Overview
In recent years, the requirements for corrosion resistance of valves in working conditions have become higher and higher. For example, valves used in chemical systems, sewage systems, and power plants using seawater cooling have improved corrosion resistance to varying degrees. Therefore, it is necessary to study the corrosion resistance of conventional valve materials, especially for valves with a large diameter above DN1000 used for seawater. Increase. This article discusses the corrosion resistance of commonly used cast iron materials for large-diameter valves.
Analyze
The corrosion of cast iron is usually chemical or electrochemical, and its corrosion state may be homogeneous, localized, stressed, and intergranular. Engineering requires that the corrosion should be uniform because localized, stressed and intergranular corrosion is a dangerous form of failure. The corrosion of cast iron is not only related to its chemical composition, metallographic structure, surface characteristics of parts, and other factors, but also to the composition, activity, and temperature of the corrosive medium. The corrosion rate of cast iron in the atmosphere is very slow. With the extension of time, a protective film will be formed on the surface of the casting, which will change the alloy from an active state to an inert state, and significantly reduce the corrosion rate of cast iron in the atmosphere. In the alloy cast iron with obvious grain boundaries, due to the precipitation of special carbides and other compounds along the grain boundaries, the alloying elements contained in the solid solution are reduced, and intergranular corrosion easily occurs along the grain boundaries, resulting in holes or reduced strength of the cast iron. According to the theory of electrochemical corrosion, the ideal structure should be a single-phase and uniform metallographic structure, so pure iron, ferrite, and austenitic steels have high corrosion resistance stability when other conditions are the same. However, it is impossible to obtain cast iron with a single metallographic structure in advance, and only the homogenization of the metallographic structure can be considered to improve the corrosion resistance. According to some information, nurture cast iron (cast iron after alloying) has higher corrosion resistance than ordinary gray cast iron in general weak media. At room temperature, in sulphuric acid, the stability of inoculated cast iron is the same as that of low alloy cast iron. The stability in 0.5%～5.0% acetic acid is 1.5 times higher than that of low nickel alloy cast iron. To some extent, it can be used as a substitute for low alloy cast iron.
How to improve the corrosion resistance of cast iron
To improve the ability of cast iron to resist seawater corrosion, the better way is to choose nurture cast iron (Table 1) and appropriate alloying to solve it. For example, the metallographic structure of HT250Ni2 (Table 2) is pearlite> 98%, fine flake, 75%～90% is flake graphite, length 30～120μm, binary phosphorus eutectic < 1%, single pearlite, no carbide precipitation at the grain boundary, effectively avoiding grain boundary corrosion, according to the provisions of GB 9439, N grade gray cast iron, its tensile strength is N — (n + 100), the actual tensile strength is between 250 ～ 350 MPa, and its pressure resistance can be fully guaranteed. In addition, after the nurturing treatment of molten iron, not only the matrix structure is uniformly refined, but also the internal stress is significantly reduced, which can avoid stress corrosion.
It must be emphasized that the amount of nickel and chromium added, the graphitization coefficient of nickel at the eutectic temperature is + 0.4, and the graphitization coefficient of chromium is – 1.2, so in order to prevent carbide precipitation at the grain boundary, Ni: Cr> 3:1, There should also be a margin to eliminate the effect of Mn to form carbides. The practice has proved that containing 0.5% Cr in gray cast iron will form a large number of carbides, which will make cutting difficult, so is not the more Cr content, the better.
Epilogue
The selection of cast iron valve material is nurtured with cast iron and appropriate alloys, such as HT250Ni2, can ensure that its organization is a uniform and refined single pearlite, and there is no carbide precipitation at the grain boundary, and the casting can only have small internal stress, and the surface and interior of the casting can be reduced. Consistent organization can effectively prevent local corrosion, intergranular corrosion, and stress corrosion. The average corrosion rate of the valve thickness used in seawater and reclaimed water production is ≤0.5mm/a, which prolongs the service life of the valve.
