There are two classification methods for stainless steel: one is divided into chromium stainless steel and chromium-nickel stainless steel according to the characteristics of alloy elements; the other is divided into M according to the organizational state of steel in the normalizing state. Stainless steel, F stainless steel, A stainless steel, A-F duplex stainless steel.
1. Martensitic stainless steel
Typical martensitic stainless steels include 1Cr13~4Cr13 and 9Cr18, etc. 1Cr13 steel has good processing performance. It can be deep drawing, bending, crimping and welding without preheating. 2Crl3 does not require preheating before cold deformation, but preheating is required before welding. 1Crl3 and 2Cr13 are mainly used to make corrosion-resistant structural parts such as steam turbine blades, while 3Cr13 and 4Cr13 are mainly used to make surgical scalpels and wear-resistant parts for medical devices; 9Crl8 can be used as corrosion-resistant bearings and cutting tools.
2. Ferritic stainless steel
The Cr content of ferritic stainless steel is generally 13%~30% and the carbon content is less than 0.25%. Sometimes other alloying elements are added. The metallographic structure is mainly ferrite. There is no α<=>γ transformation during heating and cooling, and it cannot be strengthened by heat treatment. Strong antioxidant properties. At the same time, it also has good hot workability and certain cold workability. Ferritic stainless steel is mainly used to make components that require higher corrosion resistance but lower strength requirements. It is widely used in the manufacturing of equipment for the production of nitric acid, nitrogen fertilizers, and pipelines used in the chemical industry.
Typical ferritic stainless steels include Crl7 type, Cr25 type and Cr28 type.
3. Austenitic stainless steel
Austenitic stainless steel was developed to overcome the insufficient corrosion resistance and excessive brittleness of Martensitic stainless steel. The basic ingredients are Crl8% and Ni8%, referred to as 18-8 steel. Its characteristic is that the carbon content is less than 0.1%, and the single-phase austenite structure is obtained by combining Cr and Ni.
Austenitic stainless steel is generally used to manufacture chemical equipment components such as nitric acid and sulfuric acid, and cryogenic equipment components in the refrigeration industry. After deformation strengthening, it can be used as stainless steel springs and watch springs.
Austenitic stainless steel has good resistance to uniform corrosion, but in terms of local corrosion resistance, there are still the following problems:
1. Intergranular corrosion of austenitic stainless steel< /p>
When austenitic stainless steel is kept at 450~850℃ or cooled slowly, intergranular corrosion will occur. The higher the carbon content, the greater the tendency of intergranular corrosion. In addition, intergranular corrosion will also occur in the heat-affected zone of the weldment. This is due to the precipitation of Cr-rich Cr23C6 on the grain boundaries. It causes a chromium-depleted area in the surrounding matrix, thereby causing corrosion of the primary battery. This intergranular corrosion phenomenon also exists 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 the steel so that the total carbon content in the steel is lower than the equilibrium state in Austria. The saturated solubility in the chromium body fundamentally solves the problem of chromium carbide (Cr23C6) precipitating on the grain boundaries. Usually the amount of carbon in steel can be reduced to less than 0.03% to meet the requirements for intergranular corrosion resistance.
(2) Adding Ti, Nb and other elements that can form stable carbides (TiC or NbC) to avoid the precipitation of Cr23C6 on the grain boundaries can prevent intergranular corrosion of austenitic stainless steel.
(3) By adjusting the ratio of austenite-forming elements and ferrite-forming elements in the steel, it has an austenite + ferrite dual-phase structure, of which ferrite accounts for 5% to 12 %. This dual-phase structure is not prone to intergranular corrosion.
(4) Using appropriate heat treatment process can prevent intergranular corrosion and obtain the best corrosion resistance.
2. Stress corrosion of austenitic stainless steel
The cracking caused by the combined effect of stress (mainly tensile stress) and corrosion is called stress corrosion cracking, referred to as SCC (StressCrackCorrosion) . Austenitic stainless steel is prone to stress corrosion in corrosive media containing chloride ions. When the Ni content reaches 8% to 10%, austenitic stainless steel has the greatest stress corrosion tendency. Continue to increase the Ni content to 45~50% and the stress corrosion tendency gradually decreases until it disappears.
