Alloying elements and their influence on properties of steel
Alloying elements and their influence on properties of steel
Steel is the most commonly used alloy of iron. The typical influence on the property of steel by the presence of alloying elements is depicted in Fig 1.
Fig 1 Effect of alloying element on steel
The following gives an overview of some of the influences of alloying elements on the properties of steel.
- Aluminium (Al) – It is used as a deoxidizer. Aluminium restricts austenite grain growth in reheated steels and is normally added to control grain size. Aluminium is very effective alloying element in controlling grain growth prior to quenching. It is an alloying element in nitriding steels.
- Boron (B) – It is added to fully killed steel to improve hardenability. Boron treated steels are produced with boron in a range of 0.0005% to 0.003%. Whenever boron is substituted in part for other alloys, it should be done only with hardenability in mind because the lowered alloy content may be harmful for some applications. Boron is a potent alloying element in steel. Boron is most effective in lower carbon steels.
- Carbon (C) – It has a major effect on the properties of steel. Carbon is the primary hardening element in steel. Hardness and tensile strength increases with the increase in carbon content up to about 0.85% C. Ductility and weldability of steel decrease with increasing per cent of carbon.
- Chromium (Cr) – It is commonly added to steel to increase corrosion resistance and oxidation resistance. It increases hardenability and improves high temperature strength. With added carbon it gives wear and abrasion resistance properties to steels. As a hardening element, chromium is normally used with a toughening element such as nickel for superior mechanical properties. Chromium is a strong carbide former element. Complex chromium iron carbides go into solution in austenite slowly hence requires higher heating time prior to quenching.
- Cobalt (Co) – Cobalt contributes to red hardness by considerably hardening ferrite through solid solution. It is a common alloying element in high grade high speed steels.
- Copper (Cu) – Copper in significant amounts is detrimental to hot working of steels. Copper has a negative effect during forge welding but it does not affect arc or oxyacetylene welding seriously. Copper can be detrimental to surface quality. Copper is beneficial to atmospheric corrosion resistance when present in amounts more than 0.30%. Weathering steels are made with copper content greater than 0.30%.
- Lead (Pb) – It is almost insoluble in liquid or solid steel. However, lead is sometimes added to carbon and alloy steels by means of mechanical dispersion during pouring to improve the machinability.
- Manganese (Mn) – It is beneficial to surface quality. It counteracts effect of brittleness from sulphur in steels. It also contributes to strength and hardness but its effect is less than carbon. The increase in strength is dependent upon the carbon content. Increasing the manganese content also decreases ductility and weldability but the effect is less than carbon. Manganese affects the hardenability of steels significantly. High Manganese and high carbon gives steel wear and abrasion resistant properties.
- Molybdenum (Mo) – It raises coarsening temperature of austenite. It increases the hardenability of steel. Molybdenum may produce secondary hardening during the tempering of quenched steels. It enhances the hot and creep strength of low alloy steels at elevated temperatures. It enhances corrosion resistance in stainless steels. It forms abrasion resistant particles.
- Nickel (Ni) – it is a ferrite strengthener. It strengthens unquenched or annealed steels. Nickel does not form carbides in steel. It remains in solution in ferrite, strengthening and toughening the ferrite phase especially at low temperatures. Nickel increases the hardenability and impact strength of steels.
- Niobium (Columbium) (Nb) – It increases yield strength and to a lesser degree the tensile strength of carbon steels. The addition of small amounts of Niobium can significantly increase the yield strength of steels. Niobium can also have a moderate precipitation strengthening effect. Its main contribution is to form precipitates above the transformation temperature and to retard the recrystallization of austenite, thus promoting a fine grain microstructure with improved strength and toughness.
- Phosphorus (P) – It increases strength and hardness and decreases ductility and notch impact toughness of steels. It increases resistance to corrosion and improves machinability in free cutting steels.. The adverse effect on ductility and toughness is greater in quenched and tempered high carbon steels. Phosphorous levels are normally controlled to low levels.
- Silicon (Si) – It is one of the main deoxidizer used while making steels. Silicon is less effective than manganese in increasing as rolled strength and hardness. Silicon is generally
detrimental to surface quality in low carbon steels. It strengthens low alloy steels. Silicon is used as alloying element for electrical and magnetic steels.
- Sulphur (S) – It decreases ductility and notch impact toughness especially in the transverse direction. Weldability of steels decreases with increasing sulphur content. Sulphur is found primarily in the form of sulphide inclusions. Sulphur levels in steels are controlled to low levels except in free cutting steels where sulphur is added to improve machinability.
- Titanium (Ti) – It is used to retard grain growth and thus improve toughness. It fixes carbon in inert particles. It reduces martensitic hardness and hardenability in medium chromium steels. It prevents formation of austenite in high chromium steels. Titanium is also used to achieve improvements in inclusion characteristics. Titanium causes sulphide inclusions to be globular rather than elongated thus improving toughness and ductility in transverse bending.
- Tungsten (W) – It forms hard abrasion resistant particles in tool steels and high speed steels. It promotes red hardness and hat strength. It increases hardenability strongly in small quantity. It opposes softening by secondary hardening.
- Vanadium (V) – It increases the yield strength and the tensile strength of carbon steels. Small amount of Vanadium increases the strength of steels significantly. It promotes fine grain and elevates coarsening temperature of austenite. It increases hardenability when dissolved. It resists tempering and causes marked secondary hardening. Vanadium is one of the primary contributors to precipitation strengthening in micro alloyed steels. When thermo mechanical processing is properly controlled, the ferrite grain size is refined and there is a corresponding increase in toughness. The impact transition temperature increases when vanadium is added. Vanadium, niobium, and titanium combine preferentially with carbon and/or nitrogen to form a fine dispersion of precipitated particles in the micro alloyed steel matrix.
- Zirconium (Zr) – It is added to killed high strength low alloy steels to achieve improvements in inclusion characteristics. Zirconium causes sulphide inclusions to be globular rather than elongated thus improving toughness and ductility in transverse bending.