The Role of 18 Alloying Elements in Steel

Chromium (Cr)
Chromium increases the hardenability of steel and has a secondary hardening effect. It can improve the hardness and wear resistance of carbon steel without making it brittle. When the content exceeds 12%, it imparts good high-temperature oxidation resistance and resistance to oxidizing corrosion, and also increases the hot strength of steel. Chromium is the primary alloying element in stainless acid-resistant steel and heat-resistant steel.
Chromium increases the strength and hardness of carbon steel in the as-rolled condition, while reducing elongation and reduction of area. When chromium content exceeds 15%, strength and hardness decrease, while elongation and reduction of area increase correspondingly. Parts made of chromium-containing steel are easy to obtain a high surface finish through grinding.
The main role of chromium in quenched and tempered structural steels is to increase hardenability, enabling the steel to achieve better comprehensive mechanical properties after quenching and tempering. In carburizing steels, it can also form chromium carbides, thereby improving the wear resistance of the material surface.
Chromium-containing spring steel is less prone to decarburization during heat treatment. Chromium improves the wear resistance, hardness, and red hardness of tool steel, and provides good tempering stability. In electric heating alloys, chromium enhances oxidation resistance, electrical resistance, and strength.

Nickel (Ni)
Nickel strengthens ferrite and refines pearlite in steel. The overall effect is an increase in strength, with little significant impact on plasticity. Generally, for low-carbon steels used in as-rolled, normalized, or annealed conditions without quenching and tempering treatment, a certain nickel content can increase strength without significantly reducing toughness. Statistically, each 1% increase in nickel can raise strength by about 29.4 MPa. As nickel content increases, the yield strength increases faster than the tensile strength, so the yield ratio of nickel-containing steel can be higher than that of ordinary carbon steel. While increasing strength, nickel is less detrimental to toughness, plasticity, and other processing properties compared to other alloying elements.
For medium-carbon steels, nickel lowers the pearlite transformation temperature, refining the pearlite structure. It also lowers the carbon content of the eutectoid point, meaning that compared to carbon steel of the same carbon content, nickel-containing steel has more pearlite, resulting in higher strength. Conversely, to achieve the same strength, the carbon content of nickel-containing steel can be appropriately reduced, thereby improving toughness and plasticity. Nickel increases the steel's resistance to fatigue and reduces its notch sensitivity. It lowers the brittle transition temperature of steel, which is extremely important for low-temperature steels. Steel with 3.5% Ni can be used at -100°C, while steel with 9% Ni can operate at -196°C. Nickel does not increase resistance to creep and is therefore generally not used as a strengthening element in heat-resistant steels.
Iron-nickel alloys with high nickel content exhibit significant changes in linear expansion coefficient with varying nickel content. This property is utilized to design and produce precision alloys and bimetallic materials with very low or specific linear expansion coefficients.
Additionally, nickel added to steel provides resistance not only to acids but also to alkalis, and offers corrosion resistance against atmosphere and salt. Nickel is one of the important elements in stainless acid-resistant steel.

Molybdenum (Mo)
Molybdenum in steel increases hardenability and hot strength, prevents temper brittleness, increases remanence and coercive force, and enhances corrosion resistance in certain media.
In quenched and tempered steels, molybdenum allows deeper hardening of larger cross-section parts, improves resistance to tempering (tempering stability), enabling parts to be tempered at higher temperatures. This more effectively eliminates (or reduces) residual stresses and improves plasticity.
In carburizing steels, besides the above effects, molybdenum also reduces the tendency for carbides to form a continuous network at grain boundaries in the carburized layer, decreases retained austenite in the carburized layer, and relatively increases surface wear resistance.
In die steels, molybdenum helps maintain relatively stable hardness and increases resistance to deformation, cracking, and wear.
In stainless acid-resistant steels, molybdenum further enhances resistance to corrosion by organic acids (like formic, acetic, oxalic acid), hydrogen peroxide, sulfuric acid, sulfurous acid, sulfates, acid dyes, bleaching powder solutions, etc. Crucially, molybdenum addition prevents the tendency for pitting corrosion caused by the presence of chloride ions.
W12Cr4V4Mo high-speed steel containing about 1% Mo possesses high wear resistance, tempered hardness, and red hardness.

