Heat Treatment of Carbon Steels

Heat Treatment of Carbon Steels

1. Annealing

   • Effects
     • Ductility
     • Soften
     • Relieve internal stresses
     • Refine the Structure by Making it Homogenous
     • Improve Cold-Working Properties
   • Process
     • Heating Above Re-Crystalisation Temperature
     • Then Cooling
     • Stages
       • 1: Recovery Phase
         • Removes crystal defects
           • Softens
           • Removes Internal Stresses
       • 2: Recrystallisation
         • Strain-Free Grains Nucleate and Grow
           • Replace Those Deformed by Internal Stresses
       • 3: Grain Growth
         • Microstructure coarsens
           • May Cause Less Than Satisfactory Mechanical Properties
   • Prepared for...
     • Shaping
     • Stamping
     • Forming

2. Normalising

   • Effects
     • Refine Grain Structure
     • When Steel is Heated to a High Temperature Carbon can Diffuse Throughout
       • More Uniform Composition
     • Improve Machinability
     • Will Then Respond to Heat Treatment in a More Uniform Way
     • Improve Ductility
     • Relieve Stresses
     • Improve Toughness
     • Small Grains Formed
       • Harder
       • Tougher
       • Normal Tensile Strength
       • Not the Maximum Ductility Achieved by Annealing
   • Process
     • Annealing Process
     • Cooled in Still Air in a Furnace
     • Heating Steel to Above its Critical Limit
     • Soaked For Short Period
   • Prepared For...
     • Machinability
     • Provides Dimensional Stability if Subjected to Further Heat Treatments

3. Hardening

   • Effects
     • Austenite to Pearlite CANNOT Occur
     • Martensite Occurs
       • Body Centred Tetragonal Lattice (BCT instead of BCC)
       • Looks Needle-like Under Microscope
     • Very Hard
     • Brittle
   • Process
     • Cooling Steel Rapidly by Quenching
       • Water of Brine
         • Most Rapid Cooling
       • Oil
         • Some Alloy Steels
       • Air
         • For Certain High Alloy Steels
   • Uses
     • Surface Hardening
       • Reason
         • Creates a Hardened Surface to Resist Wear and Tear
           • Shear Strength
             • Resist Wear and Tear
         • While Maintaining a Soft and Tough Interior or Core
           • Absorbs Shock
       • Uses
         • Gears
         • Ball Bearings
         • Train Wheels

4. Tempering

   • Effects
     • Transformation of Martensite into less brittle structures
     • Increase in Toughness
     • Decrease in Hardness
     • Relieve Stresses
     • Reduce Brittleness
   • Process
     • Reheating
   • Results
     • Transforms Martensite Into More Stable Pearlite