1. The Iron-Iron Carbide Phase Diagram
Heat Treatment of Steel

3. Features Reactions Phases present L a ferrite Bcc structure
Ferromagnetic
Fairly ductile d
Bcc structure
Paramagnetic
g austenite
Fcc structure
Non-magnetic
ductile
Fe3C cementite
Orthorhombic
Hard
brittle
Reactions
Peritectic L + d = g
Max. solubility of C in ferrite=0.022%
Max. solubility of C in austenite=2.11%
Eutectic L = g + Fe3C
Eutectoid g = a + Fe3C

4. Microstructures a g

5. Steels Classification/Nomenclature
Steel is an interstitial solid solution of carbon in iron.
Theoretically steel has a maximum of 2.11% carbon.
In practice, the amount of carbon rarely exceeds 0.8%
Classification/Nomenclature
Low carbon steels up to 0.2%C
Medium carbon steels %C
High carbon steels >0.4% C
AISI 1020:
Last two numbers indicate
Amount of carbon :0.2%C
10 indicates plain carbon steel
AISI 4340: 0.4%C
43 indicates alloy steel

6. Microstructural changes in steel on cooling for different compositions

7. Eutectoid steel

8. Hypoeutectoid steel

9. Hypereutectoid steel