_______ is a process of case hardening in which the outer layer is made hard by increasing amount of carbon.

Concept:

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

Carburizing

• Carburizing is one of the most widely used surface-hardening processes.
• The process involves diffusing carbon into low-carbon steel to form a high-carbon steel surface.
• The Carburizing process is also referred to as case hardening or case carburizing process.
• It is a heat treatment process that produces a surface that is resistant to wear while maintaining the toughness and strength of the core.
• High Carbon Steels have higher strength and hardness.
• Low-carbon steels have higher toughness.
• Increasing carbon only on the surface of low-carbon steel can give a hard surface and tough core.​

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 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 for about fifteen minutes, and then allowed it to cool down in still air. 
• The homogeneous structure provides a higher yield point, ultimate tensile strength, and impact strength with lower ductility to steel.
• The main purpose of normalizing is 
  • Refine grain size in metal, improve strength and hardness, reduce ductility
  • Remove cold work stress.
  • Remove dislocations due to hot working.