Microstructure Constituents

Microstructure Constituents

1. Austenite

   • Face-Centred Cubic
   • In Iron-Carbon Alloys
     • Evident between 723°C and 1500°C
   • Can be Retained at Room Temperature by Alloy Additions Such as Nickel or Manganese
   • Former by Cooling from a Liquid State

2. Martensite

   • Formed when Cooling Rate from Austenite is Sufficiently Fast
   • Very Hard Constituent due to the Carbon which is Trapped in the Solid Solution
   • Occurs by Sudden Diffusionless Shear Process
   • Term not limited to steel

3. Bainite

   • Formed at cooling rates slower than that for martensite formation but faster than that for ferrite and pearlite formation
   • Two Forms
     • Upper Bainite
       • Forms between 550°C and 400°C
     • Lower Bainite
       • Forms between 400°C and 250°C

4. Pearlite

   • Formed During the Slow Cooling of Iron Alloys
   • Can Begin at a Temperature of 1150°C to 723°C Depending on the Composition of the Alloy
   • Formed by Eutectoid Decomposition of Austenite Upon Cooling by Diffusing of C Atoms when Ferrite and Cementite Grow Continuously