Carbon from the interior diffuses
towards the surface, moving from high
to low concentration and continues until
the maximum depth of decarburization
is established. Because the carbon diffusion
rate increases with temperature
when the structure is fully austenitic,
MAD also increases as temperature rises
above the Ac3
. For temperatures in the
two-phase region, between the Ac1
and
Ac3
, the process is more complex.