By adding a certain amount of certain alloying elements to cast iron, alloy cast iron with higher corrosion resistance in some media can be obtained. High silicon cast iron is one of the most widely used. A series of alloy cast irons containing 10% to 16% silicon are called high silicon cast irons. Except for a few varieties that contain 10% to 12% silicon, the silicon content generally ranges from 14% to 16%. When the silicon content is less than 14.5%, the mechanical properties can be improved, but the corrosion resistance is greatly reduced. If the silicon content reaches more than 18%, although it is corrosion-resistant, the alloy becomes very brittle and is not suitable for casting. Therefore, the most widely used in industry is high silicon cast iron containing 14.5% to 15% silicon. [1]
The foreign trade names of high silicon cast iron are Duriron and Durichlor (containing molybdenum), and their chemical composition is as shown in the table below.
|
model |
Main chemical components, % | ||||||
| silicon | molybdenum | chromium | manganese | sulfur | phosphorus | iron | |
| High silicon cast iron | 〉14.25 | — | — | 0.50～0.56 | 〈0.05 | 〈0.1 | Remain |
| Molybdenum containing high silicon cast iron | 〉14.25 | 〉3 | 少量 | 0.65 | 〈0.05 | 〈0.1 | Remain |
Corrosion resistance
The reason why high-silicon cast iron with a silicon content of more than 14% has good corrosion resistance is that silicon forms a protective film composed of Not corrosion resistant.
Generally speaking, high silicon cast iron has excellent corrosion resistance in oxidizing media and certain reducing acids. It can withstand various temperatures and concentrations of nitric acid, sulfuric acid, acetic acid, hydrochloric acid at normal temperature, fatty acids and many other media. corrosion. It is not resistant to corrosion by media such as high-temperature hydrochloric acid, sulfurous acid, hydrofluoric acid, halogen, caustic alkali solution and molten alkali. The reason for the lack of corrosion resistance is that the protective film on the surface becomes soluble under the action of caustic alkali, and becomes gaseous under the action of hydrofluoric acid, which destroys the protective film.
Mechanical properties
High-silicon cast iron is hard and brittle with poor mechanical properties. It should avoid bearing impact and cannot be used to make pressure vessels. Castings generally cannot be machined other than grinding.
Machining performance
Adding some alloying elements to high silicon cast iron can improve its machining performance. Adding rare earth magnesium alloy to high-silicon cast iron containing 15% silicon can purify and degas, improve the matrix structure of the cast iron, and spheroidize the graphite, thus improving the strength, corrosion resistance and processing performance of the cast iron; for casting Performance has also improved. In addition to grinding, this high-silicon cast iron can also be turned, tapped, drilled, and repaired under certain conditions. However, it is still not suitable for sudden cooling and sudden heating; its corrosion resistance is better than that of ordinary high-silicon cast iron. , the adapted media are basically similar.
Adding 6.5% to 8.5% copper to high silicon cast iron containing 13.5% to 15% silicon can improve the machining performance. The corrosion resistance is similar to that of ordinary high silicon cast iron, but is worse in nitric acid. This material is suitable for making pump impellers and sleeves that are resistant to strong corrosion and wear. The machining performance can also be improved by reducing the silicon content and adding alloying elements. Adding chromium, copper and rare earth elements to silicon cast iron containing 10% to 12% silicon (called medium ferrosilicon) can improve its brittleness and processability. It can be turned, drilled, tapped, etc., and in many media, the corrosion resistance is still close to that of high silicon cast iron.
In medium-silicon cast iron with a silicon content of 10% to 11%, plus 1% to 2.5% molybdenum, 1.8% to 2.0% copper and 0.35% rare earth elements, the machining performance is improved, and it can be turned and resistant. The corrosion resistance is similar to that of high silicon cast iron. Practice has proved that this kind of cast iron is used as the impeller of dilute nitric acid pump in nitric acid production and the impeller of sulfuric acid circulation pump for chlorine drying, and the effect is very good.
