Time Temperature Transformation Curve
 
 
  • Austenite   This phase is only possible in carbon steel at high temperature.  It has a Face Centre Cubic (F.C.C) atomic structure which can contain up to 2% carbon in solution.
  • Ferrite  This phase has a Body Centre Cubic structure (B.C.C) which can hold very little carbon; typically 0.0001% at room temperature.  It can exist as either: alpha or delta ferrite. 
  • Carbon  A very small interstitial atom that tends to fit into clusters of iron atoms.  It strengthens steel and gives it the ability to harden by heat treatment.  It also causes major problems for welding , particularly if it exceeds 0.25% as it creates a hard microstructure that is susceptible to hydrogen cracking.  Carbon forms compounds with other elements called carbides.  Iron Carbide, Chrome Carbide etc.
  • Cementite  Unlike ferrite and austenite, cementite is a very hard intermetallic compound consisting of 6.7% carbon and the remainder iron, its chemical symbol is Fe3C.  Cementite is very hard, but when mixed with soft ferrite layers its average hardness is reduced considerably. Slow cooling gives course perlite; soft easy to machine but poor toughness.  Faster cooling gives very fine layers of ferrite and cementite; harder and tougher
  • Pearlite  A mixture of alternate strips of ferrite and cementite in a single grain.  The distance between the plates and their thickness is dependant on the cooling rate of the material;  fast cooling creates thin plates that are close together and slow cooling creates a much coarser structure possessing less toughness.  The name for this structure is derived from its mother of pearl appearance under a microscope.  A fully pearlitic structure occurs at 0.8% Carbon.  Further increases in carbon will create cementite at the grain boundaries, which will start to weaken the steel.


 

a) 
 
 

b) 
 
 

c) 
 

d) 

Mixture of ferrite and pearlite grains; temperature below A1, therefore microstructure not significantly affected.

Pearlite transformed to Austenite, but not sufficient temperature available to exceed the A3 line, therefore not all ferrite grains transform to Austenite.  On cooling, only the transformed grains will be normalized. 

Temperature just exceeds A3 line, full Austenite transformation.  On cooling all grains will be normalized

Temperature significantly exceeds A3 line permitting grains to grow.  On cooling, ferrite will form at the grain boundaries, and a course pearlite will form inside the grains.  A course grain structure is more readily hardened than a finer one, therefore if the cooling rate between 800°C to 500°C is rapid, a hard microstructure will be formed.  This is why a brittle fracture is most likely to propagate in this region.
 

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