The main alloying elements in stainless steel and their functions
The alloying elements in stainless steel mainly play a role in the following aspects: ① Promoting the formation and stability of the passive film on the steel; ② Increase the electrode potential of steel; ③ Adjust the microstructure of the steel; ④ Reduce or eliminate the inhomogeneity of the structure in steel and enhance the stability of the structure; ⑤ Balance or reduce the adverse effect of carbon on corrosion resistance; ⑥ Strengthen the matrix of the steel and adjust its mechanical properties; ⑦ Improve the performance of cold and hot working processes.
1. Chromium (Cr
Among the alloying elements in stainless steel, chromium is the most important one. Chromium in stainless steel not only significantly promotes the formation and stability of the passivation film, but also significantly increases the electrode potential of iron-based solid solutions. According to the electrochemical theory of corrosion, when iron-based solid solutions are added, only when the chromium content reaches a certain value, that is, when the molar ratio of chromium to iron reaches 1/8, 2/8, 3/8... When n/ 8 is reached, the electrode potential of the iron-based solid solution will increase dramatically, and the corrosion resistance of the steel will be significantly improved (see Figure 1).
Picture
As shown in Figure 1, when the molar ratio of chromium to iron reaches the n/ 8 rule, the first sudden change value occurs. That is, when the molar ratio of chromium to iron is 12.5%, the potential of iron can jump from -0.56V to 0.2V, thereby passivating the steel. Steel has better corrosion resistance in weakly corrosive media such as the atmosphere. If the role of carbon and chromium in steel is not taken into account, 11.7% (by mass) is the minimum chromium requirement for stainless steel. However, since carbon is an inevitable element in steel, it can form a series of chromium carbides with chromium. To ensure that the chromium content in the solid solution of steel is no less than 11.7% (by mass), the chromium content in steel is usually appropriately increased. This is the reason why the chromium content in practical stainless steel is no less than 13% (by mass). If it is desired to further enhance the corrosion resistance of steel (such as resistance to boiling nitric acid corrosion), a higher chromium content is required.
2. Nickel (Ni
Nickel is the most important element in austenitic stainless steel. Nickel itself is an excellent corrosion-resistant material. However, due to its high price and some other factors, its application is somewhat limited. Nickel is a strong austenitic stabilizing element. However, to obtain a pure austenitic structure in low-carbon nickel steel, the w (Cr) must reach over 24%, and only when the w (Cr) reaches over 27% can the steel improve its corrosion resistance in certain media. However, when nickel and chromium coexist in steel, the role of nickel undergoes significant changes. For instance, in ferritic stainless steel with w (Cr) = 17%, after adding about 2% (by mass fraction) of nickel, it becomes martensitic stainless steel. When w (Cr) is increased to around 8%, a single-phase austenitic structure can generally be obtained, which is the widely used 18-8 type austenitic stainless steel. It has better corrosion resistance than ferritic stainless steel and martensitic stainless steel with the same chromium content. Moreover, it has better processing performance, welding performance, plasticity at low temperatures and impact toughness. It can be seen from this that the application of nickel in stainless steel usually combines with chromium to better function, to change the structure of stainless steel, and thereby greatly improve the mechanical properties, processing performance and corrosion resistance in certain corrosive media of stainless steel.
3. Carbon (C
Carbon is the main constituent element of steel. The content and distribution form of carbon in steel determine the microstructure and properties of steel. Carbon has a significant impact on the structure, mechanical properties and corrosion resistance of stainless steel and can be said to play a leading role. Carbon is an element that strongly expands the austenite region and stabilizes the austenite structure. Its effect is approximately 30 times that of nickel. The role of carbon in improving the structure and mechanical properties of stainless steel is beneficial. However, carbon is a strong carbide-forming element. It has a high affinity for chromium and can easily combine with chromium in stainless steel to form compounds, reducing the chromium in the solid solution and thus affecting the corrosion resistance of the steel. Especially when the chromium carbides precipitate along the grain boundaries, it can cause chromium-poor zones at that location, leading to intergranular corrosion and having a detrimental effect on the corrosion resistance of stainless steel. With the advancement of metallurgical technology, in response to the demand for corrosion resistance of stainless steel, an increasing number of new low-carbon and ultra-low-carbon stainless steel grades have emerged. As can be seen from the above, carbon plays both beneficial and harmful roles in stainless steel. This point should be noted when choosing and using stainless steel, as well as formulating the correct heat treatment methods.
4. Molybdenum (Mo
Molybdenum is the most important element for improving the pitting corrosion resistance of austenitic stainless steel and duplex stainless steel. Adding molybdenum to stainless steel can enhance the passivation effect of the steel, thereby improving its corrosion resistance. However, molybdenum is an element that forms ferrite. If the steel is to obtain an austenitic structure, the influence of molybdenum in the steel should be taken into consideration. Therefore, in austenitic stainless steels containing molybdenum, the nickel content has been appropriately increased to balance the effect of molybdenum. When molybdenum is present in martensitic stainless steel, the quenching temperature should be appropriately increased during heat treatment to ensure that the molybdenum-containing carbides are fully dissolved.
