Alloying Elements in Steel
In order to improve and enhance certain properties of steel and to impart special characteristics, specific elements are intentionally added during the smelting process. These are known as alloying elements. Commonly used alloying elements include chromium, nickel, molybdenum, tungsten, vanadium, titanium, niobium, zirconium, cobalt, silicon, manganese, aluminum, copper, boron, and rare earth elements. In some cases, phosphorus, sulfur, and nitrogen also act as alloying elements.
Chromium (Cr)
Chromium increases the hardenability of steel and has a secondary hardening effect, which improves the hardness and wear resistance of carbon steel without making it brittle. When the chromium content exceeds 12%, the steel gains good high-temperature oxidation resistance and resistance to oxidative corrosion, and its thermal strength is also enhanced.
Chromium is the main alloying element in stainless, acid-resistant, and heat-resistant steels. It improves the strength and hardness of carbon steel in the rolled state while reducing elongation and section shrinkage. However, when the chromium content exceeds 15%, the strength and hardness decrease, and elongation and section shrinkage increase accordingly.
Steel parts containing chromium can be easily ground to achieve a high-quality surface finish. In tempered structures, chromium's main role is to enhance hardenability, giving the steel better overall mechanical properties after quenching and tempering. In carburized steel, it can form chromium-containing carbides, thereby improving surface wear resistance.
Chromium-containing spring steel is less prone to decarburization during heat treatment. Chromium also improves the wear resistance, hardness, and red hardness of tool steels and offers good tempering stability. In electrothermal alloys, chromium enhances oxidation resistance, electrical resistance, and strength.
Nickel (Ni)
Nickel strengthens ferrite and refines pearlite in steel, with the overall effect of increasing strength while having little effect on plasticity. Generally speaking, for low-carbon steel that does not require tempering and is used in the rolled, normalized, or annealed condition, adding a certain amount of nickel can increase the strength of the steel without significantly reducing its toughness. According to statistics, every 1% increase in nickel content can improve strength by approximately 29.4 MPa.
As nickel content increases, the yield strength of steel increases faster than its tensile strength, so nickel-containing steels can exhibit higher yield ratios compared to ordinary carbon steels. While nickel enhances steel strength, it has less negative impact on toughness, plasticity, and other processing properties compared to many other alloying elements.
For medium-carbon steels, nickel lowers the pearlite transformation temperature, resulting in finer pearlite. It also reduces the carbon content required for pearlite formation, so for the same carbon content, nickel-containing steels have more pearlite than regular carbon steels. As a result, pearlitic-ferritic nickel steels have higher strength than carbon steels with the same carbon content. Conversely, to achieve the same strength, the carbon content of nickel-containing steel can be reduced, thereby improving its toughness and plasticity.
Nickel also improves steel's fatigue resistance and reduces its notch sensitivity. It lowers the ductile-to-brittle transition temperature, which is especially important for low-temperature applications. Steel with 3.5% nickel can be used at −100 °C, while steel with 9% nickel can function at −196 °C. However, nickel does not enhance creep resistance, so it is generally not used as a strengthening element in heat-resistant steels.
In high-nickel iron-nickel alloys, the coefficient of linear expansion changes significantly with nickel content. This property is used to design and produce precision alloys and bimetallic materials with a very low or specific coefficient of linear expansion.
Additionally, nickel in steel provides resistance not only to acids but also to alkalis, atmospheric conditions, and salts. Nickel is one of the key elements in stainless and acid-resistant steels.
Molybdenum (Mo)
Molybdenum in steel improves hardenability and hot strength, prevents temper brittleness, and increases remanent magnetism, coercivity, and corrosion resistance in certain environments. In tempered steels, molybdenum allows parts with larger cross-sections to harden more deeply and uniformly. It also improves the steel's temper resistance and thermal stability, enabling parts to be tempered at higher temperatures. This helps eliminate (or reduce) residual stresses more effectively and improves plasticity.
In carburized steel, molybdenum not only provides these benefits but also reduces the tendency for continuous carbide networks to form along grain boundaries in the carburized layer. It also decreases the amount of residual austenite, thereby relatively increasing the wear resistance of the surface layer.
In forging dies, molybdenum helps maintain relatively stable hardness and enhances resistance to deformation, cracking, and wear.
