Phase diagrams and the lever rule
Which statement about the eutectoid point of the Fe-Fe₃C phase diagram is correct?
- AAt about 0.76 wt% C and 727 °C, γ → α + Fe₃C occurs and pearlite forms
- BAt about 4.3 wt% C and 1147 °C, L → γ + Fe₃C occurs and lamellar pearlite forms
- CAt about 0.022 wt% C and 912 °C, γ → α occurs and bainite forms
Show answer and explanation
Answer: A. At about 0.76 wt% C and 727 °C, γ → α + Fe₃C occurs and pearlite forms
The eutectoid point of the Fe-Fe₃C system is at about 0.76 wt% C and 727 °C, where austenite (γ) transforms into pearlite, a lamellar structure of ferrite (α) and cementite (Fe₃C). The reaction at about 4.3 wt% C and 1147 °C is the eutectic reaction involving the liquid, and the structure it produces is ledeburite, not pearlite. 0.022 wt% C is the maximum solubility of carbon in α at 727 °C, and 912 °C is the γ → α transformation temperature of pure iron. Bainite does not appear on the equilibrium phase diagram; it is a nonequilibrium structure that forms from undercooled austenite in an intermediate temperature range. Steels with less carbon than the eutectoid composition are called hypoeutectoid, and those with more are called hypereutectoid.
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