Stainless and acid-resistant steel is referred to as stainless steel. It is composed of two parts: stainless steel and acid-resistant steel. In short, the steel that can resist atmospheric corrosion is called stainless steel, and the steel that can resist chemical medium corrosion is called acid-resistant steel.

Generally speaking, steel with a content of more than 12% has the characteristics of stainless steel. Stainless steel can be divided into five categories according to the microstructure after heat treatment: ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, austenitic-ferritic stainless steel, and precipitation carbide stainless steel.

There are many classification methods for stainless steel materials, among which classification according to the metallographic structure is the most common.

One: Classification by chemical composition

Basically, it can be divided into two major systems: chromium series stainless steel (such as ferrite series, martensite series) and chromium-nickel series stainless steel (such as austenite series, abnormal series, precipitation hardening series).

Two: Classification by metallographic organization

Can be divided into austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, duplex stainless steel, precipitation hardening stainless steel.

Three: according to the type of corrosion resistance classification

It can be divided into stress corrosion-resistant stainless steel, pitting corrosion-resistant stainless steel, intergranular corrosion-resistant stainless steel, etc.

Four: Classification by functional characteristics

It can be divided into free-cutting stainless steel, non-magnetic stainless steel, low-temperature stainless steel, and high strength stainless steel.

basal tissue classification

1. Ferritic stainless steel. Contains 12% to 30% chromium. Its corrosion resistance, toughness, and weldability increase with the increase of chromium content, and its resistance to chloride stress corrosion is better than other types of stainless steel.

2. Austenitic stainless steel. It contains more than 18% chromium, about 8% nickel and a small amount of molybdenum, titanium, nitrogen and other elements. It has good comprehensive performance and is resistant to corrosion by various media.

3. Austenitic-ferritic duplex stainless steel. It has the advantages of both austenitic and ferritic stainless steels, and has superplasticity.

4. Martensitic stainless steel. High strength, but poor plasticity and weldability.

There are nearly 100 kinds of stainless steels that have been incorporated into various standards in the world, and with the advancement of technology and the development of industry and agriculture, new stainless steel grades are still increasing. For a known grade of stainless steel, the chromium equivalent [Cr] and nickel equivalent [Ni] can be calculated according to its chemical composition, and then the microstructure and properties of the steel can be roughly estimated by using the Schaeffler-Delong stainless steel structure diagram.

Austenitic stainless steel:

Stainless steel with austenitic structure at normal temperature. When the steel contains about 18% Cr, 8%~10% Ni, and about 0.1% C, it has a stable austenite structure. Austenitic chromium-nickel stainless steel includes the well-known 18Cr-8Ni steel and high Cr-Ni series steel developed on this basis by increasing the content of Cr and Ni and adding Mo, Cu, Si, Nb, Ti and other elements. Austenitic stainless steel is non-magnetic and has high toughness and plasticity, but its strength is low, it is impossible to strengthen it through phase transformation, and it can only be strengthened through cold working.
Such as adding S, Ca, Se, Te and other elements, it has good machinability. In addition to the corrosion resistance of oxidizing acid medium, this kind of steel can also resist the corrosion of sulfuric acid, phosphoric acid, formic acid, acetic acid, urea, etc. if it contains elements such as Mo and Cu. If the carbon content in this type of steel is less than 0.03% or contains Ti and Ni, its resistance to intergranular corrosion can be significantly improved. High-silicon austenitic stainless steels have good corrosion resistance to nitric acid. Due to the comprehensive and good comprehensive properties of austenitic stainless steel, it has been widely used in various industries.

Ferritic Stainless Steel:

Stainless steel mainly composed of ferrite in the use state. The chromium content is between 11% and 30%, and it has a body-centered cubic crystal structure. This type of steel generally does not contain nickel, and sometimes contains a small amount of Mo, Ti, Nb and other elements. This type of steel has the characteristics of large thermal conductivity, small expansion coefficient, good oxidation resistance, and excellent stress corrosion resistance. It is mostly used in the manufacture of atmospheric resistance. , Parts corroded by water vapor, water and oxidizing acid. This type of steel has disadvantages such as poor plasticity, significantly reduced post-weld plasticity and corrosion resistance, which limits its application. The application of out-of-furnace refining technology (AOD or VOD) can greatly reduce interstitial elements such as carbon and nitrogen, thus making this type of steel widely used.

Austenitic-ferritic duplex stainless steel

It is a stainless steel with about half of the austenite and ferrite. In the case of low C content, the Cr content is 18%~28%, and the Ni content is 3%~10%. Some steels also contain alloying elements such as Mo, Cu, Si, Nb, Ti, and N. This type of steel has the characteristics of both austenitic and ferritic stainless steels. Compared with ferrite, it has higher plasticity and toughness, no room temperature brittleness, and significantly improved intergranular corrosion resistance and welding performance. The 475 ℃ brittleness and high thermal conductivity of element stainless steel have the characteristics of superplasticity. Compared with austenitic stainless steel, it has high strength and significantly improved resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel has excellent pitting corrosion resistance and is also a nickel-saving stainless steel.

Martensitic stainless steel

Stainless steel whose mechanical properties can be adjusted by heat treatment, in layman’s terms, is a type of hardenable stainless steel. The typical grade is Cr13 type, such as 20Cr13, 30Cr13, 40Cr13 and so on. After quenching, the hardness is high, and different tempering temperatures have different combinations of strength and toughness. It is mainly used for steam turbine blades, tableware, and surgical instruments. According to the difference of chemical composition, martensitic stainless steel can be divided into martensitic chromium steel and martensitic chromium-nickel steel. According to the different structure and strengthening mechanism, it can also be divided into martensitic stainless steel, martensitic and semi-austenitic (or semi-martensitic) precipitation hardening stainless steel and maraging stainless steel.

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