Eutectoid mixture
Eutectoid mixture is a fine, mechanically mixed phase structure at the exact composition of carbon and iron. In steels, it forms the classic pearlite structure.
Physical-Chemical Mechanism of Action
Eutectoid mixture forms at a specific chemical composition of carbon and iron. It forms directly from a solid phase without a phase transition through melting and cooling. This process is typical for steels with a specific carbon content.
Metallurgical Relevance & Risks in Welding
The eutectoid mixture has high toughness and good mechanical properties, but it can cause problems with inaccurate compositions. Too high a carbon content can lead to glass-hard martensite, which causes cracks and hardness issues in the weld pool.
Workshop Practice & Action Instructions
To obtain a eutectoid mixture in steels, the composition must be controlled precisely. With a carbon content of about 0.77%, the classic pearlite structure forms. Welding temperatures should be set so that rapid cooling (t8/5 < 5 s) does not occur, as this leads to martensite. Slow cooling (t8/5 > 25 s) can coarsen the grain and reduce toughness.