Which one is better, 201 stainless steel, 202 stainless steel, 301, 302 or 304?
Issuing time:2021-03-04 15:40
1. What is stainless steel?
Stainless steel is a type of steel. Steel refers to steel containing less than 2% of carbon (C), and more than 2% is iron. The addition of chromium (Cr), nickel (Ni), manganese (Mn), silicon (Si), titanium (Ti), molybdenum (Mo) and other alloying elements during the smelting process improves the performance of the steel and makes the steel more resistant to corrosion. (that is, not rusted) is what we often call stainless steel.
What exactly are "steel" and "iron", what are their characteristics, and what is their relationship? How did the 304, 304L, 316, and 316L we usually call come from, and what are the differences between them?
Steel: A material with iron as the main element, carbon content generally below 2%, and other elements. ——GB/T 13304-91 "Steel Classification"
Iron: A metallic element with atomic number 26. Iron materials have strong ferromagnetism, good plasticity and thermal conductivity.
Stainless steel: resistant to weak corrosive media such as air, steam, and water, or stainless steel. The commonly used steel grades are 304, 304L, 316, 316L, which are 300 series steels of austenitic stainless steel.
2. Why does stainless steel have different steel grades?
In the smelting process of stainless steel, due to the different varieties of alloying elements added, the amount of addition of different varieties is different. Their characteristics are also different. In order to distinguish them, they are crowned with different steel grades. The following is the content table of "alloying elements" of different steel grades of common decorative stainless steels for reference only: chemical composition (mass fraction, %)
Usually divided into:
Usually, according to the metallographic structure, ordinary stainless steel is divided into three categories: austenitic stainless steel, ferritic stainless steel, and martensitic stainless steel. On the basis of these three basic metallographic structures, for specific needs and purposes, dual-phase steels, precipitation-hardening stainless steels and high-alloy steels with an iron content of less than 50% are derived.
1. Austenitic stainless steel
The matrix is dominated by austenite structure (CY phase) of face-centered cubic crystal structure, non-magnetic, and is mainly strengthened by cold working (and may lead to certain magnetic properties) stainless steel. The American Iron and Steel Institute is designated by numbers in the 200 and 300 series, such as 304.
2. Ferritic stainless steel
The matrix is dominated by the ferrite structure ((a phase) of the body-centered cubic crystal structure, which is magnetic and generally cannot be hardened by heat treatment, but can be slightly strengthened by cold working. The American Iron and Steel Institute is marked with 430 and 446.
3. Martensitic stainless steel
The matrix is a martensitic structure (body-centered cubic or cubic), magnetic, and its mechanical properties can be adjusted by heat treatment. The American Iron and Steel Institute is designated by the numbers 410, 420 and 440. Martensite has an austenite structure at high temperature, and when cooled to room temperature at an appropriate rate, the austenite structure can be transformed into martensite (ie, hardened).
4. Austenitic-ferritic (duplex) stainless steel
The matrix has both austenite and ferrite two-phase structure, and the content of the less-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, dual-phase steel has high strength, and the resistance to intergranular corrosion and chloride stress corrosion and pitting corrosion are significantly improved.
5. Precipitation hardening stainless steel
The matrix is austenite or martensitic structure and can be hardened by precipitation hardening. The American Iron and Steel Institute is marked with a 600 series number, such as 630, which is 17-4PH.
Generally speaking, in addition to alloys, the corrosion resistance of austenitic stainless steel is relatively excellent. In a less corrosive environment, ferritic stainless steel can be used. In a mildly corrosive environment, if the material is required to have high For strength or high hardness, martensitic stainless steel and precipitation hardening stainless steel can be used.
Features and Uses
1. During the rolling process of the steel mill machinery, the rolls are slightly deformed by heat, which leads to deviations in the thickness of the rolled plates. Generally, the middle is thick and the two sides are thin. When measuring the thickness of the board, the state stipulates that the middle part of the board head should be measured.
