Question

Which process is used to convert mild steel to high carbon steel?
1. Carburising 
2. Annealing 
3. Normalising
4. Tempering 

Answer 1

Correct Answer - Option 1 : Carburising 

Concept:

• Heat treatment is an operation involving heating and cooling of a metal or alloy so as to obtain certain desirable properties.
• A few important heat treatment processes are:
  • Annealing
  • Normalizing
  • Hardening
  • Tempering
• Case hardening is a method used to harden the outer surface of low-carbon steel while leaving the center or core soft and ductile. Case hardening involves heating the metal to its critical temperature in some carbonaceous material.
• The following methods are commonly used:
  • Pack method or Carburising 
  • Cyaniding
  • Nitriding
  • Induction Hardening
  • Flame hardening
• Carburization is a heat treatment process in which iron or steel absorbs carbon while the metal is heated in the presence of a carbon-bearing material, such as charcoal or carbon monoxide.
• Due to the presence of mild steel in the carbon atmosphere, mild steel converts to high carbon steel
• The intent is to make the metal surface harder.

Annealing

• The specimen is heated beyond upper critical temperature and held it there for some time and then cooled slowly in the furnace.
• It is used to refine grain size due to phase recrystallization and produce uniformity.
• After Annealing structure became large-grained pearlite.
• We will be able to improve the properties of cast and forged steels before machining.

Normalizing

• The specimen is heated beyond the upper critical temperature and is cooled in still air.
• The structure will now become fine equiaxial pearlite.
• It is used to restore the ductility of cold and hot worked material without appreciable alteration in their properties.

Tempering

• The specimen is reheated to temperature below lower critical temperature followed by any desired rate of cooling.
• In this process, martensite is produced in which iron carbide will be present in the matrix of ferrite.