Classification of stainless steel

Classification of stainless steel
According to the main chemical composition, it can be divided into chromium stainless steel, chromium-nickel stainless steel, chromium-manganese-nitrogen stainless steel, chromium-nickel-molybdenum stainless steel, ultra-low carbon stainless steel, high-molybdenum stainless steel, high-purity stainless steel, etc.; according to the performance characteristics and uses of steel, such as nitric acid-resistant (nitric acid grade) stainless steel,
according to the main chemical composition, it can be divided into chromium stainless steel, chromium-nickel stainless steel, chromium-manganese-nitrogen stainless steel, chromium-nickel-molybdenum stainless steel, ultra-low carbon stainless steel, high-molybdenum stainless steel, high-purity stainless steel, etc.; according to the performance characteristics and uses of steel, such as nitric acid-resistant (nitric acid grade) stainless steel, sulfuric acid-resistant stainless steel, pitting-resistant stainless steel, stress-resistant stainless steel, high-strength stainless steel, etc.
Classification according to the functional characteristics of steel, such as low-temperature stainless steel, non-magnetic stainless steel, free-cutting stainless steel, superplastic stainless steel, etc. It is usually classified by metallographic structure. According to the metallographic structure, it is classified into: ferrite (F) type stainless steel, martensite (M) type stainless steel, austenite (A) type stainless steel, austenite-ferrite (AF) type duplex stainless steel, austenite-martensite (AM) type duplex stainless steel and precipitation hardening (PH) type stainless steel.
The above classification is only based on the matrix structure of steel. In addition to the three basic types of stainless steel structures mentioned above, there are also transitional duplex stainless steels such as martensite-ferrite, austenite-ferrite, austenite-martensite, and precipitation hardening stainless steels with martensite-carbide structures.


Ferritic steel
Low-carbon chromium stainless steel containing more than 14% chromium, chromium stainless steel containing more than 27% chromium and any carbon content, and stainless steel with molybdenum, titanium, niobium, silicon, aluminum, tungsten, vanadium and other elements added to the above composition. The elements that form ferrite in the chemical composition are absolutely dominant, and the matrix structure is ferrite. The structure of this type of steel in the quenched (solid solution) state is ferrite, and a small amount of carbides and intermetallic compounds can be seen in the annealed and aged state. Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28, etc. belong to this category. Ferritic stainless steel has good corrosion resistance and oxidation resistance due to its high chromium content, but its mechanical and process properties are poor. It is mostly used in acid-resistant structures with low stress and as oxidation-resistant steel

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This type of steel is in the y+a (or δ) two-phase state at high temperature, and yM transformation occurs during rapid cooling. Ferrite is still retained, and the structure at room temperature is martensite and ferrite. Due to the different composition and heating temperature, the amount of ferrite in the structure can vary from a few percent to tens of percent. 0Crl3 steel, lCrl3 steel, 2Cr13 steel with chromium on the upper limit and carbon on the lower limit, Cr17Ni2 steel, Cr17wn4 steel, and many steel grades in many modified 12% chromium heat-resistant steels (this type of steel is also called heat-resistant stainless steel) developed on the basis of ICrl3 steel, such as Cr11MoV, Cr12WMoV, Crl2W4MoV, 18Crl2WMoVNb, etc., all belong to this category.
Ferrite-martensitic steel can be partially strengthened by quenching, so higher mechanical properties can be obtained. However, their mechanical properties and process properties are largely affected by the content and distribution of ferrite in the structure. This type of steel belongs to two series according to the chromium content in the composition: 12~14% and 15~18%. The former has the ability to resist the atmosphere and weakly corrosive media, and has good shock absorption and a small linear expansion coefficient; the latter has corrosion resistance comparable to that of ferrite acid-resistant steel with the same chromium content, but to a certain extent also retains some of the shortcomings of high-chromium ferrite steel.

Martensitic steel
This type of steel is in the y phase region at normal quenching temperature, but their y phase is only stable at high temperatures, and the M point is generally around 3OO℃, so it transforms into martensite when cooled. This type of steel includes 2Cr13, 2Cr13Ni2, 3Cr13 and some modified 12% chromium heat-resistant steels, such as 13Cr14NiWVBA, Cr11Ni2MoWVB steel, etc. The mechanical properties, corrosion resistance, process properties and physical properties of martensitic stainless steel are similar to those of ferrite-martensitic stainless steel containing 12~14% chromium. Since there is no free ferrite in the organization, the mechanical properties are higher than those of the above steels, but the overheat sensitivity during heat treatment is lower.