2. The reason for the tolerance is that according to the market and customer needs, it is generally divided into large tolerance and small tolerance: for example
- 304 stainless steel -
304 stainless steel is a common steel. As a widely used steel, it has good corrosion resistance, heat resistance, low temperature strength and mechanical properties; good hot workability such as stamping and bending, no heat treatment hardening phenomenon (non-magnetic, Then use the temperature -196 ℃ ~ 800 ℃).
Scope of application:
Household goods (1 and 2 tableware, cabinets, indoor pipes, water heaters, boilers, bathtubs)
Auto parts (windshield wipers, mufflers, moldings)
Medical equipment, building materials, chemicals, food industry, agriculture, ship parts
- 304L stainless steel -
As a low-carbon 304 steel, its corrosion resistance is similar to that of 304 steel under normal conditions, but after welding or after stress relief, its resistance to grain boundary corrosion is strong; it can also keep good without heat treatment. High corrosion resistance, operating temperature -196℃～800℃.
Scope of application:
It is used in field open-air machines in chemical, coal and petroleum industries that require high resistance to grain boundary corrosion, heat-resistant parts of building materials and parts with difficult heat treatment.
- 316 stainless steel -
316 stainless steel has excellent corrosion resistance, atmospheric corrosion resistance and high temperature strength due to the addition of molybdenum, and can be used under harsh conditions; excellent work hardening (non-magnetic).
Scope of application
Seawater equipment, chemical, dye, paper, oxalic acid, fertilizer and other production equipment; photography, food industry, coastal facilities, ropes, CD rods, bolts, nuts.
- 316L stainless steel -
As a low carbon series of 316 steel, in addition to having the same characteristics as 316 steel, its resistance to grain boundary corrosion is excellent.
Scope of application:
Products with special requirements for resistance to grain boundary corrosion.
What kind of stainless steel is not easy to rust?
There are three main factors that affect stainless steel corrosion:
1. The content of alloying elements.
Generally speaking, the content of chromium is 10.5%, and the steel 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 is 8-10%, and the content of chromium is 18-20%. Such stainless steel will not rust under normal circumstances.
2. The smelting process of the manufacturer will also affect the corrosion resistance of stainless steel.
A large stainless steel plant with good smelting technology, advanced equipment and advanced technology can ensure the control of alloying elements, the removal of impurities, and the control of the cooling temperature of the billet, so 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 mills have backward equipment and backward technology. During the smelting process, impurities cannot be removed, and the products produced will inevitably rust.
3. The external environment, the climate is dry and ventilated, and the environment is not easy to rust.
The air humidity is high, continuous rainy weather, or environmental areas with high pH in the air are prone to rust. 304 stainless steel, if the surrounding environment is too bad, it will rust.
How to deal with rust spots on stainless steel?
1. Chemical method
Use pickling paste or spray to assist the re-passivation of the rusted part to form a chromium oxide film to restore the 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 treatment, re-polish with polishing equipment, 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
Sandblast cleaning, blast cleaning with glass or ceramic particles, annihilation, brushing and polishing. It is possible to mechanically wipe away contamination from previously removed material, polishing material, or submerged 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 in 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.
Why does stainless steel also rust?
Why does stainless steel also rust? When brown rust spots (spots) appear on the surface of stainless steel pipes, people are surprised: "Stainless steel does not rust, and if it rusts, it is not stainless steel, and there may be a problem with the steel." In fact, this is a one-sided misconception about the lack of understanding of stainless steel. Stainless steel will also rust under certain conditions.
Stainless steel has the ability to resist atmospheric oxidation—that is, rust resistance, and also has the ability to corrode in media containing acids, alkalis, and salts—that is, corrosion resistance. However, the size of its anti-corrosion ability is changed with the chemical composition of the steel itself, the protection state, the use conditions and the type of environmental media. For example, 304 steel pipe has excellent anti-corrosion ability in a dry and clean atmosphere, but if it is moved to the seaside area, it will rust quickly in the sea fog containing a lot of salt; and 316 steel pipe performs well . Therefore, it is not any kind of stainless steel that is resistant to corrosion and rust in any environment.