The corrosion resistance of stainless steel decreases with the increase of carbon content. Therefore, the carbon content of most stainless steel is low, with a maximum of no more than 1.2%. The Wc (carbon content) of some steels is even lower than 0.03% (such as 00Cr12 ). The main alloying element in stainless steel is Cr (chromium). Only when the Cr content reaches a certain value, the steel has corrosion resistance. Therefore, stainless steel generally has a Cr (chromium) content of at least 10.5%. Stainless steel also contains Ni, Ti, Mn, N, Nb, Mo, Si, Cu and other elements.
Stainless steel is often divided according to its organizational state: martensitic steel, ferritic steel, austenitic steel, austenitic-ferritic (duplex) stainless steel and precipitation hardened stainless steel. In addition, it can be divided according to its composition: chromium stainless steel, chromium-nickel stainless steel, chromium-manganese-nitrogen stainless steel, etc.
1. Ferritic stainless steel: containing 12% to 30% chromium. Its corrosion resistance, toughness and weldability increase with the increase of chromium content, and its chloride stress corrosion resistance is better than other types of stainless steel. Belonging to this category are Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28, etc. Because of its high chromium content, ferritic stainless steel has relatively good corrosion resistance and oxidation resistance, but poor mechanical and process properties. It is mostly used in acid-resistant structures with little stress and as anti-oxidation steel. This type of steel can resist corrosion from the atmosphere, nitric acid and salt water solutions, and has good high-temperature oxidation resistance and small thermal expansion coefficient. It is used in nitric acid and food factory equipment, and can also be used to make parts that work at high temperatures, such as gas turbine parts. .
2. Austenitic stainless steel: contains more than 18% chromium, and also contains about 8% nickel and a small amount of molybdenum, titanium, nitrogen and other elements. It has good comprehensive performance and can withstand corrosion from various media. Commonly used grades of austenitic stainless steel include 1Cr18Ni9, 0Cr19Ni9, etc. The Wc of 0Cr19Ni9 steel is <0.08%, and the steel number is marked as "0". This type of steel contains a large amount of Ni and Cr, which makes the steel in an austenitic state at room temperature. This type of steel has good plasticity, toughness, weldability, corrosion resistance and non-magnetic properties. It has good corrosion resistance in both oxidizing and reducing media. It is used to make acid-resistant equipment, such as corrosion-resistant containers and equipment linings, transportation Pipes, nitric acid-resistant equipment parts, etc., and can also be used as the main material of stainless steel clocks and jewelry. Austenitic stainless steel generally adopts solution treatment, that is, the steel is heated to 1050~1150℃, and then water-cooled or air-cooled to obtain a single-phase austenite structure.
3. Austenitic-ferritic duplex stainless steel: It has the advantages of austenitic and ferritic stainless steel and has superplasticity. Stainless steel with approximately half austenite and ferrite structures each. 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 steel. Compared with ferrite, it has higher plasticity and toughness, no room temperature brittleness, significantly improved intergranular corrosion resistance and welding performance, while maintaining iron content. The solid stainless steel is brittle at 475°C, has high thermal conductivity, and has superplasticity and other characteristics. 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.
4. Martensitic stainless steel: high strength, but poor plasticity and weldability. Commonly used grades of martensitic stainless steel include 1Cr13, 3Cr13, etc. Because of their high carbon content, they have high strength, hardness and wear resistance, but slightly poor corrosion resistance. They are used for applications with higher requirements on mechanical properties and corrosion resistance. Some parts with general requirements, such as springs, turbine blades, hydraulic valves, etc. This type of steel is used after quenching and tempering.
5. Precipitation hardening stainless steel: The matrix is austenite or martensite structure. Common grades of precipitation hardening stainless steel include 04Cr13Ni8Mo2Al, etc. It is stainless steel that can be hardened (strengthened) through precipitation hardening (also known as age hardening).
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