Stainless Steel is the abbreviation of stainless acid-resistant steel. Steel types that are resistant to weak corrosive media such as air, steam, and water or are stainless are called stainless steel; and steel that is resistant to chemical corrosive media (acid, alkali, salt, etc.) The type of steel that corrodes is called acid-resistant steel.
Due to the difference in chemical composition between the two, the former is not necessarily resistant to corrosion by chemical media, while the latter is generally stainless. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel.
Usually divided into: according to metallographic structure:
Usually, ordinary stainless steel is divided into three categories according to its metallographic structure: austenitic stainless steel, ferritic stainless steel, and martensitic stainless steel. On the basis of these three basic metallographic structures, duplex steels, precipitation hardening stainless steels and high alloy steels with an iron content of less than 50% are derived for specific needs and purposes.
1. Austenitic stainless steel
The matrix is mainly austenite structure (CY phase) with a face-centered cubic crystal structure. It is non-magnetic and is mainly strengthened by cold working (and may lead to certain magnetism). The American Iron and Steel Institute uses numbers in the 200 and 300 series, such as 304.
2. Ferritic stainless steel
The matrix is mainly made of ferrite structure (a phase) with a body-centered cubic crystal structure. It is magnetic and generally cannot be hardened by heat treatment, but cold working can make it slightly strengthened. The American Iron and Steel Institute is labeled 430 and 446.
3. Martensitic stainless steel
The matrix is a martensitic structure (body-centered cubic or cubic), which is magnetic and its mechanical properties can be adjusted through heat treatment. The American Iron and Steel Institute is labeled with numbers 410, 420 and 440. Martensite has an austenite structure at high temperatures, and when cooled to room temperature at an appropriate rate, the austenite structure can transform into martensite (i.e., hardened).
4. Austenitic-ferritic (duplex) stainless steel
The matrix has both austenite and ferrite two-phase structures, of which the content of the smaller phase matrix is generally greater than 15%. It is a stainless steel that is magnetic and can be strengthened by cold working. 329 is a typical duplex stainless steel. Compared with austenitic stainless steel, duplex steel has high strength, and its resistance to intergranular corrosion, chloride stress corrosion and pitting corrosion is significantly improved.
5. Precipitation hardened stainless steel
Stainless steel whose matrix is austenite or martensite and can be hardened through precipitation hardening. The American Iron and Steel Institute uses 600 series numbers, such as 630, which is 17-4PH.
Generally speaking, in addition to alloys, austenitic stainless steel has excellent corrosion resistance. In less corrosive environments, ferritic stainless steel can be used. In mildly corrosive environments, if the material is required to have high For strength or high hardness, martensitic stainless steel and precipitation hardened stainless steel can be used.
Features and uses
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Surface Technology
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Thickness distinction
1. Because during the rolling process of steel mill machinery, the rollers are slightly deformed when heated, resulting in deviations in the thickness of the rolled plates, which are generally thicker in the middle and thinner on both sides. When measuring the thickness of a board, the state stipulates that the middle part of the board head should be measured.
2. The reason for tolerance is based on market and customer needs. It is generally divided into large tolerance and small tolerance, for example:
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What kind of stainless steel is not easy to rust?
There are three main factors affecting stainless steel corrosion:
1. Content of alloy elements
Generally speaking, steel with a chromium content of 10.5% is less likely to rust. The higher the chromium and nickel content, the better the corrosion resistance. For example, the nickel content of 304 material is 8% to 10%, and the chromium content reaches 18% to 20%. Such stainless steel will not rust under normal circumstances.
2. The smelting process of the production company will also affect the corrosion resistance of stainless steel.
Large stainless steel plants with good smelting technology, advanced equipment and advanced processes can ensure the control of alloy elements, removal of impurities and control of billet cooling temperature. Therefore, the product quality is stable and reliable, with good intrinsic quality and not easy to rust. On the contrary, some small steel plants have backward equipment and backward processes. During the smelting process, impurities cannot be removed, and the products produced will inevitably rust.
3. External environment, dry climate and good ventilation are not easy to rust
However, areas with high air humidity, continuous rainy weather, or environments with high pH in the air are prone to rust. Made of 304 stainless steel, it will rust if the surrounding environment is too poor.
How to deal with rust spots on stainless steel?
1. Chemical method
Use pickling paste or spray to assist in re-passivating the rusted parts to form a chromium oxide film to restore corrosion resistance. After pickling, in order to remove all contaminants and acid residues, it is very important to rinse properly with clean water. After all processing, use polishing equipment to polish again and seal with polishing wax. For local slight rust spots, you can also use a 1:1 mixture of gasoline and engine oil to wipe off the rust spots with a clean rag.