In stainless and acid-resistant steels, molybdenum further improves corrosion resistance to organic acids (such as formic acid, acetic acid, oxalic acid), as well as to hydrogen peroxide, sulfuric acid, sulfurous acid, sulfates, acidic dyes, and bleaching powder solutions. In particular, the addition of molybdenum prevents pitting corrosion caused by the presence of chloride ions.
W12Cr4V4Mo high-speed steel, which contains about 1% molybdenum, exhibits good wear resistance, temper hardness, and red hardness.
Tungsten (W)
Tungsten partially dissolves into iron to form a solid solution in steel, in addition to forming carbides. Its effects are similar to those of molybdenum but are generally less pronounced on a mass fraction basis. Tungsten's primary role in steel is to increase temper stability, red hardness, hot strength, and wear resistance due to its formation of carbides. Therefore, it is mainly used in tool steels, such as high-speed steel and hot-forging die steel.
In high-quality spring steels, tungsten forms refractory carbides, and when tempered at higher temperatures, it can slow the aggregation of carbides and maintain high strength at elevated temperatures. Tungsten also reduces the steel's superheat sensitivity, increases hardenability, and improves hardness.
For example, 65SiMnWA spring steel, after hot rolling and air cooling, achieves high hardness. In oil, spring steel with a 50 mm² cross-section can be quenched and used to withstand heavy loads and heat (up to 350 °C), making it suitable for important springs subjected to impact. 30W4Cr2VA, a high-strength, heat-resistant, high-quality spring steel, exhibits large hardenability. When quenched at 1050–1100 °C and tempered at 550–650 °C, it achieves a tensile strength of 1470–1666 MPa. It is mainly used for springs operating at high temperatures (up to 500 °C).
Due to the addition of tungsten, the wear resistance and machinability of steel can be significantly improved. Therefore, tungsten is a key alloying element in tool steels.
Vanadium (V)
Vanadium has a strong affinity for carbon, nitrogen, and oxygen, with which it forms stable compounds. In steel, vanadium mainly exists in the form of carbides. Its primary role is to refine the microstructure and grain size of steel, thereby improving its strength and toughness. When dissolved in solid solution at high temperatures, vanadium increases hardenability; conversely, when present as carbides, it reduces hardenability.
Vanadium increases the tempering stability of hardened steel and produces a secondary hardening effect. Except in high-speed tool steels, the vanadium content in steel is generally kept below 0.5%.
In ordinary low-carbon alloy steels, vanadium refines grains, improves strength and yield ratio, enhances low-temperature properties after normalizing, and improves weldability. In alloy structural steels, vanadium generally reduces hardenability under normal heat treatment conditions; therefore, it is often used together with manganese, chromium, molybdenum, tungsten, and other elements.
In tempered steels, vanadium mainly improves strength and yield ratio, refines grains, and reduces sensitivity to overheating. In carburized steels, due to its grain-refining ability, vanadium allows quenching directly after carburizing without the need for secondary quenching.
In spring steels and bearing steels, vanadium improves strength and yield ratio, especially increasing the proportional limit and elastic limit, and reduces sensitivity to decarburization during heat treatment, thereby improving surface quality. Bearing steels containing vanadium exhibit high carbon dispersion and good service performance.
In tool steels, vanadium refines grains, reduces sensitivity to overheating, increases tempering stability and wear resistance, thereby extending tool life.
Titanium (Ti)
Titanium has a very strong affinity for nitrogen, oxygen, and carbon, and a stronger affinity for sulfur than iron. Therefore, it is an effective deoxidizing and degassing agent, as well as an element that fixes nitrogen and carbon. Although titanium is a strong carbide-forming element, it does not form complex compounds with other elements. Titanium carbide is strong, stable, and not easily decomposed; it can only slowly dissolve into solid solution in steel when heated above 1000°C. Before dissolving into solid solution, titanium carbide particles help prevent grain growth.
Because the affinity between titanium and carbon is much greater than that between chromium and carbon, titanium is commonly used in stainless steel to fix carbon and prevent chromium depletion at grain boundaries, thus reducing or eliminating intergranular corrosion.
Titanium is also a strong ferrite-forming element that significantly raises the A1 and A3 transformation temperatures of steel. In ordinary low-alloy steels, titanium improves plasticity and toughness. By fixing nitrogen and sulfur and forming titanium carbides, it increases the strength of steel. Normalizing and grain refinement, combined with carbide precipitation, significantly improve the plasticity and impact toughness of steel.
