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Types and characteristics of stainless steel

发布日期:2018-12-21  总浏览:

Stainless steel has two kinds of classification: one is according to the characteristics of alloying elements, divided into chromium stainless steel and chromium nickel stainless steel; The other one is divided into M stainless steel, F stainless steel, A stainless steel, A - F dual - phase stainless steel according to the microstructure of steel under normal state.

Martensitic stainless steel


The typical martensitic stainless steel has 1Cr13~4Cr13 and 9Cr18, etc


1Cr13 steel has good processing performance. Deep blanking, bending, flanging and welding may be carried out without preheating. 2Crl3 is not required to preheat before cold deformation, but it is required to preheat before welding. 1Crl3 and 2Cr13 are mainly used to make corrosion resistant structural parts such as turbine blades, etc., while 3Cr13 and 4Cr13 are mainly used to make ***** instrument surgical knives and wear resistant parts. 9Crl8 can be used for corrosion resistant bearing and cutter.


Ferritic stainless steel


The Cr content of ferrochrome stainless steel is generally 13%~30% and the carbon content is less than 0.25%. Other alloying elements are sometimes added. The microstructure is mainly ferrite, and there is no alpha in heating and cooling. Gamma conversion cannot be strengthened with heat treatment. Strong oxidation resistance. At the same time, it also has good thermal processing and a certain degree of cold processing. Ferritic stainless steel is mainly used to produce components with high corrosion resistance and low strength requirements. It is widely used in manufacturing equipment such as nitric acid, nitrogen fertilizer and pipelines used in chemical industry.


Typical ferritic stainless steels are Crl7, Cr25 and Cr28.


Three, austenitic stainless steel


Austenitic stainless steel is developed to overcome the deficiency of corrosion resistance and brittleness of martensitic stainless steel. The basic composition is Crl8%, Ni8% referred to as 18-8 steel. Its characteristic is that the total carbon content is less than 0.1%, and the single-phase austenite tissues are obtained by Cr and Ni coordination.


Austenitic stainless steel is generally used in the production of nitric acid, sulfuric acid and other chemical equipment components, cryogenic equipment components of the refrigeration industry and after deformation strengthening can be used as stainless steel springs and clockwork.


Austenitic stainless steel has good resistance to uniform corrosion, but in the local corrosion resistance, there are still the following problems:


1. Intergranular corrosion of austenitic stainless steel


Austenite stainless steel in 450 ~ 850 ℃ heat preservation or slow cooling, will be asked crystal corrosion. The higher the carbon content, the greater the intercrystalline corrosion tendency. In addition, intergranular corrosion can also occur in the heat affected zone of welding parts. This is due to Cr23C6 rich in Cr precipitated at the grain boundary. The formation of a chrome-poor region in the surrounding matrix, resulting in corrosion of the galvanic cell. This intergranular corrosion is also present in the ferritic stainless steel mentioned earlier.


The following methods are often used in engineering to prevent intergranular corrosion:


(1) reduce the carbon content in steel, so that the carbon content in steel is lower than the saturation solubility in austenite under the equilibrium state, that is, fundamentally solve the problem of chromium carbide (Cr23C6) precipitation on the grain boundary. Generally, the carbon content of steel can be reduced by less than 0.03% to meet the requirements of intergranular corrosion resistance.


(2) the addition of Ti, Nb and other elements that can form stable carbide (TiC or NbC) can prevent the precipitation of Cr23C6 on the grain boundary and prevent the intergranular corrosion of upper austenite stainless steel.


(3) by adjusting the proportion of austenite forming elements and ferrite forming elements in steel, the austenite + ferrite biphasic structure was obtained, in which ferrite accounted for 5% to 12%. This biphase structure is not susceptible to intercrystalline corrosion.


(4) proper heat treatment process can prevent intergranular corrosion and obtain excellent corrosion resistance.


