One, martensitic stainless steel
Typical martensitic stainless steels include 1Cr13~4Cr13 and 9Cr18, etc.
1Cr13 steel has good processing performance. Deep drawing, bending, crimping and welding can be performed without preheating. 2Crl3 does not require preheating before cold deformation, but needs to be preheated 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 medical equipment surgical scalpels and wear-resistant parts; 9Crl8 can be used as corrosion-resistant bearings and tools.
2. Ferritic stainless steel
The Cr content of ferritic stainless steel is generally 13%~30% and the combined 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 oxidation resistance. 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 high corrosion resistance and low strength requirements. It is widely used in the production of nitric acid, nitrogen fertilizer and other equipment and chemical pipelines.
Typical ferritic stainless steels are Cr17 type, Cr25 type and Cr28 type.
三, austenitic stainless steel
Austenitic stainless steel was developed to overcome insufficient corrosion resistance and excessive brittleness of Martensitic stainless steel. The basic composition is Crl8% and Ni8%, referred to as 18-8 steel. Its characteristic is that the combined carbon content is less than 0.1%, and the single-phase austenite structure is obtained by the combination of Cr and Ni.
Austenitic stainless steel is generally used for 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 springs and clock springs after deformation strengthening.
Austenitic stainless steel has good resistance to uniform corrosion, but there are still the following problems in terms of local corrosion resistance:
1. Intergranular corrosion of austenitic stainless steel
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 for 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. The surrounding matrix produces a chromium-depleted area, which is caused by corrosion of the galvanic cell. This kind of intergranular corrosion phenomenon also exists in the aforementioned ferritic stainless steel.
The following methods are often used in engineering to prevent intergranular corrosion:
(1) Reduce the amount of carbon in the steel so that the amount of combined carbon in the steel is lower than the saturated solubility in the austenite in the equilibrium state, which fundamentally solves the problem of precipitation of chromium carbides (Cr23C6) on the grain boundaries . Generally, the amount of combined carbon in steel can be reduced to less than 0.03% to meet the requirements of intergranular corrosion resistance.
(2) Adding Ti, Nb and other elements that can form stable carbides (TiC or NbC) to avoid precipitation of Cr23C6 on the grain boundaries, which can prevent the intergranular corrosion of austenitic stainless steel.
(3) By adjusting the ratio of austenite-forming elements to ferrite-forming elements in the steel, it has a dual-phase structure of austenite + ferrite, in which ferrite accounts for 5% to 12%. This two-phase structure is not easy to produce intergranular corrosion.
(4) The use of 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 action of stress (mainly tensile stress) and corrosion is called stress corrosion cracking, or SCC (Stress Crack Corrosion) for short. Austenitic stainless steel is prone to stress corrosion in corrosive media containing chloride ions. When the Ni content reaches 8%-10%, the stress corrosion tendency of austenitic stainless steel is the largest. 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 the smelting. In addition, the content of impurities such as P, Sb, Bi, and As should be minimized. 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 microcracks meet the ferrite phase, they will not continue to grow, and the ferrite content should be about 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 strips or steel tubes. 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 2000 MPa. This is because in addition to the cold work hardening effect, deformation induced M transformation is also superimposed.
Austenitic stainless steel can be used to make stainless springs, clock springs, steel wire ropes in aerospace structures, etc. after deformation and strengthening. If welding is required after deformation, spot welding can only be used, and deformation will increase the stress corrosion tendency. And because of the partial γ->M transition, ferromagnetism is produced, which should be considered when using it (such as in instrument parts).
The recrystallization temperature changes with the amount of deformation. When the amount of deformation is 60%, the recrystallization temperature drops to 650℃. The recrystallization and annealing temperature of cold deformed austenitic stainless steel is 850~1050℃. At 850℃, it needs to be held for 3h, 1050℃ It can be burned at once, and then cooled with water.
4. Austenitic stainless steel heat treatment
Austenitic stainless steel commonly used heat treatment processes include: solution treatment, stabilization treatment and stress relief treatment.
