Materials and Structure

For the hypereutectoid composition shown by the vertical green bar the material enters the two phase austenite + cementite zone prior to cooling through the eutectoid temperature. While it cools through this zone Fe₃C is deposited on the austenite grain boundaries and the austenite composition moves to the eutectoid value.

When the alloy reaches the eutectoid temperature the untransformed austenite undergoes a transformation to pearlite and the already formed cementite remains unchanged. The microstructure is now a ploycrystalline pearlite with the hard-brittle proeutectoid cementite on the grain boundaries. This structure makes the alloy more brittle than the eutectoid pearlite structure. The micrograph shows the microstructure for an Fe - 1.4 wt % C steel, the white regions being cementite.

From: Callister, "Materials Science and Engineering," Wiley (1997)