The document discusses the iron-iron carbide phase diagram and the microstructural changes that occur in steels of different carbon compositions during heating and cooling. It outlines the phases present - ferrite, austenite, cementite - and their structures. Steels are defined as solid solutions of carbon in iron, with classifications based on carbon content: low carbon <0.2%, medium 0.2-0.4%, and high carbon >0.4%. The diagram shows eutectic, peritectic, and eutectoid reactions that occur, and how the microstructures of eutectoid, hypoeutectoid, and hypereutectoid steels change during cooling.

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

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

5. Microstructures
a g

6. Steels
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 0.2-0.4%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

7. Microstructural changes in steel on
cooling for different compositions

8. Eutectoid steel

9. Hypoeutectoid steel

10. Hypereutectoid steel