The alloying principle of stainless steel

There are many ways to improve the corrosion resistance of steel, such as coating a layer of corrosion-resistant metal on the surface, coating a non-metallic layer, electrochemical protection, and changing the corrosive environment medium. However, using alloying methods to improve the corrosion resistance of the material itself is one of the most effective measures to prevent corrosion damage. The method is as follows:

(1) Adding alloy elements to increase the electrode potential of the steel substrate, thereby improving the electrochemical corrosion resistance of the steel. In general, adding Cr, Ni, and Si to the steel can increase its electrode potential. Due to the lack of Ni, a large amount of Si will make the steel brittle. Therefore, only Cr is the commonly used element to significantly increase the electrode potential of the steel substrate.

Cr can increase the electrode potential of steel, but it is not linear, as shown in Figure 5.1. Experiments show that the electrode potential of steel has a relationship from quantitative change to qualitative change with the increase of alloying elements, which follows the 1/8 rule. When the Cr content reaches a certain value, that is, 1/8 atom (1/8, 2/8, 3/8…), there will be a sudden change in the electrode potential. Therefore, in almost all stainless steels, the Cr content is above 12.% (atoms), that is, above 11.7% (mass).

(2) Adding alloying elements makes the surface of the steel form a stable and complete purification film that is firmly combined with the steel matrix. Thereby improving the chemical resistance of steel. For example, adding Cr, Si.Al and other alloying elements to the steel to form a dense Cr2O3, SiO2, Al2O3 and other oxide film on the surface of the steel can improve the corrosion resistance of the steel.

(3) Adding alloying elements enables the steel to exist in a single-phase state at room temperature, reducing the number of micro-batteries and improving the corrosion resistance of the steel. If adding a sufficient amount of Cr or Cr-Ni, the steel can obtain single-phase ferrite or single-phase austenite at room temperature.

(4) Add Mo, Cu and other elements to improve the corrosion resistance.

(5) Adding elements such as Ti and Nb to eliminate the intergranular segregation of Cr, thereby reducing the tendency of intergranular corrosion.

(6) Adding Mn, N and other elements to replace part of Ni to obtain a single-phase austenite structure, while greatly improving the corrosion resistance of chromium stainless steel in organic acids.

Types and characteristics of stainless steel

There are two classifications of 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 stainless steel, F stainless steel, A stainless steel, and A-F duplex stainless steel according to the structure of the steel in the normalized state.

1. 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 it 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, and is widely used in the manufacture of equipment for the production of nitric acid, nitrogen fertilizer, and pipelines used in chemicals.

The typical ferritic stainless steels are Cr17, Cr25 and Cr28.

Three, austenitic stainless steel

Austenitic stainless steel was developed to overcome the 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 using 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, cryogenic equipment components in the refrigeration industry, and can be used as stainless steel springs and clockwork after deformation strengthening.

Austenitic stainless steel has good resistance to uniform corrosion, but the following problems still exist 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, make the amount of combined carbon in the steel lower than the saturated solubility in the austenite in the equilibrium state, which fundamentally solves the problem of chromium carbide (Cr23C6) precipitation on the grain boundary . 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 elements such as Ti and Nb that can form stable carbides (TiC or NbC) to avoid precipitation of Cr23C6 on the grain boundaries can prevent 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 dual-phase structure is not prone to 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 fine cracks meet the ferrite phase, they will not continue to grow, and the ferrite content should be about 6%.

3. Austenitic is used for deformation strengthening of 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 lot of deformation, the strength of 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 being deformed and strengthened. If welding is required after deformation, spot welding can only be used, and deformation increases the stress corrosion tendency. And because of the partial γ->M transition, ferromagnetism is produced, which should be considered when using (such as 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 kept for 3h, 1050℃ It can be burned at once, and then cooled with water.

4. Austenitic heat treatment for stainless steel

Commonly used heat treatment processes for austenitic stainless steel 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 solid treatment, the steel is heated to 850~880℃ and then air-cooled. At this time, the Cr carbides 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 to eliminate 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 should be carried out at 500~950℃, 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 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 relatively small. However, due to the high Cr content, σ phase is easily formed, so care should be taken when using it.