The most important 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, the content of impurities such as P, Sb, Bi, As should also be reduced as much as possible. In addition, A-F dual-phase steel can be used, which is not sensitive to stress corrosion in Cl- and OH- media. When the initial fine cracks encounter the ferrite phase and no longer continue to expand, the ferrite content should be around 6%.
3. Deformation strengthening of austenitic stainless steel
Single-phase austenitic stainless steel has good cold deformation properties and can be cold drawn into very thin steel wires and cold rolled into very thin steel wires. Thin steel strip or tube. After a large amount of deformation, the strength of the steel is greatly improved, especially when rolling in the sub-zero temperature zone, the effect is more significant. The tensile strength can reach more than 2000MPa. This is because in addition to the cold work hardening effect, deformation-induced M transformation is also superimposed.
After deformation strengthening, stainless steel can be used to make stainless springs, clock springs, steel wire ropes in aviation structures, etc. If welding is required after deformation, spot welding can only be used. Deformation increases the tendency of stress corrosion. And ferromagnetism is generated due to partial γ->M transformation, which should be considered when using it (such as in instrument parts).
The recrystallization temperature changes with the deformation amount. When the deformation amount is 60%, the recrystallization temperature drops to 650℃. The recrystallization annealing temperature of cold deformed austenitic stainless steel is 850~1050℃, 850℃ It needs to be kept warm for 3 hours, burned through at 1050°C, and then cooled with water.
4. Heat treatment of austenitic stainless steel
Commonly used heat treatment processes for austenitic stainless steel include: solution treatment, stabilization treatment and stress relief treatment.
(1) Solid solution treatment. The main purpose of heating the steel to 1050~1150℃ and then water quenching is to dissolve the carbides in the austenite and keep this state at room temperature, so that the corrosion resistance of the steel will be greatly improved. As mentioned above, in order to prevent intergranular corrosion, solution treatment is usually used to dissolve Cr23C6 in austenite and then rapidly cooled. For thin-walled parts, air cooling can be used, and water cooling is generally used.
(2) Stabilization treatment. It is usually carried out after solid solution treatment. It is often used for 18-8 steel containing Ti and Nb. After solid solution treatment, the steel is heated to 850~880℃ and then air cooled. At this time, the Cr carbide is completely dissolved and the titanium is removed. The carbides are not completely dissolved and are fully precipitated during the cooling process, making it impossible for carbon to form chromium carbides, thus effectively eliminating intergranular corrosion.
(3) Stress relief treatment. Stress relief treatment is a heat treatment process that eliminates the residual stress of steel after cold working or welding. It is generally heated to 300~350℃ and tempered. For steel that does not contain stabilizing elements Ti and Nb, the heating temperature should not exceed 450°C to avoid the precipitation of chromium carbides and cause intergranular corrosion. For cold-worked parts and welded parts of ultra-low carbon and stainless steel containing Ti and Nb, they need to be heated at 500~950°C and then slowly cooled to eliminate stress (the upper limit temperature is used to eliminate welding stress), which can reduce the tendency of intergranular corrosion and improve the quality of the steel. stress corrosion resistance.
4. Austenitic-ferritic duplex stainless steel
On the basis of austenitic stainless steel, appropriately increase the Cr content and reduce the Ni content, and cooperate with the remelting treatment , stainless steel with a dual-phase structure of austenite and ferrite (containing 40~60% δ-ferrite) can be obtained. Typical steel grades include 0Cr21Ni5Ti, 1Cr21Ni5Ti, OCr21Ni6Mo2Ti, etc. Duplex stainless steel has good weldability, does not require heat treatment after welding, and has less tendency to intergranular corrosion and stress corrosion. However, due to the high Cr content, it is easy to form σ phase, so care should be taken when using it.