Stainless steel　　

Stainless steel (Stainless Steel) is the abbreviation of stainless and acid-resistant steel. It is called stainless steel that is resistant to weak corrosive media such as air, steam, and water, or is stainless steel; and chemically resistant to chemical corrosion media (acid, alkali, salt, etc.) Corrosion steel is called acid-resistant steel.

Due to the difference in chemical composition between the two, their corrosion resistance is different. Ordinary stainless steel is generally not resistant to chemical media corrosion, while acid-resistant steel is generally non-corrosive. The term "stainless steel" not only refers to a single type of stainless steel, but also refers to more than one hundred industrial stainless steels. Each stainless steel developed has good performance in its specific application field. The key to success is to understand the purpose first, and then determine the correct steel grade. There are usually only six steel grades related to the application of building construction. They all contain 17-22% chromium, and better steel grades also contain nickel. The addition of molybdenum can further improve the atmospheric corrosion, especially the corrosion resistance of chloride-containing atmosphere.

Stainless steel refers to steel that is resistant to weak corrosive media such as air, steam, and water and chemically corrosive media such as acid, alkali, salt, etc., also known as stainless acid-resistant steel. In practical applications, steel that is resistant to corrosion by 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 non-corrosive. The corrosion resistance of stainless steel depends on the alloying elements contained in the steel.

Main brand

According to the composition, it can be divided into Cr series (400 series), Cr-Ni series (300 series), Cr-Mn-Ni (200 series), heat-resistant chromium alloy steel (500 series) and precipitation hardening series (600 series).

200 series: chromium-manganese-nickel

201,202, etc.: Substituting manganese for nickel, which has poor corrosion resistance and is widely used as a cheap substitute for the 300 series in China

300 series: chromium-nickel austenitic stainless steel

301: Good ductility, used for molded products. It can also be hardened by mechanical processing. Good weldability. Abrasion resistance and fatigue strength are better than 304 stainless steel.

302: The corrosion resistance is the same as that of 304, and the strength is better due to the relatively high carbon content.

303: It is easier to cut than 304 by adding a small amount of sulfur and phosphorus.

304: Universal model; that is, 18/8 stainless steel. Products such as: corrosion-resistant containers, tableware, furniture, railings, medical equipment. The standard composition is 18% chromium plus 8% nickel. It is a non-magnetic stainless steel that cannot change its metallographic structure by heat treatment. The GB grade is 0Cr18Ni9.

304 L: The same characteristics as 304, but low carbon so it is more corrosion resistant and easy to heat treatment, but the mechanical properties are poor. Suitable for welding and products that are not easy to heat treatment.

304 N: The same characteristics as 304, it is a nitrogen-containing stainless steel. Nitrogen is added to increase the strength of the steel.

309: It has better temperature resistance than 304, with a temperature resistance of up to 980℃.

309 S: It has a lot of chromium and nickel, so it has good heat resistance and oxidation resistance. Products such as heat exchangers, boiler components, and jet engines.

310: Excellent high temperature oxidation resistance, the highest operating temperature is 1200℃.

316: After 304, it is the second most widely used steel grade. It is mainly used in the food industry, watch jewelry, pharmaceutical industry and surgical equipment. The addition of molybdenum makes it a special corrosion-resistant structure. Because it has better resistance to chloride corrosion than 304, it is also used as "ship steel". SS316 is usually used in nuclear fuel recovery devices. 18/10 grade stainless steel usually also meets this application level.

316 L: Low-carbon so it is more corrosion resistant and easy to heat treatment. Products such as: chemical processing equipment, nuclear power generators, and refrigerant storage.

321: Except that the addition of titanium reduces the risk of corrosion of the material welds, other properties are similar to 304.

347: Add stabilizing element niobium, suitable for welding aviation appliance parts and chemical equipment.

400 series: ferritic and martensitic stainless steel, no manganese, can replace 304 stainless steel to a certain extent

408: Good heat resistance, weak corrosion resistance, 11% Cr, 8% Ni.

409: The cheapest model (British and American), usually used as a car exhaust pipe, is a ferritic stainless steel (chrome steel).

410: Martensite (high-strength chromium steel), good wear resistance, poor corrosion resistance.

416: The addition of sulfur improves the processing performance of the material.

420: "Cutting tool grade" martensitic steel, similar to the earliest stainless steel such as Brinell high chromium steel. It is also used for surgical knives, which can be very bright.

430: Ferritic stainless steel, for decoration, such as car accessories. Good formability, but poor temperature resistance and corrosion resistance.

440: High-strength cutting tool steel with slightly higher carbon content. After proper heat treatment, higher yield strength can be obtained. The hardness can reach 58HRC, which is among the hardest stainless steels. The most common application example is "razor blades". There are three commonly used models: 440A, 440B, 440C, and 440F (easy processing type).

500 series: heat-resistant chromium alloy steel.

600 series: martensitic precipitation hardening stainless steel.

Structural composition

The corrosion resistance of stainless steel decreases with the increase of carbon content. Therefore, the carbon content of most stainless steels is low, the maximum does not exceed 1.2%, and the ωc (carbon content) of some steels is even lower than 0.03% (such as 00Cr12 ). The main alloying element in stainless steel is Cr (chromium), and only when the Cr content reaches a certain value, the steel has corrosion resistance. Therefore, stainless steel generally contains at least 10.5% of Cr (chromium). Stainless steel also contains elements such as Ni, Ti, Mn, N, Nb, Mo, Si, and Cu.

Physical properties

Compared with carbon steel

1, density

The density of carbon steel is slightly higher than that of ferritic and martensitic stainless steel, and slightly lower than that of austenitic stainless steel;

2, resistivity

The resistivity is in increasing order of carbon steel, ferritic, martensitic and austenitic stainless steel;

3. The order of linear expansion coefficient is similar, with austenitic stainless steel the highest and carbon steel the smallest;

4. Carbon steel, ferritic and martensitic stainless steels are magnetic, austenitic stainless steels are non-magnetic, but they will produce magnetism when they are cold-worked and hardened to form martensite transformation. Heat treatment can be used to eliminate this martensite The tissue restores its non-magnetic properties.

Compared with carbon steel, 　　 austenitic stainless steel has the following characteristics:

1) High electrical resistivity, about 5 times that of carbon steel.

2) Large linear expansion coefficient, 40% larger than carbon steel, and as the temperature rises, the value of the linear expansion coefficient increases accordingly.

3) Low thermal conductivity, about 1/3 of carbon steel.

technical parameter

Steel number and indication method

①Use international chemical element symbols and symbols of the country to represent chemical components, and Arabic letters to represent component content:

Such as: China, Russia 12CrNi3

②Use fixed-digit numbers to represent steel series or numbers; such as: the United States, Japan, 300 series, 400 series, 200 series;

③The serial number is composed of Latin letters and sequence, which only indicates the purpose.

Chinese numbering rules

①Use element symbols

②Usage, Chinese pinyin, open hearth steel: P, boiling steel: F, killed steel: B, Class A steel: A, T8: special 8, GCr15: ball

◆Alloy steel, spring steel, such as: 20CrMnTi 60SiMn, (using a few ten thousandths to express C content)

◆Stainless steel, alloy tool steel (using a few thousandths to express C content), such as: 1Cr18Ni9 one thousandth (ie 0.1%C), stainless C≤0.08% such as 0Cr18Ni9, ultra-low carbon C≤0.03% such as 00Cr17Ni13Mo