Heat Treatment of Carbon Steels

Annealing
1. Effects
  1. Ductility
  2. Soften
  3. Relieve internal stresses
  4. Refine the Structure by Making it Homogenous
  5. Improve Cold-Working Properties
2. Process
  1. Heating Above Re-Crystalisation Temperature
  2. Then Cooling
  3. Stages
    1. 1: Recovery Phase
      1. Removes crystal defects
        Softens
        Removes Internal Stresses
    2. 2: Recrystallisation
      1. Strain-Free Grains Nucleate and Grow
        Replace Those Deformed by Internal Stresses
    3. 3: Grain Growth
      1. Microstructure coarsens
        May Cause Less Than Satisfactory Mechanical Properties
3. Prepared for...
  1. Shaping
  2. Stamping
  3. Forming
Normalising
1. Effects
  1. Refine Grain Structure
  2. When Steel is Heated to a High Temperature Carbon can Diffuse Throughout
    1. More Uniform Composition
  3. Improve Machinability
  4. Will Then Respond to Heat Treatment in a More Uniform Way
  5. Improve Ductility
  6. Relieve Stresses
  7. Improve Toughness
  8. Small Grains Formed
    1. Harder
    2. Tougher
    3. Normal Tensile Strength
    4. Not the Maximum Ductility Achieved by Annealing
2. Process
  1. Annealing Process
  2. Cooled in Still Air in a Furnace
  3. Heating Steel to Above its Critical Limit
  4. Soaked For Short Period
3. Prepared For...
  1. Machinability
  2. Provides Dimensional Stability if Subjected to Further Heat Treatments
Hardening
1. Effects
  1. Austenite to Pearlite CANNOT Occur
  2. Martensite Occurs
    1. Body Centred Tetragonal Lattice (BCT instead of BCC)
    2. Looks Needle-like Under Microscope
  3. Very Hard
  4. Brittle
2. Process
  1. Cooling Steel Rapidly by Quenching
    1. Water of Brine
      1. Most Rapid Cooling
    2. Oil
      1. Some Alloy Steels
    3. Air
      1. For Certain High Alloy Steels
3. Uses
  1. Surface Hardening
    1. Reason
      1. Creates a Hardened Surface to Resist Wear and Tear
        Shear Strength
        Resist Wear and Tear
      2. While Maintaining a Soft and Tough Interior or Core
        Absorbs Shock
    2. Uses
      1. Gears
      2. Ball Bearings
      3. Train Wheels
Tempering
1. Effects
  1. Transformation of Martensite into less brittle structures
  2. Increase in Toughness
  3. Decrease in Hardness
  4. Relieve Stresses
  5. Reduce Brittleness
2. Process
  1. Reheating
3. Results
  1. Transforms Martensite Into More Stable Pearlite

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Heat Treatment of Carbon Steels

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