INFLUENCE OF THE CONSTITUENTS ON STEEL :

The properties of steel are dependent primarily on the carbon content and to a large extent upon silicon^ manganese,) sulphur and phosphorus.

Carbon - It increases strength* elasticity (determined by yield point) and hardness and lowers the ductility and impact strength. An important fact is that above 0.3% carbon, the steel can be hardened by heating and quenching in water or oil.

Silicon - Silicon in the finished steel usually ranged from 0.05 to 0.30%, silicon is put in carbon steels to prevent them from becoming porous. It acts as a very good deoxidiser and removes the gases and oxides, prevents blowholes and thereby makes the steel tougher and harder.

Manganese - It serves as a valuable deoxidising and purifying agent. Manganese also combines with sulphur and thereby decreases the harmful effect of this element remaining in the steel.

When used in ordinary low carbon steels, manganese makes the metal ductile and of good bending qualities. The manganese content, of carbon steel is uaually less than 1.00% and commonly ranges from 0.30 to 1.00%.

Sulphur - Sulphur occurs in steel either as iron sulphide or as manganese sulphide. Iron sulphide, because of its low melting point, produces red-shortness, i.e., increased brittleness at elevated temperatures, whilst manganese sulphide does not affect so much. Therefore, manganese sulphide is less objectionable in steel than iron sulphide. The presence of a substantial amount of sulphur (sulphur inclusios) in steel unfavourably affect other properties of steel, e.g. tensile strength, yield point, fatigue limit, etc. and decreases its corrosion resistance.

Phosphorus - It imparts cold-shortness, i.e., high brittleness at normal and especially at low temperatures. It increases the tensile strength but at the same time reduces the impact strength and ductility. At a phosphorus content of slightly over 0.2% the impact strength  has a zero value. The embrittlement effect , of phosphorus, increases with a increase in the carbon content of steel, The sum of carbon and posphorus usually does not exceei 0.25%. Tool steels of the best quality do not contain more than 0.02%.

Aluminium - It may be considered as an impurity and should be within 0.05% limit as for sulphur and phosphorus But a small percentage is beneficial in the steel making process to scavenge oxygen and to "Kill" the steel, i.e., so that it do not have porous blow holes (Mn & Si also do this).

Nitrogen - Nitrogen causes embrittlement, and for this reason the average composition should generally not exceed 0.005% (the norm for the high grade steel by the open hearth process).

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