Alloy structural steels containing titanium have good mechanical properties and processability, though their main disadvantage is slightly reduced hardenability. In high-chromium stainless steels, titanium is usually added at about five times the carbon content. This addition not only improves corrosion resistance-mainly intergranular corrosion resistance-and toughness but also inhibits grain growth at high temperatures and improves the steel's weldability.
Niobium/Columbium (Nb/Cb)
Niobium and columbium often coexist with tantalum and play similar roles in steel. Niobium and tantalum partially dissolve in the solid solution and act as solid solution strengtheners. When dissolved in austenite, they significantly increase the hardenability of the steel. However, when present as carbides and oxide particles, they refine the grain structure and reduce the hardenability of the steel.
Niobium increases the tempering stability of steel and produces a secondary hardening effect. Trace amounts of niobium can increase the strength of steel without negatively affecting its plasticity or toughness. It also improves the impact toughness of steel and lowers its brittle transition temperature due to its grain-refining effect.
When the niobium content exceeds eight times the carbon content, almost all the carbon in the steel can be fixed, giving the steel good resistance to hydrogen embrittlement. In austenitic steels, niobium can prevent intergranular corrosion caused by oxidizing media.
Due to its carbon-fixing and precipitation-hardening effects, niobium improves the high-temperature performance of heat-resistant steels, such as creep strength. In ordinary low-alloy structural steels, niobium improves yield strength and impact toughness, lowers the brittle transition temperature, and benefits weldability.
In carburized and tempered alloy structural steels, niobium increases hardenability while improving toughness and low-temperature performance. It can reduce the air hardenability of low-carbon martensitic heat-resistant stainless steels, avoiding hardening and temper brittleness and improving creep strength.
Zirconium (Zr)
Zirconium is a strong carbide-forming element, and its role in steel is similar to that of niobium, tantalum, and vanadium. Adding a small amount of zirconium helps with degassing, purification, and grain refinement, which improves the low-temperature performance and stamping properties of steel. It is commonly used in the manufacture of ultra-high-strength steels for gas engines and ballistic missile structures, as well as in nickel-based high-temperature alloys.
Cobalt (Co)
Cobalt is mostly used in special steels and alloys. Cobalt-containing high-speed steels have high hardness at elevated temperatures. When added together with molybdenum to martensitic aging steels, cobalt helps achieve ultra-high hardness and good overall mechanical properties.
In addition, cobalt is an important alloying element in heat-resistant steels and magnetic materials. Cobalt reduces the hardenability of steel, so adding it alone to carbon steels will reduce the overall mechanical properties after tempering.
Cobalt strengthens ferrite; when added to carbon steel in the annealed or normalized condition, it can improve hardness, yield strength, and tensile strength. However, elongation and reduction in area are negatively affected, and impact toughness decreases as the cobalt content increases.
Cobalt is used in heat-resistant steels and alloys due to its antioxidant properties. Cobalt-based alloys also play a unique role in gas turbine applications.
Silicon (Si)
Silicon can dissolve in ferrite and austenite, improving the hardness and strength of steel. Its effect is second only to phosphorus and is stronger than manganese, nickel, chromium, tungsten, molybdenum, vanadium, and other elements. However, when the silicon content exceeds 3%, it significantly reduces the plasticity and toughness of steel.
Silicon increases the elastic limit, yield strength, and yield ratio (σs/σb), as well as the fatigue strength and fatigue ratio (σ−1/σb) of steel. This is why silicon or silicomanganese steels can be used as spring steels. Silicon reduces the density, thermal conductivity, and electrical conductivity of steel. It induces ferrite grain coarsening and reduces coercivity.
Silicon tends to reduce the anisotropy of the crystal, making magnetization easier and reducing magnetoresistance, which makes it suitable for producing electrical steel with lower hysteresis loss. Silicon improves the magnetic permeability of ferrite, so steel sheets have higher magnetic susceptibility in weaker magnetic fields. However, silicon reduces the magnetic susceptibility of steel in strong magnetic fields.
Due to its strong deoxidizing ability, silicon reduces the magnetic aging effect of iron. When steel containing silicon is heated in an oxidizing atmosphere, a thin film of SiO₂ forms on the surface, improving the steel's resistance to high-temperature oxidation.
Silicon promotes the growth of columnar crystals in cast steel, which reduces plasticity. When silicon steel is heated and cooled rapidly, its low thermal conductivity causes a large temperature difference between the interior and surface, leading to fracture.
