proeutektoid

Proeutektoid, also spelled proeutectoid, is a term used in iron–carbon metallurgy to describe a phase that forms from austenite before the eutectoid transformation during cooling. The concept is tied to the eutectoid point of the iron–carbon system, which is at about 0.76 wt% carbon and roughly 727°C, where austenite transforms into ferrite and cementite (pearlite). Proeutektoid phases arise when the alloy composition lies away from the eutectoid composition.

In hypoeutectoid steels (carbon content less than 0.76%), proeutectoid ferrite forms first as the steel cools through the austenite region, typically along grain boundaries. As cooling proceeds to the eutectoid temperature, the remaining austenite transforms into pearlite. In hypereutectoid steels (carbon content greater than 0.76%), proeutectoid cementite forms prior to the eutectoid reaction, again often at grain boundaries, with the remainder of austenite transforming to pearlite at the eutectoid temperature.

The presence of proeutectoid phases affects mechanical properties such as hardness, strength, and ductility and is influenced by the steel’s composition and cooling history. Understanding proeutektoid structures aids in interpreting heat treatments (such as annealing, normalizing, and slow cooling) and their resulting microstructures. The concept is widely used in phase-diagram analysis and practical steel processing to predict the evolution of ferrite, cementite, and pearlite in the final microstructure.