Stainless steel knowledge

Steel is a general term for iron-carbon alloys with a carbon content between 0.02% and 2.11%. More than 2.11% is iron.
The chemical composition of steel can vary greatly. Steel containing only carbon elements is called carbon steel or ordinary steel. During the smelting process of steel, alloy elements such as chromium, nickel, manganese, silicon, titanium, and molybdenum can also be added to improve the properties of the steel.
Stainless steel is steel with stainless steel and corrosion resistance as its main characteristics, with a chromium content of at least 10.5% and a maximum carbon content of no more than 1.2%.
Stainless steel will not rust?
When brown rust spots (dots) appear on the surface of stainless steel, people are surprised. They think that stainless steel will not rust. If it is rusty, it is not stainless steel. There may be something wrong with the steel. In fact, this is a one-sided wrong view due to lack of understanding of stainless steel. Stainless steel can also rust under certain conditions.
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Stainless steel has the ability to resist atmospheric oxidation--it is rustless--and also has the ability to resist corrosion in media containing acid, alkali and salt, that is, it is corrosion resistance. However, its corrosion resistance changes with the chemical composition of the steel itself, its mutual state, usage conditions and environmental medium types. Materials such as 304 have absolutely excellent corrosion resistance in a dry and clean atmosphere. However, if it is moved to a coastal area, it will soon get rusty in the sea fog containing a large amount of salt. Therefore, not any kind of stainless steel can be corrosion-resistant and rust-free at any time.
Stainless steel relies on the formation of an extremely thin, strong, fine, and stable chromium-rich oxide film (protective film) on its surface to prevent oxygen atoms from continuing to penetrate and oxidize, thereby gaining the ability to resist corrosion. Once this film is continuously damaged for some reason, oxygen atoms in the air or liquid will continue to penetrate or iron atoms in the metal will continue to separate out, forming loose iron oxide, and the metal surface will be constantly damaged. rust.
What kind of stainless steel is not easy to rust?
The main factors affecting stainless steel corrosion are the following three points.
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 enterprise
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
A dry and well-ventilated environment is less likely to rust. 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. If there are slight local 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 with glass or ceramic particles, obliterating, brushing and polishing. It is possible to mechanically remove contamination from previously removed material, polished material or buried material. Various contaminations, especially foreign iron particles, can be a source of corrosion, especially in humid environments. Therefore, mechanically cleaned surfaces should 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.
Can stainless steel be judged using magnets?
There are many people who buy stainless steel or stainless steel products and bring a small magnet with them to suck it when looking at the goods. They think that the stainless steel that cannot be attracted is good stainless steel. If there is no magnet, it will not rust. In fact, this is a wrong understanding.
The non-magnetic nature of stainless steel strips is determined by the organizational structure. During the solidification process of molten steel, stainless steel with different organizational structures such as "ferrite", "austenite" and "martensite" will be formed due to different solidification temperatures. Among them, "ferrite" "Martensitic" stainless steels are all magnetic. "Austenitic" stainless steel has good comprehensive mechanical properties, process performance and weldability, but in terms of corrosion resistance alone, magnetic "ferritic" stainless steel is stronger than "austenitic" stainless steel.
The so-called 200 series and 300 series stainless steel currently circulating in the market, which contain high manganese content and low nickel content, are not magnetic, but their performance is very different from that of 304, which contains high nickel content. On the contrary, 304 has been stretched, annealed, polished, cast, etc. Technological treatment will also cause micro-magnetism. Therefore, it is a misunderstanding and unscientific to judge the quality of stainless steel by whether the stainless steel belt is magnetic or not.
What are the commonly used stainless steel grades?
201: Manganese-substituted nickel stainless steel has certain acid and alkali resistance, high density, and is polished without bubbles. It is used in some shallow drawn products such as watch cases, decorative tubes, and industrial tubes.
202: It is a low-nickel and high-manganese stainless steel with a nickel and manganese content of about 8%. It can replace 304 under weak corrosion conditions and has high cost performance. It is mainly used in architectural decoration, highway guardrails, municipal engineering, glass handrails, highway facilities, etc.
304: General-purpose stainless steel with good corrosion resistance, heat resistance, low temperature strength and mechanical properties, and high toughness. It is used in the food industry, medical industry, industry, chemical industry, and home decoration industry.
304L: Low-carbon 304 stainless steel, used for equipment parts with corrosion resistance and formability.
316: Mo is added and has excellent high temperature corrosion resistance. It is used in seawater equipment, chemistry, food industry, and papermaking fields.
321: It has excellent high temperature stress breaking performance and high temperature creep resistance.
430: Resistant to thermal fatigue, with a smaller thermal expansion coefficient than austenite, used in household appliances and building decoration.
410: High hardness, toughness, good corrosion resistance, large thermal conductivity, small expansion coefficient, and good oxidation resistance. It is used to manufacture parts that are corrosive to the atmosphere, water vapor, water, and oxidizing acids.