5. Copper (Cu
Copper is an element that forms austenite, but its effect is not significant and has no substantial impact on the microstructure. Copper can enhance the stability of austenite. Adding copper to stainless steel is mainly to enhance its corrosion resistance in sulfuric acid, especially when added together with molybdenum, the effect is more significant. This may be related to its higher stability in sulfuric acid. In precipitation-hardening stainless steel, copper precipitates copper-rich strengthening phases due to aging treatment, which can enhance the strength of the steel. In addition, nano-copper-rich phases are precipitated in stainless steel to enhance its resistance to microbial corrosion and form antibacterial stainless steel.
6. Titanium (Ti) and niobium (Nb
Titanium and niobium are both strong carbide-forming elements and are more likely to form carbides than chromium. In stainless steel, titanium and niobium are present. The carbon in the steel preferentially combines with titanium and niobium, preventing the formation of chromium carbides and their precipitation along the grain boundaries. This ensures that there are no chromium-poor zones at the grain boundaries, effectively preventing intergranular corrosion in stainless steel. To ensure that stainless steel does not suffer from intergranular corrosion, in addition to ensuring that there is a sufficient amount of titanium and niobium in the steel, appropriate heat treatment should also be carried out to fully exert the functions of titanium and niobium. However, both titanium and niobium are elements that form ferrite, which may cause a small amount of ferrite to form in austenitic steel. If heat treatment or use is improper, δ phase may form, thereby causing brittleness and adversely affecting the processing performance.
7. Nitrogen (N
Nitrogen is an element that strongly expands the austenite zone and stabilizes the austenite structure. Its effect is 25 to 30 times that of nickel. Therefore, nitrogen can replace part of nickel, achieving the effect of nickel conservation. Nitrogen can also enhance the resistance to pitting (hole) corrosion and crevice corrosion in media containing chloride ions. However, excessive nitrogen content may cause defects such as porosity in stainless steel castings. Therefore, the amount of nitrogen added should be reasonably controlled, and its mass fraction generally should not exceed 0.2%.
8. Manganese (Mn
Manganese is an element that expands the austenite zone and stabilizes the austenite structure, and its effect is equivalent to half that of nickel. The application of manganese in stainless steel is mainly to replace a portion of nickel, especially in nickel-deficient countries where manganese is used instead of nickel to produce austenitic stainless steel. Manganese has little ability to increase the electrode potential of iron-based solid solutions, and its protective effect in forming oxide films is also very low. It has little effect on the corrosion resistance of stainless steel. When w (Cr) in steel is greater than 15%, if w (Mn) is greater than 10%, the content of δ phase in the structure will increase, which will instead have an adverse effect on the corrosion resistance and mechanical properties of the steel. Therefore, it is also necessary to pay attention to controlling the addition amount of manganese.
9. Silicon (Si
Silicon is a ferrite-forming element that can enhance the corrosion resistance, intergranular corrosion resistance and pitting corrosion resistance of stainless steel in oxidizing media. The addition of silicon can also improve the casting performance of steel. However, a high silicon content can easily promote the formation of σ phase, causing embrittlement of castings and reducing the mechanical properties of steel.
10. Aluminum (Al
Aluminum is less used in stainless steel, but its addition amount is relatively high in oxidation-resistant steel. When the aluminum content in steel reaches a certain level, it can passivate the steel and enhance its corrosion resistance in oxidizing acids. When aluminium is added to some precipitation-hardening stainless steels, it can precipitate nickel-aluminium intermetallic compounds during aging treatment, thereby strengthening the steel.
11. Sulfur (S) and Selenium (Se)
The addition of sulfur and selenium can reduce the toughness of steel and also show adverse effects on corrosion resistance, so they are generally rarely used. In some stainless steels, the sulfur content is consciously increased or selenium is added, mainly to improve the machinability of the stainless steel, especially austenitic stainless steel.
12. Tungsten (W
Tungsten is rarely used in stainless steel. With the research and development of duplex stainless steel, it has been found that tungsten plays a significant role in enhancing the crevice corrosion resistance of duplex stainless steel. It is generally believed that the function of tungsten in stainless steel is similar to that of molybdenum, which can inhibit the redissolution of the metal and thus play a role in delaying corrosion. Some studies also suggest that a certain amount of tungsten can reduce the tendency of medium-temperature embrittlement in steel.
In addition to the above-mentioned elements, some also add other elements to enhance certain properties of stainless steel. For instance, cobalt is added to increase the hardness of age-hardening stainless steel, vanadium to improve its thermal strength, and rare earth elements to improve its processability, etc.
Although the role of alloying elements in stainless steel has been explained above, in reality, stainless steel is a coexistence of multiple elements, and their influence is more complex than the individual role of each element in stainless steel. Therefore, in the actual alloying of stainless steel, not only the self-action of each element should be taken into account, but also their interaction and influence.
15358968703