The term “ferrite phase” describes metal grains having the BCC lattice structure; the main difference
between these and the soap bubble example is that the boundaries between metal grains are not
always straight.
At temperatures above 910°C, iron with the FCC structure can dissolve more than 1 wt% carbon. The
carbon atoms, being smaller than the iron atoms, fit in the spaces between the larger iron atoms as
shown in Figure 5.8, and the overall crystal structure remains as FCC. If one were to examine the
steel at 920°C under microscope, there will be no evidence of the carbon in the steel and one will
again see grains similar to pure iron ferrite at room temperature but, because of their different crystal
structure (FCC), these are called the austenite phase.