Composition: Carbon steel (0.35%C, hypoeutectoid).

Processing: Hot-rolled, furnace cooling from 870ºC.

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

This is a hypoeutectoid steel, therefore, with less carbon than X17 and X18 specimens. At 870ºC, the material is in the austenitic field (see Fe-Fe₃C phase diagram). Upon cooling, primary or proeutectoid ferrite precipitates in the shape of roughly equiaxial grains. The are visible as white crystals located at the grain boundaries of the original austenite grains. The carbon rejected from the ferrite diffuses into the austenite, which eventually reaches the eutectoid composition (0.8%C) and transforms into a coarse lamellar aggregate of ferrite and cementite. This is known as coarse pearlite and appears darker in these micrographs.

Fe-Fe₃C diagram

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