Types of stainless steel and features
There are two classifications of stainless steel: one is based on the characteristics of the alloy elements into chromium stainless steel and chrome-nickel stainless steel; the other one is based on the state of the steel in the normalized state organization is divided into M stainless steel, F stainless steel, A stainless steel , A an F duplex stainless steel.
A martensitic stainless steel
Typical of martensitic stainless steel with good processing performance 1Cr13 ~ 4Cr13 and 9Cr18 1Cr13 steel and other. Without preheated deep drawing, bending, curling and welding. 2Crl3 before cold deformation does not require preheating, but be preheated before welding, 1Crl3,2Cr13 mainly used to produce corrosion resistant structural parts such as turbine blades, etc., and is mainly used to make medical devices 3Cr13,4Cr13 scalpel and wear parts; 9Crl8 do corrosion of bearings and tools.
Second, ferritic stainless steel
The amount of Cr-containing ferritic stainless steel is generally 13% to 30% combined carbon content less than 0.25%. Sometimes also joined other alloying elements. Microstructure mainly ferrite, heating and cooling process is not α <=> γ transformation, can not be strengthened by heat treatment. Strong antioxidant. It also has good hot workability and cold workability certain. Ferritic stainless steel is mainly used to make demands for higher corrosion resistance and strength requirements of the lower member, is widely used in pipe manufacturing nitric acid, nitrogen and other equipment and chemicals used and so on.
Typical ferritic stainless steel has Crl7 type, Cr25 and Cr28 type type.
Three, austenitic stainless steel
Austenitic stainless steel for corrosion resistance is overcome Markov insufficient and excessive brittleness and develop. The basic components of Crl8%, Ni8% referred 18-8 steel. Its characteristics are combined carbon content is less than 0.1%, the use of Cr, Ni with a single phase austenite.
Austenitic stainless steel is generally used for chemical manufacturing nitric acid, sulfuric acid and other equipment components, frozen industrial cryogenic equipment components and can be used by the deformation strengthening steel springs and clockwork and so on.
Austenitic stainless steel has good resistance to uniform corrosion performance, but in terms of localized corrosion, there are still the following questions:
1, between austenitic stainless steel corrosion
Albright for stainless steel in the 450 ~ 850 ℃ holding or slow cooling will occur intergranular corrosion. Carbon content, the greater the tendency to intergranular corrosion. In addition, the heat-affected zone of welded parts will appear intergranular corrosion. This is due to the grain boundary precipitation of Cr-rich Cr23C6. So that the surrounding matrix generating chromium depleted zone, thereby forming a galvanic cell corrosion caused. This corrosion phenomenon between the aforementioned ferritic stainless steels also exist.
Often used on the project in several ways to prevent intergranular corrosion:
(1) reduce the amount of carbon in the steel, the steel combined carbon content below the saturation solubility in austenite at equilibrium, that is, from a fundamental solution of chromium carbide (Cr23C6) on the grain boundaries of the problem . Usually combined carbon content in steel fell 0.03% to meet resistance to intergranular corrosion requirements.
(2) was added Ti, Nb, etc. to form stable carbides (TiC or NbC) elements, to avoid grain boundary precipitation Cr23C6, to the anti between austenitic stainless steel to intergranular corrosion.
(3) by adjusting the ratio of austenitic and ferritic elements forming elements are formed, it has austenitic + ferritic phase organization, which accounts for 5% of ferrite to 12%. This organization is not easy to produce duplex intergranular corrosion.
(4) the use of appropriate heat treatment process can prevent intergranular corrosion, get the best corrosion resistance.
2, austenitic stainless steel stress corrosion
Stress cracking (mainly tensile stress) and the combined effects of corrosion caused by stress corrosion cracking is called, referred to SCC (StressCrackCorrosion). Prone to stress corrosion cracking of austenitic stainless steel in corrosive media containing chlorine ions. When the amount of Ni containing 8% to 10% of austenitic stainless steel stress corrosion tendency largest Ni content continues to increase to 45 to 50% stress corrosion tendency gradually decreases until it disappears.
The main way to prevent stress corrosion cracking of austenitic stainless steel is joined Si2 ~ 4% and will be controlled from the smelting N content of 0.04% or less. It should also minimize the content of P, Sb, Bi, As and other impurities. Another choice of AF duplex steel, it Cl- and OH- media is not sensitive to stress corrosion. When the initial fine cracks after experiencing the ferrite phase is no longer continue to expand, the ferrite content should be around 6%.
3, austenitic stainless steel for deformation strengthening
Single phase austenite stainless steel has good cold deformation properties, can be cold drawn into a very thin wire, into a thin strip or cold-rolled steel. After a lot of distortion, greatly improve the strength of steel, especially when rolling at minus temperature zone, the effect is more significant. Tensile strength of up to 2000MPa above. This is because in addition to the effect of work hardening, but also superimposed deformation induced by M transition.
S for stainless steel after deformation strengthening used to make stainless spring, clocks spring, rope and other aviation structures. For deformation after welding, spot welding process can only deformation increases the stress corrosion tendency. And because some γ-> M transition produced ferromagnetism in use (such as parts of the instrument) should be considered.
Recrystallization temperature changes with deformation when deformation is 60%, the recrystallization temperature down to 650 ℃ cold deformation of austenitic stainless steel recrystallization annealing temperature of 850 ~ 1050 ℃, 850 ℃ need insulation 3h, 1050 ℃ When you can burn through, and then cooled.
4, austenitic stainless steel for heat treatment
Austenitic stainless steels commonly used in heat treatment process are: solution treatment, stabilization process and to stress treatment.
(1) solution treatment. After the steel is heated to 1050 ~ 1150 ℃ water quenching, the main purpose is to make carbide dissolved in austenite, and retain this status to room temperature, so that the corrosion resistance of the steel will be greatly improved. As described above, in order to prevent intergranular corrosion, usually the solution treatment, so that Cr23C6 dissolved in austenite, and then rapidly cooled. For thin-walled parts can be air-cooled, water-cooled general.
(2) the stabilization process. Is generally carried out after the solution treatment, commonly containing Ti, Nb steel 18-8, after the solid processing, the steel is heated to 850 ~ 880 ℃ air cooling after incubation, this time complete dissolution of Cr carbide, and titanium removal carbide does not completely dissolve, and sufficiently precipitate during cooling, so that carbon can no longer be formed of chromium carbide, thereby effectively eliminating the intergranular corrosion.
(3) to deal with stress. To deal with stress is to eliminate steel cold or heat treatment process after welding residual stress is generally heated to 300 ~ 350 ℃ tempered. For free of stabilizing elements Ti, Nb steel, the heating temperature does not exceed 450 ℃, in order to avoid precipitation of chromium carbides cause intergranular corrosion. For ultra-low carbon and cold parts and weldments containing Ti, Nb stainless steel, to be in 500 ~ 950 ℃, heated and then slowly cooled, eliminating stress (eliminate welding stress take maximum temperature), can reduce the tendency to intergranular corrosion and improve steel The stress corrosion resistance.
Fourth, austenitic - ferritic duplex stainless steel
On the basis of austenitic stainless steel, the appropriate increase in the Cr content and reduce the Ni content and solution treatment and return match, having obtained austenitic and ferritic duplex organizations (including 40 ~ 60% δ- ferrite ) of stainless steel are typical 0Cr21Ni5Ti, 1Cr21Ni5Ti, OCr21Ni6Mo2Ti so on. Duplex stainless steel has good weldability, after welding without heat treatment, and intergranular corrosion, stress corrosion tendency is also smaller. However, as with high Cr content, easy to form σ phase, should be used to pay attention. |
