All metals react with oxygen in the atmosphere to form an oxide film on the surface. Unfortunately, the iron oxide formed on ordinary carbon steel continues to oxidize, causing the rust to continuously expand, eventually forming holes. The carbon steel surface can be protected by plating with paint or oxidation-resistant metals (for example, zinc, nickel and chromium), but, as is known, this protection is only a thin film. If the protective layer is destroyed, the steel below will begin to rust. Chromium is the basic element that makes the stainless steel corrosion resistant. When the chromium content in the steel reaches about 12%, the chromium reacts with the oxygen in the corrosive medium and forms a surface on the steel. A very thin layer of oxide film (self-passivating film) prevents further corrosion of the steel substrate. In addition to chromium, the commonly used alloying elements are nickel, molybdenum, titanium, niobium, copper, nitrogen, etc., to meet the various requirements of the stainless steel structure and performance requirements. Chemical composition The corrosion resistance of stainless steel decreases with increasing carbon content. Therefore, most stainless steels have a low carbon content, and some steels have a wC even lower than 0.03% (eg, 00Cr12). The main alloying element in stainless steel is Cr. Only when the Cr content reaches a certain value, the steel has corrosion resistance. Therefore, stainless steel generally wCr are above 13%. Stainless steel also contains elements such as Ni, Ti, Mn, N, and Nb. Stainless steel is often classified according to the state of organization: martensitic steel, ferritic steel, austenitic steel and so on. In addition, it can be divided into: chromium stainless steel, chromium nickel stainless steel and chromium manganese nitrogen stainless steel.
1, ferritic stainless steel: chromium containing 12% to 30%. Its corrosion resistance, toughness and weldability increase with the increase of chromium content, and its resistance to chloride stress corrosion is superior to other types of stainless steel.
Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28 and the like belong to this class. Ferritic stainless steels have high corrosion resistance and oxidation resistance because of their high chromium content, but their mechanical properties and process performance are poor. They are mostly used in acid-resistant structures with less stress and as anti-oxidation steels. This kind of steel can resist the corrosion of atmosphere, ** and saline solution, and has the characteristics of high temperature anti-oxidation performance, small thermal expansion coefficient, etc. It is used in ** and food factory equipment, but also can make parts working under high temperature, such as gas turbine Parts and so on.
2, austenitic stainless steel: chromium is greater than 18%, also contains about 8% of nickel and a small amount of molybdenum, titanium, nitrogen and other elements. Good overall performance, resistant to many media corrosion. The commonly used grades of austenitic stainless steels include 1Cr18Ni9 and 0Cr19Ni9. The wC of the 0Cr19Ni9 steel is less than 0.08% and the steel number is marked as "0". This type of steel contains a large amount of Ni and Cr, making the steel austenitic at room temperature. This type of steel has good plasticity, toughness, weldability and corrosion resistance, and 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, conveying pipes, and resistance to chemicals. ** Equipment parts etc. Austenitic stainless steels are generally treated by solid solution, ie steel is heated to 1050 to 1150°C and then water cooled to obtain single-phase austenite structure.
3, austenitic-ferritic duplex stainless steel: both the advantages of austenitic and ferritic stainless steel, and has superplasticity.
Austenite and ferrite are about half of stainless steel. In the case where the C content is low, the Cr content is 18% to 28%, and the Ni content is 3% to 10%. Some steels also contain alloying elements such as Mo, Cu, Si, Nb, Ti, and N. This type of steel combines the characteristics of austenitic and ferritic stainless steels. Compared with ferrite, it has higher plasticity and toughness, no brittleness at room temperature, significantly improved resistance to intergranular corrosion and welding performance, and at the same time retains iron. The 475°C brittleness and high thermal conductivity of the ferritic stainless steel make it superplastic. Compared with austenitic stainless steels, they have high strength and resistance to intergranular corrosion and chloride stress corrosion resistance. Duplex stainless steel has excellent pitting corrosion resistance and is also a nickel-stainless steel.
4, martensitic stainless steel: high strength, but poor plasticity and weldability.
The commonly used grades of martensitic stainless steels are 1Cr13 and 3Cr13. Due to the high carbon content, they have higher strength, hardness and wear resistance, but they are slightly inferior in corrosion resistance. They are used for high mechanical properties and corrosion resistance. General requirements for some parts, such as springs, turbine blades, hydraulic press valves. This kind of steel is used after quenching and tempering.
1, ferritic stainless steel: chromium containing 12% to 30%. Its corrosion resistance, toughness and weldability increase with the increase of chromium content, and its resistance to chloride stress corrosion is superior to other types of stainless steel.
Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28 and the like belong to this class. Ferritic stainless steels have high corrosion resistance and oxidation resistance because of their high chromium content, but their mechanical properties and process performance are poor. They are mostly used in acid-resistant structures with less stress and as anti-oxidation steels. This kind of steel can resist the corrosion of atmosphere, ** and saline solution, and has the characteristics of high temperature anti-oxidation performance, small thermal expansion coefficient, etc. It is used in ** and food factory equipment, but also can make parts working under high temperature, such as gas turbine Parts and so on.
2, austenitic stainless steel: chromium is greater than 18%, also contains about 8% of nickel and a small amount of molybdenum, titanium, nitrogen and other elements. Good overall performance, resistant to many media corrosion. The commonly used grades of austenitic stainless steels include 1Cr18Ni9 and 0Cr19Ni9. The wC of the 0Cr19Ni9 steel is less than 0.08% and the steel number is marked as "0". This type of steel contains a large amount of Ni and Cr, making the steel austenitic at room temperature. This type of steel has good plasticity, toughness, weldability and corrosion resistance, and 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, conveying pipes, and resistance to chemicals. ** Equipment parts etc. Austenitic stainless steels are generally treated by solid solution, ie steel is heated to 1050 to 1150°C and then water cooled to obtain single-phase austenite structure.
3, austenitic-ferritic duplex stainless steel: both the advantages of austenitic and ferritic stainless steel, and has superplasticity.
Austenite and ferrite are about half of stainless steel. In the case where the C content is low, the Cr content is 18% to 28%, and the Ni content is 3% to 10%. Some steels also contain alloying elements such as Mo, Cu, Si, Nb, Ti, and N. This type of steel combines the characteristics of austenitic and ferritic stainless steels. Compared with ferrite, it has higher plasticity and toughness, no brittleness at room temperature, significantly improved resistance to intergranular corrosion and welding performance, and at the same time retains iron. The 475°C brittleness and high thermal conductivity of the ferritic stainless steel make it superplastic. Compared with austenitic stainless steels, they have high strength and resistance to intergranular corrosion and chloride stress corrosion resistance. Duplex stainless steel has excellent pitting corrosion resistance and is also a nickel-stainless steel.
4, martensitic stainless steel: high strength, but poor plasticity and weldability.
The commonly used grades of martensitic stainless steels are 1Cr13 and 3Cr13. Due to the high carbon content, they have higher strength, hardness and wear resistance, but they are slightly inferior in corrosion resistance. They are used for high mechanical properties and corrosion resistance. General requirements for some parts, such as springs, turbine blades, hydraulic press valves. This kind of steel is used after quenching and tempering.
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