A specific microstructural phase in iron-carbon alloys containing exactly 4.3 percent carbon, which solidifies directly from a liquid at the eutectic temperature. As it cools, it transforms into a hard, brittle mixture of pearlite and cementite. It is the defining structure of white cast iron, giving the metal extreme wear resistance but making it virtually impossible to machine.
Heard on the job
The cooling rate was too fast on that thin section, so we ended up with massive ledeburite instead of a machinable gray iron.