Phase Diagrams

· A hyper-eutectoid alloy iron precipitates iron carbide on the Austenite grain boundaries when it is cooled from the γ-phase to the eutectoid temperature. The remaining Austenite then transforms to Pearlite at the eutectoid temperature.
· The pro-eutectoid iron carbide (Cementite) on the grain boundaries of the Pearlite is brittle and makes the room temperature hyper-eutectoid alloy a brittle material.
· The lever rule for the average composition,
C₁ = 1.2 wt % C, at (723 - ε) C gives:
W_carbide = (1.2 - 0.02)/(6.67 - 0.02) = 18 wt %,
W_αFe = (6.67 - 1.2)/(6.67 - 0.02) = 82 wt %.

From: Callister, "Materials Science and
Engineering," Wiley (1994)