Tungsten (W)
In steel, tungsten partly forms carbides and partly dissolves in iron to form solid solutions. Its effect is similar to molybdenum, but generally less pronounced on a weight percentage basis. The main purpose of tungsten in steel is to increase tempering stability, red hardness, hot strength, and wear resistance due to carbide formation. Therefore, it is mainly used in tool steels, such as high-speed steels and hot forging die steels.
In high-quality spring steels, tungsten forms refractory carbides. During tempering at higher temperatures, it slows down carbide agglomeration, maintaining higher high-temperature strength. Tungsten also reduces the overheating sensitivity of steel, increases hardenability, and improves hardness. 65SiMnWA spring steel achieves high hardness after hot rolling and air cooling. Sections up to 50mm² can be fully hardened in oil. It can be used for important springs subjected to heavy loads, heat (≤350°C), and impact. 30W4Cr2VA high-strength heat-resistant spring steel has high hardenability. After quenching at 1050-1100°C and tempering at 550-650°C, its tensile strength reaches 1470-1666 MPa. It is mainly used for springs operating at high temperatures (≤500°C).
The addition of tungsten significantly improves the wear resistance and machinability of steel, making it a primary element in alloy tool steels.

Vanadium (V)
Vanadium has a strong affinity for carbon, nitrogen, and oxygen, forming stable corresponding compounds. It exists mainly as carbides in steel. Its primary role is to refine the steel structure and grains, increasing strength and toughness. When dissolved in austenite at high temperatures, it increases hardenability; conversely, when present as carbides, it decreases hardenability. Vanadium increases the tempering stability of quenched steel and produces a secondary hardening effect. Vanadium content in steel (except high-speed tool steels) generally does not exceed 0.5%.
In ordinary low-alloy steels, vanadium refines grains, improves strength, yield ratio, and low-temperature properties after normalizing, and enhances weldability.
In alloy structural steels, since vanadium generally reduces hardenability under common heat treatment conditions, it is often used in combination with elements like manganese, chromium, molybdenum, and tungsten. In quenched and tempered steels, vanadium mainly increases strength and yield ratio, refines grains, and reduces overheating sensitivity. In carburizing steels, because it refines grains, the steel can be quenched directly after carburizing without a second quench.
In spring and bearing steels, vanadium increases strength and yield ratio, particularly the proportional limit and elastic limit. It reduces sensitivity to decarburization during heat treatment, thereby improving surface quality. Bearing steels containing vanadium (like chromium steels) have finely dispersed carbides and good service performance.
In tool steels, vanadium refines grains, reduces overheating sensitivity, increases tempering stability and wear resistance, thereby extending tool life.

Titanium (Ti)
Titanium has a strong affinity for nitrogen, oxygen, and carbon, and a stronger affinity for sulfur than iron. Therefore, it is an effective deoxidizer and degasser, and an effective element for fixing nitrogen and carbon. Although titanium is a strong carbide-forming element, it does not form complex compounds with other elements. Titanium carbide has strong bonding, is stable, and difficult to decompose. In steel, it dissolves slowly into the solid solution only when heated above 1000°C. Before dissolution, titanium carbide particles inhibit grain growth. Because the affinity between titanium and carbon is much greater than that between chromium and carbon, titanium is often used in stainless steel to fix carbon, eliminating or reducing chromium depletion at grain boundaries, thereby mitigating intergranular corrosion.
Titanium is also a strong ferrite former, significantly raising the A1 and A3 temperatures of steel. In ordinary low-alloy steels, titanium improves plasticity and toughness. By fixing nitrogen and sulfur and forming titanium carbide, it increases steel strength. Normalizing refines grains, and carbide precipitation significantly improves plasticity and impact toughness. Alloy structural steels containing titanium have good mechanical and processing properties; their main drawback is slightly lower hardenability.
In high-chromium stainless steels, titanium is usually added in amounts about 5 times the carbon content. This not only improves corrosion resistance (mainly against intergranular corrosion) and toughness but also prevents grain growth at high temperatures and improves weldability.

