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 is called carbon steel or plain steel. Alloying elements such as chromium, nickel, manganese, silicon, titanium, and molybdenum can also be added to steel during the smelting process to improve the properties of the steel.
Stainless steel is a steel with stainless and corrosion resistance as the main characteristics, and the chromium content is at least 10.5%, and the carbon content does not exceed 1.2%.
01
Stainless steel must not rust?
When brown rust spots (spots) appear on the surface of stainless steel, people are greatly surprised, thinking that stainless steel will not rust, and if it gets rusty, it is not stainless steel. In fact, this is a one-sided misconception about the lack of understanding of stainless steel. Stainless steel also can get rusty under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation—stainlessness, and also has the ability to resist corrosion in media containing acids, alkalis, and salts—corrosion resistance. But the size of its resistance to corrosion with the chemical composition of its steel itself, mutual state, working conditions and surrounding medium type and change. Like 304 steel pipes, in the atmosphere of dry cleansing, absolute good resistant to corrosion ability is arranged, but it is moved on to riviera, in containing the sea fog of a large amount of salts, and will soon get rusty. Therefore, not any kind of stainless steel can resist corrosion and rust at any time.
Stainless steel relies on a layer of extremely thin, firm and dense stable chromium-rich oxide film (protective film) formed on its surface to prevent oxygen atoms from continuing to infiltrate and oxidize, thereby obtaining the ability to resist corrosion. Once there is some reason, this film is continuously damaged, and the oxygen atoms in the air or liquid will continuously infiltrate or the iron atoms in the metal will continuously separate out to form loose iron oxide, and the metal surface will be continuously damaged. rust.
02
What kind of stainless steel is not easy to rust?
The main factors affecting the corrosion of stainless steel are the following three points.
1) The content of alloying elements
Generally speaking, steel with a chromium content of 10.5% is not easy to rust. The higher the content of chromium and nickel, the better the corrosion resistance. For example, the content of nickel in 304 material should be 8% to 10%, and the content of chromium should reach 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 enterprise will also affect the corrosion resistance of stainless steel. Large stainless steel factories with good smelting technology, advanced equipment and advanced technology can guarantee the control of alloy elements, the removal of impurities, and the control of billet cooling temperature. Therefore, the product quality is stable and reliable, with good internal quality and is not easy to rust. On the contrary, some small steel mills have backward equipment and backward technology. 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 not easy to rust. And the environment area with high air humidity, continuous rainy weather, or high pH in the air is easy to rust. 304 stainless steel, if the surrounding environment is too bad, it will rust.
03
How to deal with rust spots on stainless steel?
1) chemical method
Use pickling cream or spray to assist the re-passivation of the rusted parts to form a chromium oxide film to restore the corrosion resistance. After pickling, in order to remove all pollutants and acid residues, it is very important to rinse properly with clean water. After all treatment, re-polish with polishing equipment and seal with polishing wax. For those with slight rust spots locally, 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
Sand blasting, shot blasting with glass or ceramic particles, obliteration, brushing and polishing. It is possible to mechanically wipe away contamination from previously removed material, polishing material or obliterating material. All kinds of contamination, especially foreign iron particles, can be a source of corrosion, especially in humid environments. Therefore, mechanically cleaned surfaces should be properly cleaned 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-polish with polishing equipment after mechanical cleaning and seal with polishing wax.
04
Can you judge stainless steel with a magnet?
Many people go to buy stainless steel or stainless steel products, and they bring a small magnet with them. When they look at the goods, they think that the ones that cannot be sucked are good stainless steel. There will be no rust without magnetism. In fact, this is a wrong understanding.
The structure of the stainless steel strip is determined by the structure. During the solidification process, due to the different solidification temperatures, stainless steel with different structures such as “ferrite”, “austenite” and “martensite” will be formed. Among them, “ferrite” “Martensitic” stainless steel is magnetic. The “austenitic” stainless steel has good comprehensive mechanical properties, process performance and weldability, but only in terms of corrosion resistance, the magnetic “ferritic” stainless steel is stronger than the “austenitic” stainless steel.
The so-called 200 series and 300 series stainless steels with high manganese content and low nickel content currently in the market are not magnetic, but their performance is far from that of 304 with high nickel content. On the contrary, 304 has been stretched, annealed, polished, cast, etc. Process treatment will also be slightly magnetic, so it is a misunderstanding and unscientific to judge the quality of stainless steel with or without magnetism.
05
What are the commonly used grades of stainless steel?
201: Manganese-nickel stainless steel, with certain acid and alkali resistance, high density, polished without bubbles, used in watch cases, decorative pipes, industrial pipes and other shallow stretched products.
202: It belongs to low-nickel and high-manganese stainless steel, with nickel and manganese content of about 8%. Under weak corrosion conditions, it can replace 304. It is cost-effective and is mainly used in architectural decoration, highway guardrails, municipal engineering, glass handrails, road facilities, etc.
304: General-purpose stainless steel with good corrosion resistance, heat resistance, low-temperature strength, good 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 in equipment parts with corrosion resistance and formability.
316: Added Mo, excellent high temperature corrosion resistance, used in seawater equipment, chemistry, food industry, and papermaking fields.
321: It has excellent high temperature stress breaking performance and high temperature creep resistance performance.
430: heat fatigue resistance, thermal expansion coefficient is smaller than austenite, used in household appliances, architectural decoration.
410: High hardness, toughness, good corrosion resistance, high thermal conductivity, small expansion coefficient, and good oxidation resistance. It is used to manufacture parts that are corrosive to atmosphere, water vapor, water, and oxidative acids.
Post time: Jan-29-2023