State the phase regions labelled A, B, C and D. Describe the reaction and write the equation that occurs at point X.

Here are the solutions for Question Four.

i) State the phase regions labelled A, B, C and D.

• A: Austenite ($\gamma$)
• B: Austenite ($\gamma$) + Cementite (Fe$_3$C)
• C: Ferrite ($\alpha$) + Austenite ($\gamma$)
• D: Ferrite ($\alpha$) + Cementite (Fe$_3$C)

ii) Describe the reaction and write the equation that occurs at point X.

Description: Point X represents the eutectoid reaction. This is a solid-state transformation where, upon cooling, a single solid phase (Austenite) transforms isothermally into two new solid phases (Ferrite and Cementite) that are intimately mixed in a lamellar structure known as Pearlite. Upon heating, the reverse reaction occurs.

Equation:

$$\gamma \text{ (Austenite)} \xrightarrow{\text{cooling}} \alpha \text{ (Ferrite)} + \text{Fe}_3\text{C} \text{ (Cementite)}$$