2. Mechanical method
Sandblasting, blasting, obliterating, brushing and polishing with glass or ceramic particles. It is possible to mechanically remove contamination from previously removed material, polished material or buried material. All kinds of contamination, especially foreign iron particles, can be a source of corrosion, especially in humid environments. Therefore, mechanically cleaned surfaces should preferably be cleaned regularly under dry conditions. The use of mechanical methods can only clean the surface and cannot change the corrosion resistance of the material itself. Therefore, it is recommended to re-polishing with polishing equipment after mechanical cleaning and sealing with polishing wax.
Commonly used stainless steel grades and performance of instruments
1. 304 stainless steel. It is one of the most widely used austenitic stainless steels. It is suitable for manufacturing deep-drawn formed parts and acid pipelines, containers, structural parts, various instrument bodies, etc. It can also be used to manufacture non-magnetic and low-temperature equipment and part.
2. 304L stainless steel. Ultra-low carbon austenitic stainless steel was developed to solve the problem of serious intergranular corrosion tendency of 304 stainless steel under some conditions due to the precipitation of Cr23C6. Its sensitized state resistance to intergranular corrosion is significantly better than that of 304 stainless steel. Except for slightly lower strength, other properties are the same as 321 stainless steel. It is mainly used for corrosion-resistant equipment and components that require welding and cannot be solution treated. It can be used to manufacture various instrument bodies, etc.
3. 304H stainless steel. The internal branch of 304 stainless steel has a carbon mass fraction of 0.04% to 0.10%, and its high temperature performance is better than that of 304 stainless steel.
4. 316 stainless steel. Molybdenum is added to 10Cr18Ni12 steel to make the steel have good resistance to reducing media and pitting corrosion resistance. In seawater and various other media, the corrosion resistance is better than 304 stainless steel, and it is mainly used for pitting corrosion-resistant materials.
5. 316L stainless steel. Ultra-low carbon steel has good resistance to sensitized intergranular corrosion and is suitable for manufacturing welded parts and equipment with thick cross-sections, such as corrosion-resistant materials in petrochemical equipment.
6. 316H stainless steel. The internal branch of 316 stainless steel has a carbon mass fraction of 0.04% to 0.10%, and its high temperature performance is better than that of 316 stainless steel.
7. 317 stainless steel. Its resistance to pitting corrosion and creep resistance is better than that of 316L stainless steel, and it is used to manufacture petrochemical and organic acid corrosion-resistant equipment.
8. 321 stainless steel. Titanium-stabilized austenitic stainless steel, which adds titanium to improve intergranular corrosion resistance and has good high-temperature mechanical properties, can be replaced by ultra-low carbon austenitic stainless steel. Except for special occasions such as high temperature or hydrogen corrosion resistance, it is not recommended for general use.
9. 347 stainless steel. Niobium-stabilized austenitic stainless steel, adding niobium to improve intergranular corrosion resistance, the corrosion resistance in acid, alkali, salt and other corrosive media is the same as 321 stainless steel, the welding performance is good, it can be used as corrosion-resistant material and resistant material. Hot steel is mainly used in thermal power and petrochemical fields, such as making containers, pipes, heat exchangers, shafts, furnace tubes in industrial furnaces, and furnace tube thermometers.
10. 904L stainless steel. Super complete austenitic stainless steel is a super austenitic stainless steel invented by the Finnish company OUTOKUMPU. Its nickel mass fraction is 24%~26%, the carbon mass fraction is less than 0.02%, and it has excellent corrosion resistance. , has good corrosion resistance in non-oxidizing acids such as sulfuric acid, acetic acid, formic acid, and phosphoric acid, and also has good resistance to crevice corrosion and stress corrosion resistance. It is suitable for sulfuric acid of various concentrations below 70°C. It can withstand acetic acid of any concentration and temperature under normal pressure and has good corrosion resistance in mixed acids of formic acid and acetic acid. The original standard ASMESB-625 classified it as a nickel-based alloy, and the new standard classified it as stainless steel. China only has a similar grade of 015Cr19Ni26Mo5Cu2 steel, and a few European instrument manufacturers use 904L stainless steel as their key material. For example, the measuring tube of E+H's mass flow meter is made of 904L stainless steel, and the case of Rolex watches is also made of 904L stainless steel.
11. 440C stainless steel. Martensitic stainless steel has the highest hardness among hardenable stainless steels and stainless steels, with a hardness of HRC57. Mainly used to make nozzles, bearings, valve cores, valve seats, sleeves, valve stems, etc.
12. 17-4PH stainless steel. Martensitic precipitation hardening stainless steel, with a hardness of HRC44, has high strength, hardness and corrosion resistance and cannot be used at temperatures above 300°C. It has good corrosion resistance to the atmosphere and dilute acids or salts. Its corrosion resistance is the same as that of 304 stainless steel and 430 stainless steel. It is used to manufacture offshore platforms, turbine blades, valve cores, valve seats, sleeves, and valve stems. wait.
