Body-centered cubic iron that dissolves only about 0.02 percent carbon at most, so it is soft, ductile and strongly magnetic. In annealed or normalized low-carbon steel it shows as light grains under a nital etch, next to dark pearlite colonies. Free ferrite in a through-hardened part is a red flag: the steel was not fully austenitized or was quenched too slowly.
Core shows blocky ferrite on the mount, so either the load never made temperature or the quench was lazy.
Also heard as
- alpha iron
- free ferrite
Related terms
Pearlite
A layered mix of ferrite and cementite that forms when austenite cools slowly through the eutectoid range.
Austenite
The face-centered cubic form of iron that dissolves carbon and must form before steel can be hardened.
Iron-carbon diagram
The phase map of iron and carbon that shows which structures are stable at each temperature and carbon level.
Slack quench
A quench too slow to make full martensite, leaving pearlite, bainite or ferrite and low hardness.