Martensitic-Carbide Steel
The carbon content of the eutectic point of Fe-C alloy is 0.83%. In stainless steel, due to the chromium, the S point shifts to the left. Steels containing 12% chromium and more than 0.4% carbon (Figure 11-3), and steels containing 18% chromium and more than 0.3% carbon (Figure 11-3) are all hypereutectoid steels. When this type of steel is heated at normal quenching temperature, the secondary carbides cannot be completely dissolved in austenite, so the structure after quenching is composed of martensite and carbides.
There are not many stainless steel grades belonging to this category, but they are some stainless steels with relatively high carbon content, such as 4Crl3, 9Cr18, 9Crl8MoV, 9Crl7MoVCo steel, etc. 3Crl3 steel with a carbon content close to the upper limit may also have such a structure when quenched at a lower temperature. Due to the high carbon content, although the above three steel grades such as 9Cr18 contain more chromium, their corrosion resistance is only equivalent to that of stainless steel containing 12~14% germanium. The main use of this type of steel is for parts that require high hardness and wear resistance, such as cutting tools, bearings, springs and medical equipment.

Austenitic steel
This type of steel contains more elements that expand the y zone and stabilize austenite. At high temperatures, it is all y phase. When cooled, because the Ms point is below room temperature, it has an austenitic structure at room temperature. 18-8, 18-12, 25-20, 20-25Mo and other chromium-nickel stainless steels, and low-nickel stainless steels that replace part of nickel with manganese and add nitrogen, such as Cr18Mnl0Ni5, Cr13Ni4Mn9, Cr17Ni4Mn9N, Cr14Ni3Mnl4Ti steels, etc. belong to this category.
Austenitic stainless steel has many advantages mentioned above. Although its mechanical properties are also relatively low, and it cannot be strengthened by heat treatment like ferritic stainless steel, its strength can be improved by cold working deformation and work hardening. The disadvantage of this type of steel is that it is sensitive to intergranular corrosion and stress corrosion, which needs to be eliminated through appropriate alloy additives and process measures.

Austenite-ferrite steel
This type of steel is in an austenite-ferrite duplex state because the effect of expanding the y zone and stabilizing austenite elements is not enough to make the steel have pure austenite structure at room temperature or very high temperature. The amount of ferrite can also vary in a large range due to different compositions and heating temperatures.
There are many stainless steels belonging to this category, such as low-carbon 18-8 chromium-nickel steel, 18-8 chromium-nickel steel with titanium, niobium and molybdenum, especially ferrite can be seen in the organization of cast steel, in addition to chromium-manganese stainless steel (such as Cr17Mnll) with chromium content greater than 14~15% and carbon less than 0.2%, and most of the chromium-manganese-nitrogen stainless steels currently studied and applied. Compared with pure austenitic stainless steel, this type of steel has many advantages, such as higher yield strength, higher resistance to intergranular corrosion, low sensitivity to stress corrosion, less tendency to produce hot cracks during welding, good casting fluidity, etc. The disadvantages are poor pressure processing performance, greater tendency to pitting corrosion, easy to produce c-phase brittleness, and weak magnetism under strong magnetic fields. All these advantages and disadvantages come from ferrite in the organization.

Austenitic martensitic steel
The Ms point of this type of steel is lower than room temperature. After solution treatment, it is an austenitic organization, which is easy to form and weld. Usually two process methods can be used to make it undergo martensitic transformation. One is to heat the steel at 700-800 degrees after solution treatment, so that austenite is transformed into a medium-stable state due to the precipitation of chromium carbide, and the Ms point rises above room temperature, and it transforms into martensite when cooled; the other is to directly cool the steel to between the Ms and Mf points after solution treatment, so that austenite is transformed into martensite. The latter method can obtain higher corrosion resistance, but the interval time from solution treatment to deep cooling should not be too long, otherwise the strengthening effect of deep cooling will be reduced due to the aging stabilization of austenite. After the above treatment, the steel is aged at 400-500 degrees to further strengthen the precipitated intermetallic compounds. Typical steel grades of this type of steel are 17Cr-7Ni-A1, 15Cr-9Ni-A1, 17Cr-5Ni-Mo, 15Cr-8Ni-Mo-A1, etc.

Semi-austenitic precipitation hardening stainless steel.
This type of steel is a new type of stainless steel developed and applied in the late 1950s. Its general characteristics are high strength (C can reach 100-150) and good thermal strength. However, due to the low chromium content and the precipitation of chromium carbide during heat treatment, its corrosion resistance is lower than that of standard austenitic stainless steel. It can also be said that the high strength of this type of steel is obtained by sacrificing part of the corrosion resistance and other properties (such as non-magnetic). At present, this type of steel is mainly used in the aviation industry and rocket missile production. It is not widely used in general machinery manufacturing, and it is also classified as a series of ultra-high strength steels.