The toughness in steel increases and the brittleness decrease by a heat treatment operation called as _____.

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:

1. Annealing
2. Normalizing
3. Hardening
4. 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:

1. Carburizing
2. Cyaniding
3. Nitriding
4. Induction Hardening
5. Flame hardening

Tempering 

• Tempering is the process of annealing martensite at low temperatures.
• Tempering is a heat-treatment process consisting of reheating the hardened steel to a temperature below 400°C, followed by cooling.
• Steel in its hardened condition is generally too brittle to be used for certain functions. Therefore, it is tempered. The aims of tempering are:
  • to relieve the internal stresses
  • to regulate the hardness and toughness
  • to decrease the brittleness
  • to restore some ductility
  • to induce shock resistance

Annealing

• Annealing is a heat treatment process in which the material is taken to a high temp, kept there for some time, and then cooled in the furnace itself.
• Cooling is done slowly to avoid the distortion.
• The benefits of annealing are:
  • Relieve stresses
  • Increase softness, ductility, and toughness
  • Produce a specific microstructure
  • Improve machining characteristic

Normalizing

• Heat the steel from 30°C to 50°C above its upper critical temp, held about fifteen minutes, and then allowed to cool down in still air. 
• The homogeneous structure provides a higher yield point, ultimate tensile strength, and impact strength with lower ductility to steels.
• The main purpose of normalizing is 
  • Refine grain size in metal, improve strength and hardness, reduce ductility
  • Remove cold worked stress.
  • Remove dislocations due to hot working.