Identify the method by which steel is NOT classified.

Identify the method by which steel is NOT classified.

Right Answer is:

Based on notch toughness


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 steelsThese 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 high temperature enlargement of grains takes place.

ii) Soaking/Holding: After heating the specimen is hold 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
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