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 state of organization. In addition, it can be divided into chromium stainless steel, chromium-nickel stainless steel and chromium-manganese-nitrogen stainless steel according to the composition. There is also a special stainless steel for pressure vessels "GB24511_2009_Stainless steel plates and strips for pressure equipment".

Contains 15% 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. This category includes Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28, etc. Ferritic stainless steel has good corrosion resistance and oxidation resistance due to its high chromium content, but poor mechanical and process properties. It is mostly used for acid-resistant structures with low stress and as oxidation-resistant steel. This type of steel can resist corrosion from the atmosphere, nitric acid and saline solutions, and has the characteristics of good high-temperature oxidation resistance and low thermal expansion coefficient. It is used for nitric acid and food factory equipment, and can also be used to make parts that work at high temperatures, such as gas turbine parts.

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 can resist corrosion from a variety of media. Common grades of austenitic stainless steel include 1Cr18Ni9, 0Cr19Ni9, etc. 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 magnetism. 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. It 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 degree , and then water-cooled or air-cooled to obtain a single-phase austenitic structure.
Austenitic-ferritic duplex stainless steel
It has the advantages of both austenitic and ferritic stainless steels and has superplasticity. Austenitic and ferritic structures each account for about half of the stainless steel. Under the condition of low carbon content, the chromium (Cr) content is 18%~28%, and the nickel (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. At the same time, it also maintains the 475 degree brittleness of ferritic stainless steel and 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.

The matrix is ​​austenite or martensite structure. Common grades of precipitation hardening stainless steel include 04Cr13Ni8Mo2Al, etc. It can be hardened (strengthened) by precipitation hardening (also known as aging hardening) treatment.

High strength, but poor plasticity and weldability. Common grades of martensitic stainless steel include 1Cr13, 3Cr13, etc. Due to the high carbon content, it has high strength, hardness and wear resistance, but slightly poor corrosion resistance. It is used for some parts with high mechanical properties requirements and general corrosion resistance requirements, such as springs, turbine blades, hydraulic press valves, etc. This type of steel is used after quenching and tempering. Annealing is required after forging and stamping.

Stainless steel for pressure vessels has clear requirements for classification and code, size, shape and allowable deviation, technical requirements, test methods, inspection rules, packaging, marking and product quality certificates. Commonly used grades are 06Cr19Ni10, 022Cr17Ni12Mo2, and the digital codes are: S30408, S31603, etc. Mainly used in sanitary equipment such as food machinery and pharmaceutical machinery.