Introduction to types and grades of stainless steel materials

Introduction to types and grades of stainless steel materials
Stainless acid-resistant steel is referred to as stainless steel. It is composed of two major parts: stainless steel and acid-resistant steel. In short, steel that can resist atmospheric corrosion is called stainless steel, and steel that can resist chemical corrosion is called acid-resistant steel. Generally speaking, steel with a Wcr content greater than 12% has the characteristics of stainless steel. Stainless steel can be divided into five categories according to the microstructure after heat treatment: ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, austenitic-ferritic stainless steel and precipitation carbide stainless steel.

Austenitic stainless steel: stainless steel with austenitic structure at room temperature. When the steel contains about 18% Cr, 8%~10% Ni, and about 0.1% C, it has a stable austenitic structure. Austenitic chromium-nickel stainless steel includes the famous 18Cr-8Ni steel and the high Cr-Ni series steel developed on this basis by increasing the Cr and Ni content and adding elements such as Mo, Cu, Si, Nb, and Ti. Austenitic stainless steel is non-magnetic and has high toughness and plasticity, but has low strength. It is impossible to strengthen it through phase transformation and can only be strengthened through cold working. If elements such as S, Ca, Se, and Te are added, it has good machinability. In addition to being resistant to corrosion by oxidizing acid media, this type of steel can also resist corrosion by sulfuric acid, phosphoric acid, formic acid, acetic acid, urea, etc. if it contains elements such as Mo and Cu. If the carbon content in this type of steel is less than 0.03% or contains Ti and Ni, its intergranular corrosion resistance can be significantly improved. High-silicon austenitic stainless steel has good corrosion resistance to nitric acid. Due to its comprehensive and good comprehensive properties, austenitic stainless steel has been widely used in all walks of life. Ferritic
stainless steel: stainless steel with ferrite as the main structure when in use. The chromium content is 11%~30%, and it has a body-centered cubic crystal structure. This type of steel generally does not contain nickel, and sometimes contains a small amount of Mo, Ti, Nb and other elements. This type of steel has the characteristics of large thermal conductivity, small expansion coefficient, good oxidation resistance, excellent stress corrosion resistance, etc., and is mostly used to manufacture parts resistant to atmospheric, water vapor, water and oxidizing acid corrosion. This type of steel has the disadvantages of poor plasticity, significantly reduced plasticity and corrosion resistance after welding, which limits its application. The application of out-of-furnace refining technology (AOD or VOD) can greatly reduce interstitial elements such as carbon and nitrogen, so this type of steel is widely used.
Austenite-ferrite duplex stainless steel: It is a stainless steel with about half austenite and half ferrite. 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 austenite and ferrite stainless steel. 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℃ brittleness and high thermal conductivity of ferrite stainless steel, 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 resistance and is also a nickel-saving stainless steel.
Martensitic stainless steel: Stainless steel whose mechanical properties can be adjusted by heat treatment. In layman's terms, it is a type of hardenable stainless steel. The typical grade is Cr13, such as 20Cr13, 30Cr13, 40Cr13, etc. After quenching, the hardness is high, and different tempering temperatures have different strength and toughness combinations. It is mainly used for steam turbine blades, tableware, and surgical instruments. According to the difference in chemical composition, martensitic stainless steel can be divided into two categories: martensitic chromium steel and martensitic chromium-nickel steel. According to the difference in organization and strengthening mechanism, it can also be divided into martensitic stainless steel, martensitic and semi-austenitic (or semi-martensitic) precipitation hardening stainless steel, and martensitic aging stainless steel.
Representative steel grades, chemical composition, and room temperature mechanical properties: The representative steel grades of martensitic stainless steel are 12Cr13, 20Cr13, 30Cr13, etc.; the representative steel grades of ferritic stainless steel are 10Cr17, 10Cr17Mo, etc.; the representative steel grades of austenitic stainless steel are 06Cr19Ni10(304), 022Cr19Ni10(304L), 06Cr17Ni12Mo2(316), 022Cr17Ni12Mo2(316L), 06Cr18Ni11Ti(321), 06Cr18Ni11Nb(347), etc.; the representative steel grades of duplex stainless steel are 00Cr26Ni7Mo2Ti, etc.