2. Stress corrosion of austenitic stainless steel


Stress (mainly tensile Stress) and Corrosion cracking caused by the combination of known as Stress Corrosion cracking, hereinafter referred to as SCC (Stress Crack Corrosion). Austenitic stainless steel is prone to stress corrosion in corrosive media containing chloride ions. When the Ni content reaches 8% to 10%, the stress corrosion tendency of austenitic stainless steel is very large, and the stress corrosion tendency decreases gradually as the Ni content continues to increase ***45~50%.


The main way to prevent stress corrosion of austenitic stainless steel is to add Si2~4% and control the N content below 0.04% from smelting. In addition, P, Sb, Bi, As and other impurities should be minimized. In addition, a-f biphase steel can be selected, which is not sensitive to stress corrosion in Cl- and OH- media. The ferrite content should be about 6% when the initial microcracks do not continue to expand after encountering ferrite phase.


3. Deformation strengthening of austenitic stainless steel


Single phase austenitic stainless steel has good cold deformation performance, can be cold drawn into very fine steel wire, cold rolled into very thin steel belt or steel pipe. After a large number of deformation, the strength of steel is greatly improved, especially in the minus temperature zone rolling, the effect is more significant. The tensile strength can reach more than 2000 MPa. This is because in addition to the effect of cold hardening, deformation induced M transformation is also superimposed.


Austenitic stainless steel after deformation strengthening can be used to manufacture stainless spring, clockspring, aviation structure of the wire rope. If welding is required after deformation, only spot welding process and deformation can increase the stress corrosion tendency. Part gamma-> M conversion and ferromagnetism, in use (such as instrument parts) should be considered.


Recrystallization temperature change with form variables and, when the form of a variable is 60%, the recrystallization temperature to 650 ℃ cold deformation of austenitic stainless steel recrystallization annealing temperature is 850 ~ 1050 ℃, 850 ℃ heat preservation 3 h, burn to 1050 ℃, and water.


Austenitic heat treatment of stainless steel


Austenitic stainless steel commonly used heat treatment processes are: solid solution treatment, stabilization treatment and stress treatment.


(1) solid solution treatment. Steel heating to 1050 ~ 1150 ℃ after water quenching, the main purpose is to make the carbon dissolved in austenite, and the state reserves to room temperature, so that the corrosion resistance of steel will have improved greatly. As mentioned above, in order to prevent crystal corrosion, Cr23C6 is usually dissolved in austenite by solid solution, and then rapidly cooled. Air cooling can be used for thin wall parts and water cooling is generally used.


(2) stabilization treatment. It's usually done after solid solution treatment, often used for 18-8 steel containing Ti, Nb, solid processing, steel heating to 850 ~ 880 ℃ air cooling after heat preservation, the Cr carbide completely dissolved, and titanium carbide is not completely dissolved, and full precipitation in the cooling process, making it impossible for the carbon and chromium carbide formation, thus effectively eliminate the intergranular corrosion.


(3) stress reduction. To deal with stress is to eliminate the residual stresses of the steel after cold working or welding general heating to 300 ~ 350 ℃ tempering heat treatment process. For excluding stabilizing element Ti, Nb steel, heating temperature does not exceed 450 ℃, so as to avoid precipitation of chromium carbide caused by intergranular corrosion. For ultra-low carbon and stainless steel containing Ti, Nb of cold working parts and components, in 500 ~ 950 ℃, heating, and then slow cooling, eliminate stress, eliminate the welding stress limit temperature), can reduce the intercrystalline corrosion and stress corrosion resistance of steel.


Austenitic ferrite duplex stainless steel


On the basis of austenitic stainless steel, increase the Cr content and reduce the Ni content, and with the back melting treatment, you can get a stainless steel with austenitic and ferrite biphasic structure (including 40-60% ferrite), typical steel number is 0Cr21Ni5Ti, 1Cr21Ni5Ti, OCr21Ni6Mo2Ti and so on. Duplex stainless steel has good weldability, does not need heat treatment after welding, and its intergranular corrosion, stress corrosion tendency is small. But because of the high Cr content, it is easy to form sigma phases.


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