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Type and characteristics of stainless steel

Time:2018-09-26Views:376

Stainless steel has two kinds of classification: one kind is according to the alloy element characteristic, divides into the chromium stainless steel and the chromium nickel stainless steel;The other type is divided into M stainless steel, F stainless steel, A stainless steel and A - F two-phase stainless steel according to the organization state of the steel under normal fire.


Martensitic stainless steel


Typical martensite stainless steels include 1Cr13~4Cr13 and 9Cr18


Good processing performance of 1Cr13 steel.Deep drawing, bending, edge rolling and welding without preheating.Preheating is not required before cold deformation of 2Crl3, but preheating is required 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 medical instrument surgical knife and wear-resistant parts.9Crl8 can be used as corrosion-resistant bearings and cutters.


Ferritic stainless steel


The Cr content of ferritin stainless steel is generally 13%~30% carbon content is less than 0.25%.Other alloying elements are sometimes added.The metallographic structure is mainly ferrite, which is not formed during heating and cooling.A. c. conversion cannot be strengthened by 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 make components with high corrosion resistance and low strength. It is widely used to make 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


Austenite stainless steel is developed to overcome the deficiency of corrosion resistance and brittleness of martensite stainless steel.The basic components are Crl8%, Ni8% short 18-8 steel.The results showed that the carbon content was less than 0.1%.


Austenitic stainless steel is commonly used in the manufacture of chemical equipment components such as nitric acid and sulfuric acid, cryogenic equipment components in the refrigeration industry, and can be used as stainless steel spring and clock spring after deformation reinforcement.

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


1. Intercrystalline 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 more intergranular erosion tendency.In addition, intercrystalline corrosion can also occur in the heat affected zone of welding parts.This is due to the Cr - rich Cr23C6 precipitated on the grain boundary.The poor chromium zone of the surrounding matrix is formed to corrode the galvanic cell.This intercrystalline corrosion is also present in the aforementioned ferritic stainless steel.


In engineering, the following methods are often used to prevent intergranular corrosion:


(1) reduce the carbon content in the steel, so that the carbon content in the steel is lower than the saturated solubility in austenite under the equilibrium state, that is, fundamentally solve the problem of chromium carbide (Cr23C6) precipitating on the grain boundary.Generally, the carbon content of steel decreases below 0.03% to meet the requirement of intercrystalline corrosion resistance.


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


(3) by adjusting the proportion of austenite forming elements and ferrite forming elements in steel, the austenite + ferrite body double phase structure can be obtained, among which ferrite accounts for 5%-12%.This kind of double - phase structure is not easy to produce intercrystalline corrosion.

(4) proper heat treatment can prevent intercrystalline 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 steels are prone to stress corrosion in chloride containing corrosive medium.When the Ni content reaches 8% to 10%, the austenitic stainless steel stress corrosion tendency is very large, and the stress corrosion tendency decreases gradually until the Ni content increases to 45~50%.


The main way to prevent stress corrosion of austenitic stainless steels is to add Si2~4% and control N content below 0.04% from smelting.In addition, the content of impurities such As P, Sb, Bi and As should be reduced As far As possible.In addition, a-f double phase steel can be selected, which is not sensitive to stress corrosion in Cl- and OH- media.When the initial micro crack encounters ferrite phase, the ferrite content should be about 6%.


3. Austenitic deformation reinforcement of stainless steel


Single-phase austenitic stainless steel has good cold deformation performance, can be cold-drawn into very thin steel wire, cold rolled into very thin steel strip or steel pipe.After a lot of deformation, the strength of steel is greatly improved, especially when it is rolled in the sub-zero temperature zone.Tensile strength can reach above 2000 MPa.This is because in addition to the effect of cold hardening, the addition of deformation induced M changes.


Austenitic stainless steel can be used to produce stainless spring, clockwork and wire rope in aviation structure.If welding is required after deformation, only spot welding process and deformation can be adopted to increase the stress corrosion tendency.Ferromagnetism due to partial intraday ->M conversion should be considered when used, such as in instrument parts.


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.

Heat treatment of austenitic stainless steel


Heat treatment of austenitic stainless steels include solution treatment, stabilization treatment and stress reduction 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, Cr23C6 is usually dissolved in austenite to prevent crystal corrosion, 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) de-stress treatment.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.


Two - phase austenitic - ferrite stainless steel


On the basis of austenitic stainless steel, appropriate increase of Cr content and reduction of Ni content can be obtained, which can be combined with the remelting treatment to obtain the stainless steel with austenite and ferritic dual-phase tissue (containing 40~60% cu-ferrite). Typical steel Numbers include 0Cr21Ni5Ti, 1Cr21Ni5Ti, OCr21Ni6Mo2Ti and so on.Biphasic stainless steel has good weldability, no heat treatment is needed after welding, and its intercrystalline corrosion and stress corrosion tendency are small.However, due to the high Cr content, easy to form the screen phase, use should be paid attention to.



   

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