Types and characteristics of stainless steel

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 alloying elements; the other is divided into M stainless steel, F stainless steel, A stainless steel according to the structure of the steel in the normalized state. , A-F duplex stainless steel.

1. Martensitic stainless steel

Typical martensitic stainless steels are 1Cr13~4Cr13 and 9Cr18, etc.

1Cr13 steel has good processing performance. Can be deep drawn, bent, crimped and welded without preheating. 2Cr13 does not require preheating before cold deformation, but needs to be preheated before welding. 1Cr13 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 for corrosion-resistant bearings and cutting tools.

Second, 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, and there is no α<=>γ transformation during heating and cooling, so it cannot be strengthened by heat treatment. Strong antioxidant. 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 and lower strength requirements.

Typical ferritic stainless steels are Crl7, Cr25 and Cr28.

Third, austenitic stainless steel

Austenitic stainless steel is developed to overcome the insufficient corrosion resistance and excessive brittleness of martensitic stainless steel. The basic components are Crl8%, Ni8% referred to as 18-8 steel. It is characterized in 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 in the manufacture of chemical equipment components such as nitric acid and sulfuric acid, components of cryogenic equipment in the refrigeration industry, and can be used as stainless steel springs and clockwork springs after deformation and strengthening.

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

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 can 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 is caused by the formation of a chromium-depleted zone in the surrounding matrix, thereby forming a corrosion galvanic cell. This intergranular corrosion phenomenon also exists in the aforementioned ferritic stainless steels.

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

(1) Reduce the carbon content in the steel, so that the combined carbon content in the steel is lower than the saturated solubility in the austenite in the equilibrium state, that is, the problem of the precipitation of chromium carbides (Cr23C6) on the grain boundary is fundamentally solved . Usually, the combined carbon content in steel can be reduced to less than 0.03% to meet the requirements of anti-intergranular corrosion performance.

(2) Add elements such as Ti and Nb that can form stable carbides (TiC or NbC) to avoid the precipitation of Cr23C6 on the grain boundary, which can prevent the intergranular corrosion of austenitic stainless steel.

(3) By adjusting the ratio of austenite-forming elements and ferrite-forming elements in the steel, it has a dual-phase structure of austenite + ferrite, of which ferrite accounts for 5% to 12%. This dual-phase structure is not prone to intergranular corrosion.

(4) The use of appropriate heat treatment process can prevent intergranular corrosion and obtain good 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% to 10%, the stress corrosion tendency of austenitic stainless steel is large, and the stress corrosion tendency of the austenitic stainless steel is gradually reduced until it disappears by continuing to increase the Ni content to 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, the content of impurities such as P, Sb, Bi, As should be minimized. In addition, A-F dual-phase steel can be selected, which is not sensitive to stress corrosion in Cl- and OH- media. When the initial microcrack encounters the ferrite phase, it will not continue to expand, 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 pipes. After a large amount of deformation, the strength of the steel is greatly improved, especially when it is rolled in the sub-zero temperature region, the effect is more significant. The tensile strength can reach more than 2000 MPa. This is because the deformation-induced M transformation is superimposed in addition to the cold work hardening effect.

Austenitic stainless steel can be used to manufacture stainless springs, clock springs, wire ropes in aviation structures, etc. after deformation and strengthening. If welding is required after deformation, only spot welding process can be used, and the 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 amount of deformation. When the amount of deformation is 60%, the recrystallization temperature drops to 650℃. The recrystallization annealing temperature of cold deformed austenitic stainless steel is 850~1050℃. At 850℃, it needs to be kept for 3h and 1050℃. It can be burnt through for a while, then water-cooled.

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) Solution treatment. The main purpose of water quenching after heating the steel to 1050~1150℃ 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 cool. For thin-walled parts, air cooling can be used, and water cooling is generally used.

(2) Stabilization treatment. It is generally carried out after solution treatment, and is often used for 18-8 steel containing Ti and Nb. After solid treatment, the steel is heated to 850~880 ° C for heat preservation and then air-cooled. At this time, the carbides of Cr are completely dissolved, and the titanium is removed. The carbides are not completely dissolved, and are fully precipitated during the cooling process, so that it is impossible for carbon to form chromium carbides again, 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 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 cold-worked parts and welded parts of ultra-low carbon and stainless steel containing Ti and Nb, it needs to be heated at 500~950 ° C, and then slowly cooled to eliminate stress (the upper limit temperature for welding stress is eliminated), which can reduce the tendency of intergranular corrosion and improve the quality of 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 reflow 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 has less tendency to intergranular corrosion and stress corrosion. However, due to the high Cr content, it is easy to form a σ phase, so care should be taken when using it.