Effects of the alloying elements on the properties of steels

Carbon (C): decreases the ductility, formability, weldability increases the strength and hardenability.

Manganese (Mn): increases the strength, shock resistance, toughness, hardenability, weldebility, hot formability, no change in ductility. In addition Mn is a strong austenite former by reducing the eutectoid temperature below to room temperature. Handfield steel with 1% C and 12% Mn has strong deformation hardening ability allowing increase in strength in service (helmets, railway equipments, rock crushers jaws, shovel dippers, etc.)

Silicon (Si): increases strength, decreases the weldability, magnetic losses, oxide formation affinity, no change in ductility. In addition Si has higher affinity to O than carbon therefore used as deoxizing agent (semi-killed steels). It is also austenite former agent leading the nucleation of austenite grain in many size yielding finer grain size.

Chromium (Cr): as the Cr content increases, strength, hardenability, corrosion resistance, high temperature strength, decreases the oxide formation tendency. (forms a very coherent oxide layer on the surface preventing further oxidation– in stainless steels). It is also strong carbide former as an essential factor behaving as a strong second phase particle, therefore, obstructs the dislocation motion particularly at elevated temperatures. Also nitride former and used in nitriding steels.

Nickel (Ni): increases the strength, toughness (even at sub zero temperatures), hardenability, no change in ductility. It is an austenite former, therefore, widely used in austenitic stainless steels.

Molybdenum (Mo): increases the hardenability, high temperature strength, decreases the risk for temper embritterment (~0.5 % Mo). Since the melting point of molybdenum carbide is very high, it provides high temperature strength which is very useful in some HSS (high speed steel) tools.

Vanadium (V) and Tungsten (W): as the V and W content; increases the strength, high temperature strength, wear resistance, since both are strong carbide formers widely used in HSS tools.

Copper (Cu): restricted to max. 0.35%. Up to 0.2 % provides some resistance against to atmospheric corrosion. Not desired in spring steels.

Aluminum (Al): used as a grain refinement agent especially in the form of AlN particles. It is also deoxidizing agent used in killed steels. Also increases nitridability (Used in nitriding steels).

Zirconium (Zr), Titanium (Ti), Niobium (Nb) and Tantalum (Ta): Strong carbide formers even better than Cr. Therefore commonly used in austenitic stainless steel to free the Cr and thus further increase the corrosion resistance. Their even small concentration (~0.5 % ) can forms small carbides at grain boundaries providing very fine grain size which is the reason to high strength and ductility of low alloy (HSLA) steel, commonly used in automotive industry.

Phosphorus (P): decreases the toughness, impact resistance, cold formability, weldebility increase the corrosion resistance. Its contend is limited 0.035% max. in quality steels.

Sulfur (S): The excess sulfur reduces the ability for hot (900oC) deformation of steel forming the brittle FeS phase at the grain boundaries (hot brittleness). The solubility of S is higher than C therefore it restricts the formation of pearlite in the zones with higher S contents, leading a banded structure of pearlite and ferrite. (Macroscopy experiment: flow lines). This causes severe anisotropy in the mechanical prop of steel therefore S content is limited 0.035%. However, 0.3% S may be added to free cutting steels to increase the chip formation thus the machinability.

Oxygen (O): Produces iron oxide at grain boundaries leading high brittleness in steels. Therefore steels are deoxidized with Si and Al to avoid the risk.

Hydrogen (H): It is smallest element therefore can penetrate inside of the steels along their grain boundaries. The formation of H2 molecules creates pressure at the grain boundaries. When this is associated with the external stresses acting on the material, brittle fracture may occur which is called hydrogen embitterment. This is particularly very harmful for high strength steels.

Nitrogen (N): Increases the tendency of aging and strain aging of low carbon steels. Thus the distinct yield point becomes apparent and the strength increase and ductility decrease. The nitrides of Cr, Al etc. increases high temperature strength of steels. It is used in PH (precipitation hardening) stainless steels for this purpose.