Stainless steel can be seen everywhere in life, and there are various types, which are hard to distinguish. Today, the editor will share an article with you to explain the knowledge points here.
Stainless steel is the abbreviation of stainless acid-resistant steel. Steels that are resistant to weak corrosive media such as air, steam, and water or have stainless properties are called stainless steels; and steels that are resistant to chemical corrosive media (acids, alkalis, salts, etc.) are called acid-resistant steels. Stainless steel refers to steel that is resistant to weak corrosive media such as air, steam, and water and chemically corrosive media such as acids, alkalis, and salts, also known as stainless acid-resistant steel. In practical applications, steel that is resistant to weak corrosive media is often called stainless steel, and steel that is resistant to chemical media is called acid-resistant steel. Due to the difference in chemical composition between the two, the former is not necessarily resistant to chemical media corrosion, while the latter is generally rust-resistant. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel.
Common classification: Usually divided according to metallographic structure: Usually, according to metallographic structure, ordinary stainless steel is divided into three categories: austenitic stainless steel, ferritic stainless steel, and martensitic stainless steel. Based on these three basic metallographic structures, duplex steel, precipitation hardening stainless steel and high alloy steel with an iron content of less than 50% have been derived for specific needs and purposes. 1. Austenitic stainless steel. The matrix is mainly austenitic structure (CY phase) with a face-centered cubic crystal structure, non-magnetic, and mainly strengthened by cold working (and may cause certain magnetism). The American Iron and Steel Association uses 200 and 300 series numbers, such as 304.
2. Ferritic stainless steel. The matrix is mainly ferrite structure (a phase) with a body-centered cubic crystal structure, magnetic, generally cannot be hardened by heat treatment, but can be slightly strengthened by cold working. The American Iron and Steel Association uses 430 and 446 as labels. 3. Martensitic stainless steel. The matrix is martensitic (body-centered cubic or cubic), magnetic, and the mechanical properties of stainless steel can be adjusted by heat treatment. The American Iron and Steel Association uses 410, 420 and 440 digital markings. Martensite has an austenitic structure at high temperatures. When cooled to room temperature at an appropriate rate, the austenitic structure can be transformed into martensite (i.e. hardened). 4. Austenitic-ferritic (duplex) stainless steel. The matrix has both austenite and ferrite phases, in which the content of the lesser phase matrix is generally greater than 15%. It 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 are significantly improved. 5. Precipitation hardening stainless steel. Stainless steel with a matrix of austenite or martensite structure that can be hardened by precipitation hardening treatment. The American Iron and Steel Institute uses 600 series numbers to mark, such as 630, i.e. 17-4PH. Generally speaking, except for alloys, austenitic stainless steel has excellent corrosion resistance. In environments with low corrosion resistance, ferritic stainless steel can be used. In mildly corrosive environments, if the material is required to have high strength or high hardness, martensitic stainless steel and precipitation hardening stainless steel can be used. Characteristics and uses
Surface process thickness distinction 1. Because the rollers are slightly deformed by heat during the rolling process of the steel mill machinery, the thickness of the rolled plate is deviated, generally thick in the middle and thin on both sides. When measuring the thickness of the plate, the state stipulates that the middle part of the plate head should be measured. 2. The reason for the tolerance is based on market and customer needs, generally divided into large tolerance and small tolerance: For example, what kind of stainless steel is not easy to rust? There are three main factors that affect the corrosion of stainless steel: 1. The content of alloy elements. Generally speaking, steel is not easy to rust when the chromium content is 10.5%. The higher the chromium-nickel content, the better the corrosion resistance. For example, the nickel content of 304 material is 8-10%, and the chromium content reaches 18-20%. Such stainless steel Steel will not rust under normal circumstances.
