Austenitic stainless steel refers to stainless steel with austenitic structure at room temperature. The stable austenitic structure is obtained when Cr, Ni and C are about 18%, 8%~25% and 0.1% respectively. Austenitic chrome-nickel stainless steel includes the famous 18Cr-8Ni steel and the high Cr-Ni series steel developed on the basis of increasing Cr and Ni content and adding Mo, Cu, Si, Nb, Ti and other elements. Austenitic stainless steel is non-magnetic and has high toughness and plasticity, but the strength is low, it is impossible to strengthen through phase change, can only be strengthened through cold processing, such as adding S, Ca, Se, Te and other elements, it has good machinability.
characteristic
In addition to the corrosion resistance of oxidizing acid medium, this kind of steel can also resist the corrosion of sulfuric acid, phosphoric acid and formic acid, acetic acid and urea if it contains Mo, Cu and other elements. If the carbon content of such steel is less than 0.03% or contains Ti and Ni, the resistance to intergranular corrosion can be significantly improved. High silicon austenitic stainless steel has good corrosion resistance to concentrated nitric acid. Austenitic stainless steel has been widely used in all walks of life because of its comprehensive and good comprehensive properties.
classification
Physical classification
Stainless steel colloquially, stainless steel is not easy to rust steel, in fact, part of the stainless steel, both stainless, and acid resistance (corrosion resistance). Stainless steel's anti-rust and corrosion resistance is due to the formation of chromium-rich oxide film (passivation film) on its surface. This resistance to rust and corrosion is relative. The test shows that the corrosion resistance of steel in the atmosphere, water and other weak medium and nitric acid and other oxidizing medium increases with the increase of chromium content in steel. When the chromium content reaches a certain percentage, the corrosion resistance of steel changes, that is, from easy rust to not easy rust, from no corrosion resistance to corrosion resistance. There are many ways to classify stainless steel. According to the room temperature structure, there are martensitic, austenitic, ferrite and duplex stainless steel; According to the main chemical composition classification, basically can be divided into chromium stainless steel and chromium nickel stainless steel two systems; According to the use, there are nitric acid resistant stainless steel, sulfuric acid resistant stainless steel, seawater resistant stainless steel and so on. According to the corrosion resistance type, it can be divided into pitting corrosion resistant stainless steel, stress corrosion resistant stainless steel, intergranular corrosion resistant stainless steel and so on. According to the functional characteristics and classification can be divided into no magnetic stainless steel, easy cutting stainless steel, low temperature stainless steel, high strength stainless steel and so on. Because stainless steel has excellent corrosion resistance, formability, compatibility and strength and toughness in a wide range of temperature, it has been widely used in heavy industry, light industry, household goods industry and building decoration and other industries.
Chemical classification
Ferritic stainless steel having a predominantly ferritic texture when in service. The chromium content is 11%~30%, with body - centered cubic crystal structure. This kind of steel generally does not contain nickel, and sometimes contains a small amount of Mo, Ti, Nb and other elements. This kind of steel has the characteristics of large thermal conductivity, small expansion coefficient, good oxidation resistance, excellent stress corrosion resistance and so on. It is mostly used in the manufacture of parts resistant to atmospheric, water vapor, water and oxidizing acid corrosion. This kind of steel has some disadvantages, such as poor plasticity, reduced plasticity after welding and corrosion resistance, which limit its application. The outside furnace refining technology (AOD or VOD) can greatly reduce the carbon, nitrogen and other gap elements, so that this kind of steel has been widely used.
Austenitic - ferritic stainless steel is a stainless steel with about half austenitic and half ferritic structure. In the case of low C content, Cr content is 18%-28%, and Ni content is 3%-10%. Some steels also contain Mo, Cu, Si, Nb, Ti, N and other alloying elements. This kind of steel has the characteristics of austenitic and ferritic stainless steel, compared with ferritic, plasticity, toughness is higher, no room temperature brittleness, intercrystalline corrosion resistance and welding performance are significantly improved, but also maintain the ferritic stainless steel 475℃ brittleness and high thermal conductivity, with the characteristics of superplasticity. Compared with austenitic stainless steel, it has higher strength and better resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel has excellent corrosion resistance, is also a nickel stainless steel.
The mechanical properties of martensitic stainless steel can be adjusted by heat treatment. Generally speaking, it is a kind of hardened stainless steel. Typical brand is Cr13, such as 2Cr13, 3Cr13, 4Cr13, etc. High hardness after quenching, different tempering temperature has different strength and toughness combination, mainly used in steam turbine blades, tableware, surgical instruments. According to the difference of chemical composition, martensitic stainless steel can be divided into martensitic chromium steel and martensitic chromium nickel steel two types. According to the different structure and strengthening mechanism, it can also be divided into martensitic stainless steel, martensitic and half austenitic (or half martensitic) precipitation hardening stainless steel and martensitic aging stainless steel.
constituent
On the basis of the composition of 18-8 stainless steel evolution, there are mainly important developments in the following aspects:
1) Add Mo to improve pitting corrosion and crevice corrosion resistance;
2) Reducing C or adding Ti and Nb to reduce intergranular corrosion tendency;
3) Adding Ni and Cr to improve oxidation resistance and strength at high temperature;
4) Adding Ni to improve stress corrosion resistance;
5) Add S and Se to improve the machinability and component surface accuracy.
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