(1) Solution treatment. The steel is heated to 1050~1150℃ and then water quenched. The main purpose is to dissolve the carbide in austenite and keep this state to room temperature, so that the corrosion resistance of the steel will be greatly improved. As mentioned above, in order to prevent intergranular corrosion, solid solution treatment is usually used to dissolve Cr23C6 in austenite, and then rapidly cool. Air cooling can be used for thin-walled parts, and water cooling is generally used.
(2) Stabilization treatment. It is usually carried out after solution treatment, which is usually used for 18-8 steel containing Ti and Nb. After solidification treatment, heat the steel to 850~880℃ and then cool it in air. At this time, the carbides of Cr are completely dissolved and the titanium is removed. The carbide is not completely dissolved, and it is found out during the cooling process that it is impossible for the carbon to form chromium carbides, thus effectively eliminating the 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. Generally, it is heated to 300~350℃ and tempered. For steels that do not contain stabilizing elements Ti and Nb, the heating temperature should not exceed 450°C to avoid precipitation of chromium carbides and cause intergranular corrosion. For ultra-low carbon and cold-worked parts and welded parts of stainless steel containing Ti and Nb, heating at 500~950℃, and then slow cooling, to eliminate stress (elimination of welding stress takes the upper limit temperature), which can reduce the tendency of intergranular corrosion and improve the steel The stress corrosion resistance.
Fourth, 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 to obtain a dual-phase structure with austenite and ferrite (containing 40~60% δ-ferrite ) Stainless steel, typical steel grades are 0Cr21Ni5Ti, 1Cr21Ni5Ti, OCr21Ni6Mo2Ti, etc. Duplex stainless steel has good weldability, does not require heat treatment after welding, and its tendency to intergranular corrosion and stress corrosion is also small. However, due to the high Cr content, σ phase is easily formed, so care should be taken when using it.
Corrosion resistance of various stainless steels
304 is a versatile stainless steel, which is widely used to make equipment and parts that require good overall performance (corrosion resistance and formability).
301 stainless steel shows obvious work hardening during deformation, and is used in various occasions requiring higher strength.
302 stainless steel is essentially a variant of 304 stainless steel with higher carbon content, which can be made to obtain higher strength through cold rolling.
302B is a kind of stainless steel with high silicon content, it has high resistance to high temperature oxidation.
303 and 303Se are free-cutting stainless steels containing sulfur and selenium, respectively, and are used in occasions where free cutting and high surface gloss are mainly required. 303Se stainless steel is also used to make parts that require hot upsetting, because under such conditions, this stainless steel has good hot workability.
304L is a variant of 304 stainless steel with lower carbon content and is used where welding is required. The lower carbon content minimizes the precipitation of carbides in the heat-affected zone near the weld, and the precipitation of carbides may cause stainless steel to produce intergranular corrosion (welding erosion) in certain environments.
304N is a nitrogen-containing stainless steel. Nitrogen is added to increase the strength of the steel.
305 and 384 stainless steels contain higher nickel, and their work hardening rate is low. They are suitable for various occasions that require high cold formability.
308 stainless steel is used to make welding rods.
309, 310, 314 and 330 stainless steel have relatively high nickel and chromium content, in order to improve the oxidation resistance and creep strength of the steel at high temperatures. While 30S5 and 310S are variants of 309 and 310 stainless steel, the difference is that the carbon content is lower in order to minimize the carbides precipitated near the weld. 330 stainless steel has particularly high resistance to carburization and thermal shock resistance.
Type 316 and 317 stainless steel contain aluminum, so the resistance to pitting corrosion in marine and chemical industrial environments is much better than that of 304 stainless steel. Among them, the 316 stainless steel variants include low-carbon stainless steel 316L, nitrogen-containing high-strength stainless steel 316N, and free-cutting stainless steel 316F with higher sulfur content.
321, 347 and 348 are stainless steel stabilized with titanium, niobium plus tantalum, and niobium, respectively, and are suitable for welding components used at high temperatures. 348 is a kind of stainless steel suitable for the nuclear power industry, and has certain restrictions on the combined amount of tantalum and drill.