2. The smelting process of the production enterprise will also affect the corrosion resistance of stainless steel. Large stainless steel plants with good smelting technology, advanced equipment and advanced technology can ensure the control of alloy elements, removal of impurities and control of billet cooling temperature. Therefore, the product quality is stable and reliable, the internal quality is good, and it is not easy to rust. On the contrary, some small steel plants have backward equipment and backward technology. Impurities cannot be removed during the smelting process, and the products produced will inevitably rust. 3. External environment, dry and well-ventilated environment is not easy to rust. However, areas with high air humidity, continuous rainy weather, or high acidity and alkalinity in the air are prone to rust. 304 stainless steel will also rust if the surrounding environment is too bad. How to deal with rust spots on stainless steel? 1. Chemical method: Use pickling paste or spray to assist the rusted parts to re-passivate to form a chromium oxide film to restore its corrosion resistance. After pickling, it is very important to rinse with clean water properly to remove all pollutants and acid residues. After all treatments, re-polish with polishing equipment and seal with polishing wax. For those with slight rust on the part, you can also use a 1:1 gasoline and oil mixture with a clean rag to wipe off the rust. 2. Mechanical sandblasting cleaning, glass or ceramic particle shot blasting cleaning, annihilation, brushing and polishing. It is possible to wipe off the pollution caused by previously removed materials, polishing materials or annihilation materials by mechanical methods. All kinds of pollution, especially foreign iron particles, may become a source of corrosion, especially in a humid environment. Therefore, the mechanically cleaned surface should be cleaned formally under dry conditions. The mechanical method can only clean the surface, but cannot change the corrosion resistance of the material itself. Therefore, it is recommended to re-polish with polishing equipment after mechanical cleaning and seal with polishing wax. Common stainless steel grades and properties of instruments 1. 304 stainless steel. It is one of the austenitic stainless steels with a large application volume and the widest range of use. It is suitable for manufacturing deep-drawn forming parts and acid pipelines, containers, structural parts, various instrument bodies, etc. It can also manufacture non-magnetic and low-temperature equipment and components. 2. 304L stainless steel. Ultra-low carbon austenitic stainless steel was developed to solve the problem that 304 stainless steel has a serious tendency to intergranular corrosion under some conditions due to the precipitation of Cr23C6. Its sensitized intergranular corrosion resistance 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 cannot be solution treated after welding, and can be used to manufacture various instrument bodies. 3. 304H stainless steel. The internal branch of 304 stainless steel, with a carbon mass fraction of 0.04%-0.10%, has better high-temperature performance than 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. In seawater and various other media, the corrosion resistance is better than that of 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-section dimensions, such as corrosion-resistant materials in petrochemical equipment. 6. 316H stainless steel. Internal branch of 316 stainless steel, carbon mass fraction is 0.04%-0.10%, high temperature performance is better than 316 stainless steel. 7. 317 stainless steel. It has better pitting and creep resistance than 316L stainless steel, and is used to manufacture petrochemical and organic acid corrosion resistant equipment. 8. 321 stainless steel. Titanium stabilized austenitic stainless steel, adding 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 generally not recommended. 9. 347 stainless steel. Niobium stabilized austenitic stainless steel, adding niobium to improve intergranular corrosion resistance, corrosion resistance in acid, alkali, salt and other corrosive media is the same as 321 stainless steel, with good welding performance, can be used as corrosion resistant material and heat resistant steel, mainly used in thermal power and petrochemical fields, such as making containers, pipelines, heat exchangers, shafts, furnace tubes in industrial furnaces and furnace tube thermometers. 10. 904L stainless steel. Super fully austenitic stainless steel is a super austenitic stainless steel invented by Outokumpu of Finland. Its nickel mass fraction is 24% to 26%, and its carbon mass fraction is less than 0.02%. It has excellent corrosion resistance and good corrosion resistance in non-oxidizing acids such as sulfuric acid, acetic acid, formic acid, and phosphoric acid. It also has good resistance to crevice corrosion and stress corrosion. It is suitable for various concentrations of sulfuric acid below 70°C, and has good corrosion resistance in acetic acid of any concentration and any temperature and mixed acid of formic acid and acetic acid under normal pressure. The original standard ASMESB-625 classified it as a nickel-based alloy, and the new standard classifies it as stainless steel. China only has similar grade 015Cr19Ni26Mo5Cu2 steel, and a few European instrument manufacturers use 904L stainless steel as the key material. For example, the measuring tube of E+H's mass flowmeter 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 steel and stainless steel, with a hardness of HRC57. It is mainly used to make nozzles, bearings, valve cores, valve seats, sleeves, valve stems, etc. 12. 17-4PH stainless steel. Martensitic precipitation hardening stainless steel has 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 make offshore platforms, turbine blades, valve cores, valve seats, sleeves, valve stems, etc. In the instrumentation profession, combined with versatility and cost issues, the conventional selection order of austenitic stainless steel is 304-304L-316-316L-317-321-347-904L stainless steel, among which 317 is rarely used, 321 is not recommended, 347 is used for high temperature corrosion resistance, and 904L is only the default material for some components of individual manufacturers. 904L is generally not actively selected in the design. In the design and selection of instruments, there are usually occasions where the instrument material is different from the pipeline material, especially in high temperature conditions. Special attention should be paid to whether the selection of instrument material meets the design temperature and design pressure of process equipment or pipelines. For example, if the pipeline is high temperature chromium-molybdenum steel, and the instrument is stainless steel, there is a high probability of problems at this time, and the temperature and pressure table of the relevant material must be consulted. In the design and selection of instruments, stainless steel of various systems, series, and grades is often encountered. When selecting, it is necessary to consider the problem from multiple angles based on the specific process medium, temperature, pressure, stress-bearing components, corrosion, cost, etc.
