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Phase diagrams and the lever rule

When a 1.2 wt% C hypereutectoid steel is slowly cooled from the austenite region, what microstructure is obtained at room temperature?

  1. AProeutectoid ferrite and pearlite
  2. BProeutectoid cementite and pearlite
  3. CPearlite only
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Answer: B. Proeutectoid cementite and pearlite

In a hypereutectoid steel, with more carbon than the eutectoid composition of 0.76 wt% C, crossing the Acm line during cooling first precipitates proeutectoid cementite as a network along the austenite grain boundaries. On reaching 727 °C, the remaining austenite (about 0.76 wt% C) becomes pearlite through the eutectoid reaction, so the room-temperature microstructure is proeutectoid cementite and pearlite. Proeutectoid ferrite and pearlite is the microstructure of a hypoeutectoid steel, and pearlite alone forms in a eutectoid steel. Because a grain-boundary cementite network causes brittleness, tool steels and similar grades are given a spheroidizing anneal to make the cementite globular.

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