Classification of steels:

1) Based on the chemical composition:

i) Low carbon steels: 

• Composition: 0%C to 0.25 % C.
• Microstructure: Predominantly α - ferrite and small quantities of pearlite.
• Properties: Outstanding ductility and toughness. good machinability and weldability, high formability, toughness, high ductility, etc.
• Applications: Automobile body components, structural shapes, pipes, sheets, etc.

ii) Medium carbon steels: 

• Composition: From 0.25% to 0.55% C.
• Microstructure: α - ferrite and pearlite.
• Properties: Stronger than low–carbon steel but less tough than it.
• Applications: Railway wheels & tracks, gears, etc.

iii) High carbon steels: 

• Composition: From 0.55% up to 2.1 % C
• Microstructure: Fe3C, Pearlite(C >0.8%), - ferrite and pearlite (C < 0.8%).
• Properties: Hardness, strongest, and least ductile compared to Low carbon steels.
• Applications: Knives, hack saw blades, chisels, hammers, drills, dies, machine tool cutters, punches, etc.

2) Based on Notch toughness:

Notch toughness: Notch toughness is an indication of the capacity of the steel to absorb energy when a stress concentrator or notch is present.

i) Class H steels: These are usually used for primary structure members, piling, jacket braces and legs, and deck beams. Because this class of steel has a good record of application in welded structures at service temperatures above freezing.

ii) Class N steels: These are used where the service temperature is 10°C to 0°C.

iii) Class C steels: These are used in subfreezing service temperatures (lower than 0°C), as in the North Sea or another cold climate. Because in the colder regions the temperature can reach –40°C

3) Based on manufacturing method:

i) Bessemer steel method:

• The principle of the Bessemer Converter is the removal of impurities from the iron by oxidation and the air is being blown through the molten iron.
• The furnace is made of steel with fire clay bricks to resist heat.
• The impurities manganese(mn) and Silicon(Si) are converted into their respective oxides and that can be expelled out.

?ii) Electric Arc Furnace Method:

• it is an extremely hot enclosed region, where heat is produced employing electrodes for melting certain materials such as steel (scrap) without changing the electrochemical properties of the material(metal).
• The electric arc produced between the electrodes and the metal is used for melting the metal(scrap).

?Heat treatment of steels: Heat treatment is the secondary process applied to steel to improve mechanical properties without changing the chemical properties but only change in grain structure.

Steps involved in heat treatment:

i) Heating: The specimen is heated up to a certain high temperature and during heating initially, residual stresses are relieved and at the high-temperature enlargement of grains takes place.

ii) Soaking/Holding: After heating the specimen is held for some time to get uniform grain formation, the holding time or soaking time depends on the size of the specimen.

iii) Cooling: After holding the specimen is cooled in different ways based on the requirement like slow cooling or fast cooling.

The heat treatment processes are:

• Hardening
• Annealing
• Normalizing
• Tempering