Iron Carbon Diagram

Iron and Steel

This section consists of the following:

The important Steel Making Processes

The Production of Cast Iron

The Structure of Plain Carbon Steel; influence of Carbon Content; Equilibrium Diagrams.

Classification, Properties and Uses of Carbon Steels, Cast Iron and Alloy Wheels.

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A Simplified version of the Iron-Carbon Equilibrium Diagram is shown below

1 – Austenite and Ferrite.

2 – Austenite.

3 – Austenite and Cementite.

4 – Pearlite and Ferrite.

5 – Pearlite and Cementite.

Describe the structural changes that occur in 0.4% carbon steel as it cooled slowly from 900 °C.

At 900oC, above the upper critical temperature, the carbon is dissolved in

a solid solution called austenite. This has a face-centre cubic (FCC)

structure above 910oC.

As slow cooling takes place to below the lower critical temperature of

723oC, ferrite and soft pearlite (ferrite and thin layers of cementite) are

formed. This makes the material as soft as possible. A body-centred cubic (BCC)

form exists below 810oC

Describe the structural changes that occur in 0.4% carbon steel as it quenched from 900 °C.

The resulting material is called Martensite. Martensite has a hard needle-like structure giving a strong but brittle

material. The distortion in the structure occurs because of the rapid quenching in hardening where

excess carbon does not come out of the solution.