X17

Carbon steel (0.8%C) Annealed

Composition: Carbon steel (0,8%C, eutectoid).

Processing: Hot-rolled, Annealing (800ºC - 1h), Slow cooling in furnace.

Etching: 10-15 s immersion in 2% nital.

Hot-rolling eliminates the as cast structure, resulting in a recrystallized material.

This is carbon steel of eutectoid composition. Therefore, annealing at 800ºC puts the material in the γ or austenite field (see diagram). This phase is an interstitial solid solution of C in Fe γ. Slow cooling results in a eutectoid transformation, where austenite decomposes into ferrite or α phase and cementite (Fe₃C). The lamellar aggregate that is formed is known as pearlite. The amount of α and cementite in pearlite can be calculated by applying the lever rule in the Fe-Fe₃C diagram. At low magnifications, it is still possible to see the contours of the original austenite grains. At higher magnification, both ferrite and cementite appear in white colour. However, the small width of the lamellae makes cementite to appear darker in colour. It is worth mentioning that the etching process leaves cementite slightly above the ferrite matrix due to preferential dissolution of the latter.

At some regions, pearlite adopts a degenerated or spheroidal morphology due to the slow cooling conditions. This is because spheres minimize the amount of surface energy and the overall energy of the system.