, the disc disc can open and close the valve by rotating only 90° around the axis.
Butterfly valve simple structure, small size, light weight. It consists of only a few parts. And only need to rotate 90° can be fast open and close, easy to operate. The valve has good fluid control characteristics.
Butterfly valve in fully open position, butterfly plate thickness is the only resistance when the medium flows through the valve body. The pressure drop through the valve is therefore small. When the valve is closed, the disc turns towards the closed position and the flow velocity of the medium decreases gradually. The valve has good flow control characteristics.
The stainless steel butterfly valve made of CF8M has rusted during use. Austenitic stainless steel after normal heat treatment, the structure should be austenitic at room temperature, corrosion resistance is very good. In order to analyze the cause of the butterfly valve corrosion, the sample on it is analyzed.
The samples were subjected to chemical composition analysis (to determine whether they met the standard requirements), metallographic inspection, heat treatment process test and SEM analysis.
The metallographic sample was cut from the butterfly valve with rust phenomenon. After grinding and polishing, the sample was corroded with ferric chloride aqueous solution. The metallographic structure was observed and analyzed on the neophot-32 metallographic mirror. Theoretically, austenitic stainless steel should obtain homogeneous austenitic structure after normal heat treatment. There are two ways of determining what the second precipitate in the tissue is: the phase and the carbide. There are different conditions for the formation of phase and carbide, but there is a common characteristic of austenitic stainless steel: susceptibility to intergranular corrosion.
The method of noise is used to identify the phase. Alkaline red blood salt solution (red blood salt 10g + potassium hydroxide 10g + water 100ml) was used. After boiling the sample in the reagent for 2~4 min, the ferrite was yellow, the carbide was corroded, the austenite was bright, and the phase changed from brown to black. The sample cut from the butterfly valve was boiled in alkaline red blood saline solution for 4 min by the above method, and observed under the visible microscope. The original morphology of the precipitating substance was maintained, and no obvious change was found. Therefore, it is decided to use heat treatment method to further test the face analysis.
The phase is an intermetallic compound with roughly equal proportions of iron and chromium atoms. Chemical composition, ferrite, cold deformation and temperature variation all affect the formation of phase to varying degrees. The method of heat treatment was used to identify phase. The staining method was used to observe the change of precipitated phase under microscope. The phase is generally formed at a long-term aging temperature of 500~800℃. This is because aging at higher temperatures is beneficial to chromium diffusion. The phase will begin to dissolve at a temperature of at least 920℃. It is eliminated by heating at a stable temperature above the phase. Although the time required to form an phase is very long, the removal of an phase generally requires only a short period of heating. Based on this theory, a heat treatment process was developed to observe whether the precipitated phase in the tissue could be eliminated. The sample cut from the butterfly valve was heated to 940℃ for 30 min, and then observed and analyzed on the neophot-32 metallographic microscope. After heat treatment, the precipitated phase did not disappear and remained the original morphology, which proved that the precipitated phase in the tissue may not be phase.
There is a phase in steel which cannot be identified by any staining method, but it can be identified by SEM analysis. Since is known to be an iron and chromium compound with chromium content of 42%~48%, the composition and content of unknown phase are determined by EDS qualitative and quantitative analysis, thus the unknown phase is determined.
The RESULTS of EDS analysis show that the chromium content of the precipitate is 33.6%, significantly higher than the Cr content of 16.3% in the matrix, while the chromium content of phase is 42%~48%, thus denying that the precipitate is phase. It is considered that the precipitated phase in the tissue of stainless steel butterfly valve is not phase. After SEM observation, the precipitated phase is eutectic structure, which is carbide dominated by chromium.
Stainless steel butterfly valve material for ni-Cr austenitic stainless steel, this material is generally used in the state of solid solution. Austenitic stainless steel has good corrosion resistance in a wide range of corrosive media, especially in the atmosphere. The causes of corrosion of stainless steel butterfly valve are analyzed as follows:
According to the results of the above tests, it can be determined that the precipitated phase of butterfly valve material is not phase. Therefore, the corrosion of butterfly valve is not caused by phase.
Through SEM observation, it was confirmed that the precipitated phase in the butterfly valve structure was carbides dominated by chromium, and this eutectic structure was distributed along grain boundaries. The RESULTS of EDS analysis show that the content of chromium carbide on grain boundary is obviously higher than that on the substrate. This carbide is of type M23C6. With the precipitation of carbide, and can not get chromium diffusion supplement, in the form of chromium carbide along the austenite grain boundary precipitation, in the carbide around the formation of a chromium-poor area, so that austenite stainless steel grain boundary is easy to be corroded. So the carbide precipitates along the grain boundary is the main cause of the corrosion of butterfly valve.
after the solid solution of austenitic stainless steel, due to the high temperature heating when most of the carbide is dissolved, austenite saturated with a large number of carbon and chromium, and due to the subsequent rapid cooling and fixed down, so that the material has a very commercial corrosion resistance. Therefore, the heat treatment process should be strictly controlled. During the solid solution treatment, the workpiece should be heated to high retreat, so that the carbide can be fully dissolved, and then quickly cooled to obtain homogeneous Austenitic tissue. After solution treatment, if slow cooling is adopted, chromium carbide will be precipitated along the grain boundary during the cooling process, thus reducing the corrosion resistance of the material.