Silicon can reduce the weldability of steel. Because silicon's affinity for oxygen is stronger than that of iron, it easily forms low-melting-point silicates during welding, increasing the mobility of slag and molten metal, causing spattering and affecting weld quality.
Silicon is a good deoxidizer. Deoxidation with aluminum is significantly improved by adding an appropriate amount of silicon. Silicon in steel often remains as a residue introduced with raw materials during ironmaking and steelmaking. In boiling steel, the silicon content is limited to less than 0.07%, and when intentionally added, it is usually introduced via ferrosilicon alloys during steelmaking.
Manganese (Mn)
Manganese is a good deoxidizer and desulfurizer. Steel generally contains a certain amount of manganese, which can eliminate or reduce thermal embrittlement caused by sulfur, thus improving the hot-working properties of steel. Manganese forms a solid solution with iron, increasing the hardness and strength of ferrite and austenite in steel. At the same time, it is a carbide-forming element that can replace some iron atoms in carburized steel. Manganese lowers the critical transformation temperature in steel, thereby refining pearlite and indirectly improving the strength of pearlitic steel.
Manganese's ability to stabilize austenite is second only to nickel, and it also significantly increases the hardenability of steel. Various alloy steels containing up to 2% manganese, combined with other elements, have been developed. Manganese is abundant and effective in many applications, such as carbon structural steels and spring steels with high manganese content.
In high-carbon, high-manganese wear-resistant steels, the manganese content can reach 10–14%. After solution treatment, these steels have good toughness. When subjected to impact and deformation, the surface layer strengthens due to work hardening, resulting in high wear resistance.
Manganese combines with sulfur to form MnS, which has a high melting point and prevents hot brittleness caused by FeS. However, manganese tends to promote grain coarsening and increase temper brittleness sensitivity. If the steel is improperly cooled after smelting, casting, or forging, it is prone to developing white spots.
Aluminum (Al)
Aluminum is mainly used for deoxidation and grain refinement. In nitriding steels, it promotes the formation of hard, corrosion-resistant nitrided layers. Aluminum inhibits aging in mild steel and improves toughness at low temperatures. When present in higher amounts, it can improve the oxidation resistance of steel as well as its corrosion resistance in oxidizing acids and H₂S gas. Additionally, aluminum enhances the electrical and magnetic properties of steel.
Aluminum strengthens steel through solid solution strengthening, improving the wear resistance of carburized steel, fatigue strength, and core mechanical properties. In metallurgical alloys, aluminum and nickel form compounds that enhance metallurgical strength. For example, ferrochrome-aluminum alloys containing aluminum exhibit nearly constant resistance and excellent oxidation resistance at high temperatures, making them suitable for manufacturing electrometallurgical alloy materials and chrome-aluminum resistance wire.
However, excessive aluminum during steel deoxidation can cause abnormal microstructures and promote graphitization in steel. In ferritic and pearlitic steels, higher aluminum content can reduce high-temperature strength and toughness, and cause difficulties during smelting and casting processes.
Copper (Cu)
The primary role of copper in steel is to improve the atmospheric corrosion resistance of ordinary low-alloy steel, especially when used in conjunction with phosphorus. The addition of copper can also improve the strength and yield ratio of steel, while not adversely affecting its welding performance. Rail steel containing 0.20% to 0.50% copper (U-Cu) has corrosion resistance and wear resistance, with a service life 2 to 5 times longer than that of general carbon steel rails. When the copper content exceeds 0.75%, solid solution treatment followed by aging can produce an aging strengthening effect. At low copper content, its effect is similar to that of nickel, but weaker. At higher copper content, hot deformation processing becomes difficult due to copper embrittlement during hot working. Austenitic stainless steel containing 2% to 3% copper exhibits good corrosion resistance to sulfuric acid, phosphoric acid, hydrochloric acid, and stress corrosion cracking.
Boron (B)
The main role of boron in steel is to increase its hardenability, thus reducing the need for other rarer metals such as nickel, chromium, and molybdenum. For this purpose, its content is generally specified in the range of 0.001% to 0.005%. Boron can replace 1.6% of nickel, 0.3% of chromium, or 0.2% of molybdenum. However, when replacing molybdenum, it should be noted that molybdenum prevents or reduces tempering embrittlement, whereas boron has a slight tendency to promote tempering embrittlement, so boron cannot completely replace molybdenum.