Niobium/Columbium (Nb/Cb)
Niobium and tantalum (Ta) often coexist; their effects in steel are similar. Niobium and tantalum partially dissolve in the solid solution, causing solid solution strengthening. When dissolved in austenite, they significantly increase the hardenability of steel. However, when present as carbide and oxide particles, they refine grains and decrease hardenability. They increase tempering stability and have a secondary hardening effect. Trace amounts of niobium can increase steel strength without impairing plasticity or toughness. Due to grain refinement, they improve impact toughness and lower the brittle transition temperature. When the content exceeds 8 times the carbon content, they can fix almost all carbon in the steel, imparting good hydrogen resistance. In austenitic steels, they prevent intergranular corrosion in oxidizing media. By fixing carbon and precipitation hardening, they improve the high-temperature properties of heat-resistant steels, such as creep strength.
In construction-grade ordinary low-alloy steels, niobium increases yield strength and impact toughness, lowers the brittle transition temperature, and benefits weldability. In carburizing and quenched/tempered alloy structural steels, it increases hardenability while improving toughness and low-temperature properties. It reduces the air hardenability of low-carbon martensitic heat-resistant stainless steels, avoids temper embrittlement, and increases creep strength.

Zirconium (Zr)
Zirconium is a strong carbide-forming element. Its effect in steel is similar to niobium, tantalum, and vanadium. Adding small amounts acts as a degasser, purifier, and grain refiner, benefiting low-temperature properties and improving formability. It is often used in ultra-high-strength steels and nickel-based superalloys for gas turbine engines and ballistic missile structures.

Cobalt (Co)
Cobalt is mostly used in special steels and alloys. Cobalt-containing high-speed steels have high hot hardness. When added to maraging steels along with molybdenum, ultra-high hardness and good comprehensive mechanical properties can be achieved. Additionally, cobalt is an important alloying element in heat-resistant steels and magnetic materials.
Cobalt reduces the hardenability of steel. Therefore, adding it alone to carbon steel reduces the comprehensive mechanical properties after quenching and tempering. Cobalt strengthens ferrite. When added to carbon steel, it increases hardness, yield point, and tensile strength in the annealed or normalized condition, but adversely affects elongation and reduction of area. Impact toughness also decreases with increasing cobalt content. Due to its oxidation resistance, cobalt finds application in heat-resistant steels and superalloys. Cobalt-based alloys demonstrate their unique role in gas turbines.

Silicon (Si)
Silicon dissolves in ferrite and austenite, increasing the hardness and strength of steel. Its effect is second only to phosphorus and stronger than manganese, nickel, chromium, tungsten, molybdenum, vanadium, etc. However, silicon content exceeding 3% significantly reduces the plasticity and toughness of steel. Silicon increases the elastic limit, yield strength, yield ratio (σs/σb), fatigue strength, and fatigue ratio (σ-1/σb). This is why silicon or silicon-manganese steels are used as spring steels.
Silicon reduces the density, thermal conductivity, and electrical conductivity of steel. It promotes coarsening of ferrite grains and reduces coercive force. It diminishes the tendency for crystalline anisotropy, making magnetization easier and reducing magnetic reluctance. This is used to produce electrical steels, resulting in low core loss in silicon steel sheets. Silicon increases the permeability of ferrite, giving higher magnetic induction in weak magnetic fields. However, in strong magnetic fields, silicon reduces magnetic induction. Due to its strong deoxidizing power, silicon reduces magnetic aging in iron.
Silicon-containing steel forms an SiO2 film on its surface when heated in an oxidizing atmosphere, improving oxidation resistance at high temperatures.
Silicon promotes the growth of columnar crystals in cast steel, reducing plasticity. If silicon steel cools too rapidly after heating, the low thermal conductivity causes large internal-external temperature differences, leading to cracking.
Silicon reduces the weldability of steel. Because silicon has a stronger affinity for oxygen than iron, it easily forms low-melting-point silicates during welding, increasing slag and molten metal fluidity, causing spatter and affecting weld quality. Silicon is a good deoxidizer. Adding an appropriate amount of silicon along with aluminum significantly enhances the deoxidizing effect. Steel inherently contains some residual silicon from raw materials in iron and steelmaking. In rimmed steel, silicon is limited to <0.07%. When intentionally added, ferrosilicon alloy is used during steelmaking.

