According to the definition in GB/T20878-2007, stainless steel is a steel whose main characteristics are stainless and corrosion resistance, with a chromium content of at least 10.5% and a carbon content of no more than 1.2%.

Stainless steel is the abbreviation of stainless and acid-resistant steel. The steel grades that are resistant to weak corrosive media such as air, steam, and water, or have stainless properties are called stainless steel; The steel grade is called acid-resistant steel.

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

Generally speaking, the hardness of stainless steel is higher than that of aluminum alloy, and the cost of stainless steel is higher than that of aluminum alloy.

Stainless steel is often divided into martensitic steel, ferritic steel, austenitic steel, austenitic-ferritic (duplex) stainless steel and precipitation hardening stainless steel according to the organizational state. In addition, it can be divided into: chromium stainless steel, chromium-nickel stainless steel and chromium manganese nitrogen stainless steel. There are also special stainless steels for pressure vessels

Austenitic stainless steel

It contains more than 18% chromium, and also contains about 8% nickel and a small amount of molybdenum, titanium, nitrogen and other elements. Good overall performance, resistant to corrosion by various media. Common grades of austenitic stainless steel are 1Cr18Ni9, 0Cr19Ni9 and so on. The Wc of 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, which makes the steel austenitic at room temperature. This type of steel has good plasticity, toughness, weldability, corrosion resistance and non-magnetic or weak magnetic properties. It has good corrosion resistance in oxidizing and reducing media. It is used to make acid-resistant equipment, such as corrosion-resistant containers and equipment. Linings, pipelines, nitric acid-resistant equipment parts, etc., and can also be used as the main material of stainless steel watch accessories. Austenitic stainless steel generally adopts solution treatment, that is, the steel is heated to 1050-1150°C, and then water-cooled or air-cooled to obtain a single-phase austenite structure.

Ferritic Stainless Steel

Chromium 15% to 30%. 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, such as Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28, etc. Ferritic stainless steel has good corrosion resistance and oxidation resistance because of its high chromium content, but its mechanical properties and process performance are poor. It is mostly used for acid-resistant structures with little stress and as anti-oxidation steel. This type of steel can resist the corrosion of the atmosphere, nitric acid and salt solution, and has the characteristics of 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, etc.

Austenitic-ferritic duplex stainless steel

It has the advantages of both austenitic and ferritic stainless steels, and has superplasticity. Austenite and ferrite each account for about half of the stainless steel. In the case of low carbon content, the content of chromium (Cr) is 18%~28%, and the content of nickel (Ni) 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, significantly improved intergranular corrosion resistance and welding performance, while maintaining iron The body stainless steel is brittle at 475°C, has high thermal conductivity, and has 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

High strength, but poor plasticity and weldability. The commonly used grades of martensitic stainless steel are 1Cr13, 3Cr13, etc., because of the high carbon content, it has high strength, hardness and wear resistance, but the corrosion resistance is slightly poor, and it is used for high mechanical properties and corrosion resistance. General requirements on some parts. This type of steel is used after quenching and tempering. Annealing is required after forging and stamping.

Precipitation Hardened Stainless Steel

The matrix is austenite or martensite, and the commonly used grades of precipitation hardening stainless steel are 04Cr13Ni8Mo2Al and so on. It is a stainless steel that can be hardened (strengthened) by precipitation hardening (also known as age hardening).

Physical properties:

compared to carbon steel

1. Density

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

2. Resistivity

Resistivity increases in order of carbon steel, ferritic, martensitic and austenitic stainless steel;

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

4. Carbon steel, ferritic and martensitic stainless steel are magnetic, and austenitic stainless steel is non-magnetic, but it will produce magnetism when it is cold-work hardened to form martensitic phase transformation, and heat treatment can be used to eliminate this martensite tissue to restore its non-magnetic properties.

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

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

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

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