Classification of stainless steel

Based on their main chemical composition, they are classified into chromium stainless steel, chromium-nickel stainless steel, chromium-manganese-nitrogen stainless steel, etc； They can also be classified by their performance characteristics into acid-resistant stainless steels and heat-resistant stainless steels, etc ； It is usually classified based on the metallographic structure. Classified by microstructural morphology, they are: ferritic (F) stainless steels, martensitic (M) stainless steels, austenitic (A) stainless steels, austenitico-ferritic (A-F) duplex stainless steels, austenitico-martensitic (A-M) duplex stainless steels, and precipitation-hardening (PH) stainless steels. International Method for Designating Stainless Steels: The American Iron and Steel Institute uses three digits to designate various standard grades of malleable stainless steels. Among them: ① Austenitic stainless steels are designated by the numbers in the 200 and 300 series, ② Ferritic and martensitic stainless steels are indicated by the numbers in the 400 series. For example, some of the more common austenitic stainless steels are designated by the codes 201, 304, 316, and 310. Ferritic stainless steels are labeled as 430 and 446, while martensitic stainless steels are identified by the codes 410, 420, and 440C. For duplex (austenitic-ferritic) steels, ④ Stainless steels, precipitation-hardening stainless steels, and high-alloy materials with an iron content of less than 50% are usually named using proprietary names or trademarks.

Based on their main chemical composition, they are classified into chromium stainless steel, chromium-nickel stainless steel, chromium-manganese-nitrogen stainless steel, etc； They can also be classified by their performance characteristics into acid-resistant stainless steels and heat-resistant stainless steels, etc ； It is usually classified based on the metallographic structure. Classified by microstructural morphology, they are: ferritic (F) stainless steels, martensitic (M) stainless steels, austenitic (A) stainless steels, austenitico-ferritic (A-F) duplex stainless steels, austenitico-martensitic (A-M) duplex stainless steels, and precipitation-hardening (PH) stainless steels. Ferritic stainless steel has a ferritic microstructure, with a chromium content ranging from 11.5% to 32.0%. As the chromium content increases, its acid resistance also improves; the addition of molybdenum (Mo) further enhances its resistance to acid corrosion and stress corrosion. The **standard grades of this type of stainless steel include 00Cr12, 1Cr17, 00Cr17Mo, 00Cr30Mo2, etc.** Marshallite-type stainless steel has a microstructure consisting of martensite. In this type of steel, the chromium content ranges from 11.5% to 18.0%, but the carbon content can reach up to 0.6%. An increase in carbon content enhances the strength and hardness of steel. A small amount of nickel added to such steels can promote the formation of martensite, while also improving their corrosion resistance. This type of steel has poor weldability. The steel plates classified under **standard grades include 1Cr13, 2Cr13, 3Cr13, 1Cr17Ni2, etc.** Austenitic stainless steel has an austenitic microstructure. It is a stainless steel with an austenitic structure, formed by adding an appropriate amount of nickel to high-chromium stainless steel (with the mass fraction of nickel ranging from 8% to 25%). Austenitic stainless steels are based on the Cr18Ni19 iron-based alloy; depending on their various applications, they have evolved into the series of chromium-nickel austenitic stainless steels shown in Figures 1-2. Austenitic stainless steels generally belong to the category of corrosion-resistant steels, and they are the most widely used type of steel. Among them, 18-8 stainless steel is the most representative; it possesses good mechanical properties, making it easy to machine, stamp, and weld. It exhibits excellent corrosion resistance and good heat resistance in oxidizing environments. However, it is particularly sensitive to media containing chloride ions (CL-), and is prone to stress corrosion. Stainless steel type 18-8 is further divided into three grades based on the carbon content in its chemical composition: normal carbon grade (Wc≤0.15%), low carbon grade (Wc≤0.08%), and ultra-low carbon grade (Wc≤0.03%). For example, the three types of steel plates specified in our country’s **standards – 1Cr18Ni9Ti, 0Cr18Ni9, and 00Cr17Ni14M02 – fall under those three grades respectively. Many countries around the world are facing a shortage of nickel reserves. To save nickel, as early as the 1940s and 1950s, the world began to replace some of the nickel in 18-8 stainless steel with manganese and nitrogen. The steel grades that have been developed and included in **standards are 1Cr17Mn6Ni5N and 0Cr19Ni9N, among others. Austenite-ferritic stainless steels have a microstructure consisting of austenite and ferrite. Stainless steels with a ferrite volume fraction of less than 10% are steel grades developed on the basis of austenitic steels. Precipitation-hardening stainless steels can be classified into three categories based on their microstructural morphology: precipitation-hardening semi-austenitic, precipitation-hardening martensitic, and precipitation-hardening austenitic stainless steels. The standard steel grades listed in our country are 0Cr17Ni7A, 0Cr17Ni4Cu4Nb, and 0Cr15Ni7Mo2Al; they belong to the precipitation-hardening semi-austenitic stainless steels. The microstructural feature of this steel is an austenite structure together with a ferrite phase accounting for 5% to 20% by volume in the solution-treated or annealed state. After a series of heat treatment or mechanical deformation processes, the austenite in this steel transforms into martensite, and the desired high strength is achieved through age-hardening. This steel possesses excellent formability and good weldability, and can be used as a super-high strength material in the nuclear, aviation, and aerospace industries.