Manganese (Mn)
Manganese is a good deoxidizer and desulfurizer. Steel generally contains a certain amount of manganese, which eliminates or mitigates the hot shortness caused by sulfur, thereby improving the hot workability of steel.
Manganese forms solid solutions with iron, increasing the hardness and strength of ferrite and austenite in steel. It is also a carbide-forming element, replacing some iron atoms in cementite. By lowering the critical transformation temperature, manganese refines pearlite, indirectly contributing to increased strength in pearlitic steels. Manganese's ability to stabilize austenite is second only to nickel, and it also strongly increases hardenability. Manganese content up to 2% is combined with other elements to produce various alloy steels.
Manganese is widely used due to its abundance and versatility, such as in higher-manganese carbon structural steels and spring steels.
In high-carbon, high-manganese wear-resistant steels (Hadfield steel), manganese content can reach 10-14%. After solution treatment, it has good toughness. When subjected to impact deformation, the surface layer work-hardens, providing high wear resistance.
Manganese forms the relatively high-melting-point MnS with sulfur, preventing hot shortness caused by FeS. Manganese tends to increase grain coarsening and sensitivity to temper embrittlement. Improper cooling after casting or forging/rolling can easily cause flakes (hydrogen flakes) in steel.

Aluminum (Al)
Aluminum is mainly used for deoxidation and grain refinement. In nitriding steels, it promotes the formation of a hard, corrosion-resistant nitrided layer. Aluminum inhibits aging in low-carbon steels and improves low-temperature toughness. At higher contents, it enhances oxidation resistance and corrosion resistance in oxidizing acids and H2S gas, and improves electrical and magnetic properties. Aluminum has a strong solid solution strengthening effect, improving wear resistance, fatigue strength, and core mechanical properties in carburizing steels.
In superalloys, aluminum forms compounds with nickel (gamma prime), increasing high-temperature strength. Iron-chromium-aluminum alloys (FeCrAl) have near-constant electrical resistance and excellent oxidation resistance at high temperatures, making them suitable for electric heating alloys and resistance wires like Kanthal.
Excessive aluminum used for deoxidation in some steels can cause abnormal structures and promote graphitization. In ferritic and pearlitic steels, higher aluminum content reduces high-temperature strength and toughness, and poses difficulties in smelting and casting.

Copper (Cu)
The prominent role of copper in steel is to improve the atmospheric corrosion resistance of ordinary low-alloy steels, especially when used in combination with phosphorus. Adding copper also increases strength and yield ratio without adversely affecting weldability. Rail steel (U-Cu) containing 0.20-0.50% copper is not only wear-resistant but also has a corrosion life 2-5 times that of ordinary carbon rail steel.
Copper content exceeding 0.75%, after solution treatment and aging, can produce age hardening. At lower levels, its effect is similar to nickel but weaker. Higher copper content is detrimental to hot working, causing copper embrittlement during hot deformation. 2-3% copper in austenitic stainless steels can enhance corrosion resistance to sulfuric, phosphoric, and hydrochloric acids, and improve resistance to stress corrosion cracking.

