All the Classifications of Stainless Steel | AISI classification
January 11, 2023
Stainless steel can be classified in many ways. The main classification methods are classification by organization, classification by chemical composition, and classification by use.
This is the oldest and most basic method still in use today. The metallographic structure of steel is closely related to its chemical composition and heat treatment state. It is classified according to whether there is phase change when heated to high temperature (or cooled from high temperature to room temperature) and the main metallographic structure at room temperature.
According to this classification, stainless steel can be divided into
Stainless steel can be basically divided into two major systems: chromium stainless steel and chromium-nickel stainless steel. Other stainless steels are generally developed on the basis of them, such as chromium-nickel-molybdenum stainless steel.
By main saving element
Stainless steel can be divided into nickel-saving stainless steel, nickel-free stainless steel, chromium-saving stainless steel, etc.
By some characteristic elements
Divided into high silicon stainless steel, high molybdenum stainless steel, and so on.
By use
By use of the environment
It is divided into nitric acid-resistant stainless steel, sulfuric acid-resistant stainless steel, urea-resistant stainless steel, and seawater-resistant stainless steel.
By corrosion resistance
Divided into anti-stress corrosion stainless steel, anti-pitting stainless steel, anti-abrasion stainless steel, and so on.
By functional characteristics
Divided into low-temperature stainless steel, non-magnetic stainless steel, free-cutting stainless steel, superplastic stainless steel, etc.
AISI /SAE Classification
The well-known standardization bodies engaged in the work of metal materials in the United States have their own standards and grade systems.
The most common product grades are AISI (American Iron and Steel Institute) and SAE (American Society of Automotive Engineers).
The way of expressing the steel number of the AISI standard is roughly the same as that of the SAE standard steel number system.
301: Good ductility, used for molded products. It can also be hardened by mechanical processing. Good weldability. Wear resistance and fatigue strength are better than 304 stainless steel, products such as springs, steel structures, and wheel covers.
302: The corrosion resistance is the same as 304, and the strength is better due to the relatively high carbon content.
303: It is easier to cut than 304 by adding a small amount of sulfur and phosphorus.
304: Universal model. The standard composition is 18 % chromium plus 8 % nickel. Products such as corrosion-resistant containers, tableware, furniture, railings, and medical equipment. It is non-magnetic. When the impurity content is high, it will occasionally show weak magnetism after processing. This weak magnetism can only be eliminated by heat treatment. It belongs to stainless steel whose metallographic structure cannot be changed by heat treatment.
304 L: It has the same characteristics as 304, but with low carbon, it is more resistant to corrosion and easy to heat treatment, but it has poor mechanical properties and is suitable for welding and products that are not easy to heat treat.
304 N: With the same characteristics as 304, it is nitrogen-containing stainless steel. Nitrogen is added to improve the strength of the steel.
309: Compared with 304, it has better temperature resistance.
309 S: With a lot of chromium and nickel, it has a good heat resistance and oxidation resistance. Products such as heat exchangers, boiler components, and jet engines.
310 S: Contains the most chromium and nickel, so it has the best heat and oxidation resistance for heat exchangers, boiler components, and electrical equipment.
316: After 304, the second most widely used steel grade is mainly used in the food industry and surgical equipment. The addition of molybdenum enables it to obtain a special structure that is resistant to corrosion. Because of its better resistance to chloride corrosion than 304, it is also used as “marine steel”. SS316 is usually used in nuclear fuel recovery units. Especially used in chemical, seaside, and other corrosive environments, ship assembly, and building materials.
316 L: Low carbon is more resistant to corrosion and easy to heat treatment. Products such as chemical processing equipment, nuclear power generators, and refrigerant storage tanks.
321: In addition to reducing the risk of corrosion of material welds due to the addition of titanium elements, other properties are similar to 304, suitable for welding brewing equipment, steam pipes, and aerospace parts.
347: Add stabilizing element niobium, suitable for welding aircraft parts and chemical equipment.
400 Series: Ferritic and martensitic stainless steels
408: Ferritic and martensitic stainless steels.
409: Except for the addition of titanium, the cheapest model, is usually used as a car exhaust pipe, which is ferritic stainless steel (chromium steel), suitable for welding, and has a lower cost for automobile exhaust pipes and petroleum equipment.
410: Except for the addition of titanium, the cheapest model (British and American) is usually used as a car exhaust pipe, which is ferritic stainless steel (chromium steel), suitable for welding, and has a lower cost for automobile exhaust pipes and petroleum equipment.
416: The addition of sulfur improves the processing properties of the material.
430: Ferritic stainless steel, decorative, magnetic, e.g. for car accessories. Good formability, but poor temperature resistance and corrosion resistance, suitable for fasteners, tableware, and furniture. Its standard chemical composition is 16-18% chromium, and its carbon content is low. This type of stainless steel is magnetic.
434: It contains molybdenum, so its corrosion resistance is better than that of 430. It is suitable for tableware, wipers, and car decoration.
440: High-strength cutting tool steel, with slightly higher carbon content, can obtain higher yield strength after proper heat treatment, and the hardness can reach 58HRC, which is one of the hardest stainless steel. The most common application example is “razor blades”. There are three commonly used models: 440A, 440B, 440C, and 440F (easy to process).