Put simply, stainless steel is steel that does not rust easily; in fact, some types of stainless steel possess both rust resistance and acid resistance (corrosion resistance). The rust resistance and corrosion resistance of stainless steel are due to the formation of a chromium-rich oxide film (passivation film) on its surface. This resistance to rust and corrosion is relative. Tests have shown that in weak media such as air and water, as well as in oxidizing media such as nitric acid, the corrosion resistance of steel increases with the increase in the chromium content in the steel. When the chromium content reaches a certain percentage, there is a sudden change in the steel’s corrosion resistance – it goes from being prone to rusting to less prone to rusting, and from being non-corrosion resistant to corrosion resistant. There are many ways to classify stainless steel. Classified by their microstructure at room temperature, there are martensitic, austenitic, ferritic, and duplex stainless steels ； Classified by their main chemical components, they can be basically divided into two major categories: chromium stainless steels and chromium-nickel stainless steels ； Classified by application, there are nitric acid-resistant stainless steels, sulfuric acid-resistant stainless steels, seawater-resistant stainless steels, and so on. Classified by corrosion resistance type, they can be divided into pitting-resistant stainless steels, stress-corrosion-resistant stainless steels, intergranular corrosion-resistant stainless steels, and so on ； Classified by functional characteristics, they can be further divided into non-magnetic stainless steel, free-cutting stainless steel, low-temperature stainless steel, high-strength stainless steel, and so on. Due to its excellent corrosion resistance, formability, compatibility, and strength and toughness over a wide temperature range, stainless steel is widely used in industries such as heavy industry, light industry, household goods manufacturing, and building decoration. Austenitic stainless steel is a type of stainless steel that has an austenitic structure at room temperature. When steel contains about 18% Cr, 8%~10% Ni, and about 0.1% C, it has a stable austenitic structure. Austenitic chromium-nickel stainless steels include the well-known 18Cr-8Ni steel, as well as high Cr-Ni series steels that were developed by increasing the contents of Cr and Ni and adding elements such as Mo, Cu, Si, Nb, and Ti. Austenitic stainless steel is non-magnetic and possesses high toughness and plasticity, but it has low strength; it cannot be strengthened through phase transformation and can only be strengthened by cold working. When elements such as S, Ca, Se, and Te are added, it exhibits good machinability. In addition to resisting corrosion in oxidizing acidic media, such steels can also withstand corrosion by sulfuric acid, phosphoric acid, as well as formic acid, acetic acid, urea, etc., if they contain elements such as Mo and Cu. If the carbon content in such steels is below 0.03% or if they contain Ti and Ni, their resistance to intergranular corrosion can be significantly improved. Austenitic stainless steels with high silicon content exhibit good corrosion resistance in concentrated nitric acid. Due to their comprehensive and excellent overall properties, austenitic stainless steels are widely used in various industries. Ferritic stainless steel A stainless steel that is primarily composed of a ferritic structure in its operating condition. It contains 11% to 30% chromium and has a body-centered cubic crystal structure. These types of steel generally do not contain nickel; sometimes they also contain small amounts of elements such as Mo, Ti, and Nb. They feature a high thermal conductivity, a low coefficient of expansion, good oxidation resistance, and excellent resistance to stress corrosion. They are often used to manufacture components that can withstand corrosion caused by the atmosphere, water vapor, water, and oxidizing acids. Such steels have disadvantages such as poor plasticity and a significant reduction in plasticity and corrosion resistance after welding, which limits their application. The use of secondary refining technologies (AOD or VOD) enables the reduction of interstitial elements such as carbon and nitrogen, which is why these types of steel are widely used. Austenite-ferrite duplex stainless steel is a type of stainless steel in which austenite and ferrite structures each make up about half of it. At low C levels, the Cr content ranges from 18% to 28%, while the Ni content ranges from 3% to 10%. Some steels also contain alloying elements such as Mo, Cu, Si, Nb, Ti, and N. This type of steel possesses the characteristics of both austenitic and ferritic stainless steels. Compared to ferritic steels, it has higher plasticity and toughness, no room-temperature brittleness, and significantly improved resistance to intergranular corrosion as well as weldability. At the same time, it retains the 475°C brittleness and high thermal conductivity associated with ferritic stainless steels, as well as superplasticity. Compared to austenitic stainless steels, it has higher strength, as well as significantly improved resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel exhibits excellent pitting resistance and is also a nickel-saving stainless steel. Maraging stainless steel: A type of stainless steel whose mechanical properties can be adjusted through heat treatment; in simple terms, it is a type of stainless steel that can be hardened. The typical grades are of the Cr13 type, such as 2Cr13, 3Cr13, 4Cr13, etc. It has high hardness after quenching, and different tempering temperatures result in various combinations of strength and toughness; it is mainly used for steam turbine blades, tableware, and surgical instruments. Based on differences in chemical composition, martensitic stainless steels can be divided into two categories: martensitic chromium steel and martensitic chromium-nickel steel. Based on their mechanisms of formation and strengthening, they can also be classified into martensitic stainless steels, martensitic and semi-austenitic (or semi-martensitic) precipitation-hardening stainless steels, and maraging stainless steels. 304, 304L, 316, and 317 are primarily distinguished by the chemical elements they contain; for example, 304 generally contains 8–12 percent nickel, although in China it actually contains only 8 percent nickel, with the exception of 304H. 316 contains 12 percent nickel plus an additional 2 percent Mo; 316L has 14 percent nickel, with again 2 percent Mo.