Boron (B)
The main role of boron in steel is to increase hardenability, thereby saving other scarcer metals like nickel, chromium, and molybdenum. For this purpose, its content is generally specified in the range of 0.001-0.005%. It can replace 1.6% Ni, 0.3% Cr, or 0.2% Mo. Care must be taken when substituting boron for molybdenum because molybdenum prevents or reduces temper embrittlement, while boron slightly promotes it; thus, boron cannot completely replace molybdenum.
Adding boron to medium-carbon carbon steel significantly improves the properties after quenching and tempering of sections thicker than 20mm due to increased hardenability. Therefore, 40B and 40MnB steels can replace 40Cr, and 20Mn2TiB can replace 20CrMnTi carburizing steel. However, since boron's effect diminishes or disappears as the carbon content in the steel increases, when selecting boron-containing carburizing steel, it must be considered that the hardenability of the carburized case will be lower than that of the core after carburizing.
Spring steels generally require full hardenability. Since spring sections are usually small, boron-containing steels are advantageous. For high-silicon spring steels, boron's effect is more variable and less convenient to use.
Boron has a strong affinity for nitrogen and oxygen. Adding 0.007% boron to rimmed steel eliminates aging.

Rare Earths (RE)
The term "rare earth elements" generally refers to the lanthanides (15 elements from atomic number 57 to 71) plus scandium (21) and yttrium (39), totaling 17 elements. Their properties are similar and difficult to separate. The unseparated mixture is called mischmetal, which is cheaper. Rare earth elements improve the plasticity and impact toughness of forged and rolled steels, particularly significantly in cast steels. They increase the creep resistance of heat-resistant steels, electric heating alloys, and superalloys.
Rare earth elements also improve oxidation and corrosion resistance. Their effect on oxidation resistance surpasses that of silicon, aluminum, and titanium. They improve the fluidity of molten steel, reduce non-metallic inclusions, and make the steel structure denser and purer.
Adding appropriate amounts of rare earths to ordinary low-alloy steels provides good deoxidation and desulfurization, improves impact toughness (especially at low temperatures), and reduces anisotropy.
In iron-chromium-aluminum alloys (FeCrAl), rare earths increase oxidation resistance, maintain a fine grain structure at high temperatures, and improve high-temperature strength, significantly extending the service life of electric heating alloys.

Nitrogen (N)
Nitrogen partially dissolves in iron, providing solid solution strengthening and slightly increasing hardenability. Since nitrides precipitate at grain boundaries, they can increase high-temperature grain boundary strength and creep strength. By combining with other elements in steel, they cause precipitation hardening. Nitrogen does not significantly affect corrosion resistance, but surface nitriding greatly improves hardness, wear resistance, and corrosion resistance. Residual nitrogen in low-carbon steel causes aging embrittlement.

Sulfur (S)
Increasing sulfur and manganese content improves the machinability of steel. In free-cutting steels, sulfur is added as a beneficial element. Sulfur causes severe segregation, degrading steel quality. At high temperatures, it reduces plasticity and is generally a harmful element. It exists mainly as low-melting-point FeS (melting point 1190°C). The FeS-Fe eutectic has an even lower melting point (988°C). During solidification, FeS segregates at the primary grain boundaries. When steel is rolled at 1100-1200°C, the FeS at grain boundaries melts, severely weakening intergranular cohesion, leading to hot shortness. Therefore, sulfur must be strictly controlled, generally between 0.020-0.050%. To prevent sulfur-induced brittleness, sufficient manganese must be added to form higher-melting-point MnS. If sulfur content is high, SO2 generated during welding causes porosity and shrinkage cavities in the weld metal.

Phosphorus (P)
Phosphorus has a strong solid solution strengthening and work hardening effect in steel. Added as an alloying element to low-alloy structural steels, it increases strength and atmospheric corrosion resistance, but reduces cold formability. Used in combination with sulfur and manganese, it improves machinability and surface finish of machined parts, hence its high content in free-cutting steels. While phosphorus strengthens ferrite, increasing strength and hardness, its greatest harm is severe segregation, increased temper embrittleness, and a significant decrease in plasticity and toughness. This makes steel prone to brittle fracture during cold working, known as "cold shortness". Phosphorus also adversely affects weldability. Phosphorus is a harmful element and should be strictly controlled, generally not exceeding 0.03-0.04%.