The above-mentioned high-silicon cast irons have poor resistance to hydrochloric acid corrosion. Generally, they can only resist corrosion in low-concentration hydrochloric acid at room temperature. In order to improve the corrosion resistance of high silicon cast iron in hydrochloric acid (especially hot hydrochloric acid), the molybdenum content can be increased. For example, adding 3% to 4% molybdenum to high silicon cast iron with a silicon content of 14% to 16% can obtain Molybdenum-containing high-silicon cast iron will form a molybdenum oxychloride protective film on the surface of the casting under the action of hydrochloric acid. It is insoluble in hydrochloric acid, thus significantly increasing its ability to resist hydrochloric acid corrosion at high temperatures. The corrosion resistance remains unchanged in other media. This high-silicon cast iron is also called chlorine-resistant cast iron. [1]
High silicon cast iron processing
High silicon cast iron has the advantages of high hardness (HRC=45) and good corrosion resistance. It has been used as a material for mechanical seal friction pairs in chemical production. Since cast iron contains 14-16% silicon, is hard and brittle, there are certain difficulties in manufacturing it. However, through continuous practice, it has been proved that high-silicon cast iron can still be machined under certain conditions.
High silicon cast iron is processed on a lathe, the spindle speed is controlled at 70~80 rpm, and the tool feed is 0.01 mm. Before rough turning, the casting edges must be ground away. The maximum feed amount for rough turning is generally 1.5 to 2 mm for the workpiece.
The turning tool head material is YG3, and the tool stem material is tool steel.
The cutting direction is reverse. Because high-silicon cast iron is very brittle, the cutting is carried out from the outside to the inside according to the general material. In the end, the corners will be chipped and the edges will be chipped, causing the workpiece to be scrapped. According to practice, reverse cutting can be used to avoid chipping and chipping, and the final cutting amount of the light knife should be small.
Due to the high hardness of high-silicon cast iron, the main cutting edge of turning tools is different from ordinary turning tools, as shown in the picture on the right. The three types of turning tools in the picture have negative rake angles. The main cutting edge and the secondary cutting edge of the turning tool have different angles according to different uses. Picture a shows the internal and external circular turning tool, the main deflection angle A=10°, and the secondary deflection angle B=30°. Picture b shows the end turning tool, the main declination angle A=39°, and the secondary declination angle B=6°. Figure C shows the bevel turning tool, the main deflection angle = 6°.
Drilling holes in high-silicon cast iron is generally processed on a boring machine. The spindle speed is 25 to 30 rpm and the feed amount is 0.09 to 0.13 mm. If the drilling diameter is 18 to 20 mm, use tool steel with higher hardness to grind the spiral groove. (The groove should not be too deep). A piece of YG3 carbide is embedded in the drill bit head and ground to an angle suitable for drilling general materials, so drilling can be performed directly. For example, when drilling a hole larger than 20 mm, you can first drill 18 to 20 holes, and then make a drill bit according to the required size. The head of the drill bit is embedded with two pieces of carbide (YG3 material is used), and then ground into a semicircle. Enlarge the hole or turn it with a saber.
application
Due to its superior acid corrosion resistance, high silicon cast iron has been widely used for chemical corrosion protection. The most typical grade is STSil5, which is mainly used to manufacture acid-resistant centrifugal pumps, pipes, towers, heat exchangers, containers, valves and cocks, etc.
Generally speaking, high-silicon cast iron is brittle, so great care must be taken during installation, maintenance, and use. Do not hit with a hammer during installation; assembly must be accurate to avoid local stress concentration; drastic changes in temperature difference or local heating are strictly prohibited during operation, especially when starting, stopping or cleaning, the heating and cooling speed must be slow; it is not suitable to be used as pressure equipment.
It can be made into various corrosion-resistant centrifugal pumps, Nessler vacuum pumps, cocks, valves, special-shaped pipes and pipe joints, pipes, venturi arms, cyclone separators, denitrification towers and bleaching towers, concentration furnaces and pre-washing machines, etc. In the production of concentrated nitric acid, the temperature of nitric acid is as high as 115 to 170°C when used as a stripping column. The concentrated nitric acid centrifugal pump handles nitric acid with a concentration of up to 98%. It is used as heat exchanger and packed tower for mixed acid of sulfuric acid and nitric acid, and is in good condition. Heating furnaces for gasoline in refining production, acetic anhydride distillation towers and benzene distillation towers for triacetate cellulose production, acid pumps for glacial acetic acid production and liquid sulfuric acid production, as well as various acid or salt solution pumps and cocks, etc., are all used in high-efficiency applications. Silicon cast iron.
High silicon copper cast iron (GT alloy) is resistant to alkali and sulfuric acid corrosion, but not to nitric acid corrosion. It has better alkali resistance than aluminum cast iron and high wear resistance. It can be used in pumps, impellers and bushings that are highly corrosive and subject to slurry wear.
Post time: May-30-2024