It is mainly classified by chemical composition: 304 and 316 are American standards, while 0Cr17Ni7A is China’s steel standard

Austenitic stainless steels account for a large proportion of stainless steel applications, with basically those starting with 2 and 3. The corrosion resistance of those with a 3-digit code is better than that of those with a 2-digit code. Stainless steels starting with 4 are often martensitic; they have high hardness and are commonly used for making cutting tools and similar items. However, its corrosion resistance is not that good. So it’s not that corrosion resistance of numbers starting with 4 is better than that of numbers starting with 3. Two-phase stainless steel is primarily composed of austenite and ferrite; its advantage is that it has better corrosion resistance than austenitic stainless steel. However, its drawback is poor high-temperature performance; due to the presence of ferrite, the problem of brittleness at 475°C still exists. Therefore, it is usually used below 280°C, and it has strong resistance to chloride corrosion. Ferritic stainless steels are less commonly used in contacts, so it’s not clear.

Thank you. Studying*. I need some basic knowledge of codes – for example, what do 306, 316L, etc. mean?

8# hanhai taos du

Based on their main chemical composition, they are classified into chromium stainless steel, chromium-nickel stainless steel, chromium-manganese-nitrogen stainless steel, etc； They can also be classified by their performance characteristics into acid-resistant stainless steels and heat-resistant stainless steels, etc ； It is usually classified based on the metallographic structure. Classified by microstructural morphology, they are: ferritic (F) stainless steels, martensitic (M) stainless steels, austenitic (A) stainless steels, austenitico-ferritic (A-F) duplex stainless steels, austenitico-martensitic (A-M) duplex stainless steels, and precipitation-hardening (PH) stainless steels. 304 and 316 are international standards for identifying stainless steels; the specific differences lie in their chemical compositions 　　

The answer above is very correct and comprehensive.

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