In medium carbon steels, the addition of boron improves hardenability, significantly enhancing the tempering performance of steel thicker than 20 mm. Therefore, 40B and 40MnB steels can be used instead of 40Cr, and 20Mn2TiB steel can replace 20CrMnTi carburizing steel. However, the effectiveness of boron decreases or even disappears with increasing carbon content in the steel. When selecting boron-containing carburizing steels, it is important to consider that the hardenability of the carburized layer will be lower than that of the core due to this characteristic.
Spring steels generally require full hardening. Since the spring section is usually small, the use of boron-containing steel is advantageous. However, boron's effect in high-silicon spring steels is variable and inconvenient to control.
Boron has a strong affinity for nitrogen and oxygen. By adding 0.007% boron to molten steel, the aging phenomenon in steel can be eliminated.
Rare earths (Re)
Rare earth elements, as they are generally known, are the lanthanides (15 elements) plus scandium (21) and yttrium (39), a total of 17 elements in the periodic table with atomic numbers ranging from 57 to 71. They are similar in nature and not easily separated. The unseparated mixture is called mixed rare earths and is cheaper. Rare earth elements can improve the plasticity and impact toughness of forged and rolled steel, especially in cast steel. They can improve the creep resistance of heat-resistant steels, electrical alloys, and high-temperature alloys. Rare earth elements can also enhance the oxidation resistance and corrosion resistance of steel. Their effect on oxidation resistance exceeds that of elements such as silicon, aluminum, and titanium. They improve the fluidity of steel, reduce non-metallic inclusions, and make the steel structure dense and pure. The addition of appropriate rare earth elements to ordinary low alloy steel has good deoxidizing and desulfurization effects, improves impact toughness (especially low-temperature toughness), and improves anisotropic properties. Rare earth elements in ferrochrome-aluminum alloys increase the oxidation resistance of the alloy, maintain fine grains in steel at high temperatures, improve high-temperature strength, and significantly extend the life of electric heating alloys.
Nitrogen (N)
Nitrogen can be partially dissolved in iron, providing solid solution strengthening and improving hardenability, though not significantly. Due to the precipitation of nitrides at grain boundaries, it can improve the high-temperature strength of grain boundaries and increase the creep strength of steel. When chemically combined with other elements in steel, it produces a precipitation hardening effect. Nitrogen does not significantly affect the corrosion resistance of steel, but nitriding the surface of steel not only increases its hardness and wear resistance but also significantly improves corrosion resistance. Residual nitrogen in mild steel can lead to aging embrittlement.
Sulfur (S)
Increasing the content of sulfur and manganese improves the machinability of steel, and in free-cutting steels, sulfur is added as a beneficial element. Sulfur segregates heavily in steel. It deteriorates the quality of steel and reduces its plasticity at high temperatures. Sulfur is a deleterious element that exists in the form of FeS, which has a low melting point. The melting point of FeS alone is only 1190 ℃, and the eutectic temperature for the formation of eutectic crystals with iron in steel is even lower, only 988 ℃. During steel solidification, iron sulfide precipitates at the primary grain boundaries. When steel is rolled at 1100 ~ 1200 ℃, FeS at the grain boundaries will melt, greatly weakening the bond between grains, resulting in steel thermal brittleness. Therefore, sulfur should be strictly controlled, generally maintained at 0.020% to 0.050%. To prevent embrittlement due to sulfur, enough manganese should be added to form MnS with a higher melting point. If the steel contains a high sulfur content, welding may generate SO₂, which can cause porosity and looseness in the weld metal.
Phosphorus (P)
Phosphorus has a strong solid solution strengthening and cold work hardening effect in steel. As an alloying element added to low-alloy structural steel, it can improve strength and the steel's resistance to atmospheric corrosion but reduces cold stamping performance. When used in conjunction with sulfur and manganese, phosphorus can increase the machinability of steel and improve the surface quality of machined parts. For this reason, free-cutting steels often contain relatively high levels of phosphorus. Although phosphorus can improve the strength and hardness of ferrite, its greatest drawback is serious segregation, which increases tempering brittleness and significantly reduces the plasticity and toughness of steel. This leads to the so-called "cold brittleness" phenomenon, where steel undergoes brittle fracture during cold working. Phosphorus also negatively impacts weldability. Phosphorus is a harmful element and should be strictly controlled, with a typical content limit of no more than 0.03% to 0.04%.