There are more than 40 grades of austenitic stainless steel commonly used in China, and the most common one is type 18-8.
(1)1Cr17Mn6Ni15N; (2)1Cr18Mn8Ni5N; (3)1Cr18Ni9; (4)1Cr18Ni9Si3; (5)06Cr19Ni10(0Cr18Ni9); (6)00Cr19Ni10; (7)0Cr19Ni9N; (8)0Cr19Ni10NbN; (9)00Cr18Ni10N; 10)1Cr18Ni12; (11) 0Cr23Ni13; (12) 0Cr25Ni20; (13) 0Cr17Ni12Mo2; (14) 00Cr17Ni14Mo2; (15) 0Cr17Ni12Mo2N; (16) 00Cr17Ni13Mo2N; (17) 1Cr18Ni12Mo2Ti; (18) 0Cr18Ni12Mo2Ti; (19) 1Cr18Ni12Mo3Ti; (20) 0Cr18Ni12Mo3Ti; (21) 0Cr18Ni12Mo2Cu2; (22) 00Cr18Ni14Mo2Cu2; (23) 0Cr19Ni13Mo3; (24) 00Cr19Ni13Mo3; (25) 0Cr18Ni16Mo5; (26) 1Cr18Ni9Ti; (27) 0Cr18Ni10Ti; (28) 0Cr18Ni11Nb; (29) 0Cr18Ni13Si4
200 Series: Chromium-Nickel-Manganese Austenitic Stainless Steel
300 Series: Chromium-Nickel Austenitic Stainless Steel
Type 301: Good ductility, used for molded products. Can also be hardened quickly by machining. Good weldability. The wear resistance and fatigue strength are better than 304 stainless steel, and the products include: springs, steel structures, wheel covers.
Model 302: The corrosion resistance is the same as 304, and the strength is better due to the relatively high carbon content.
Model 303: By adding a small amount of sulfur and phosphorus, it is easier to cut than 304.
Model 304: General model; that is, 18/8 stainless steel. Products such as: corrosion-resistant containers, tableware, furniture, railings, medical equipment, and some mobile phone frames. The standard composition is 18% chromium plus 8% nickel. It is a non-magnetic stainless steel whose metallographic structure cannot be changed by heat treatment.
Model 304 L: Same characteristics as 304, but low carbon, so it is more corrosion-resistant and easy to heat treat, but the mechanical properties are poor, suitable for welding and products that are not easy to heat treat.
Model 304 N: Same characteristics as 304, a nitrogen-containing stainless steel, nitrogen is added to increase the strength of the steel.
Model 309: Better temperature resistance than 304.
Model 309 S: Contains a large amount of chromium and nickel, so it has good heat resistance and oxidation resistance. Products such as: heat exchangers, boiler components, and jet engines. Model 310
S: Contains the most chromium and nickel, so it has the best heat resistance and oxidation resistance. Heat exchangers, boiler components, and motor equipment.
Model 316: After 304, it is the second most widely used steel grade. It is mainly used in the food industry and surgical equipment. Molybdenum is added to give it a special corrosion-resistant structure. Because it has better chloride corrosion resistance than 304, it is also used as "marine steel". SS316 is usually used in nuclear fuel recovery devices. 18/10 grade stainless steel usually meets this application level. It is especially used in chemical, seaside and other corrosive environments, ship assembly, and building materials.
Model 316 L: Low carbon, so it is more corrosion-resistant and easy to heat treat. Products such as: chemical processing equipment, nuclear power generators, and refrigerant storage tanks.
Model 321: In addition to the addition of titanium to reduce the risk of material weld corrosion, other properties are similar to 304. It is suitable for welding brewing equipment, steam pipes, and aviation parts.
Model 347: Adding niobium, a stabilizing element, is suitable for welding aircraft equipment parts and chemical equipment.
Strictly speaking, since manganese in 200 series stainless steel plays a small role in the electrode potential of steel, the protective effect of the oxide film formed is very low and cannot play a role in corrosion resistance. Therefore, manganese-alloyed austenitic stainless steel cannot be called true "stainless steel". At present, many domestic manufacturers have reduced the content of chromium and nickel in stainless steel and increased the content of manganese for cost considerations. Experts believe that the reason why stainless steel is rust-proof is because of the presence of chromium and nickel. Reducing the content of these two components will reduce the rust resistance.
Comparison of standard grades in various countries
GB (China) ASTM (USA) JIS (Japan) DIN (Germany)
1Cr17Ni7 301 SUS301 X12CrNi177
1Cr18Ni9 302 SUS302 X12CrNi188
1Cr18Ni10 303 SUS303 X12CrNiS188 0Cr18Ni9
304 SUS304 X5CrNi189
00Cr19Ni10 304L SUS304L X2CrNi189 0Cr17Ni12Mo2
316 SUS316 X5CrNiMo1810
00Cr17Ni14Mo2 316L SUS316L X2CrNiMo1810
0Cr18Ni10Ti 321 SUS321 X10CrNiTi189
0Cr19Ni13Mo3 317 SUS317 X2CrNiMo1816

Comparison table and density table of stainless steel standard grades in various countries