Peter Rowe gave a good description of annealing.
It is easy to overheat and heat for too long when annealing which
causes grain growth.
True. more for some alloys than others, as well as the degree to
which it then causes problems.
This permanently damages the metal structure until it is remelted.
True also, unless you are able to continue cold working the metal.
While initially the larger grain size means you cannot work the metal
quite as far without risking cracking of the metal (rather like a
newly poured ingot sometimes), if you overanneal the metal at a stage
where you can still work the metal some more, deforming the crystals
again, then a subsequent careful anneal will result in smaller
So while over annealing does damage the metal structure, if you do
it early enough in your working process to allow sufficient
subsequent working, you may be able to pretty much repair the damage.
Your final crystal size may not be as small as it could have been
otherwise, but it may be just fine. But to do this, you really do have
to be deforming the crystal shapes. Rolling, forging, drawing down in
size, twisting wire, or the like. Anything that will cause it to work
harden to a goodly degree, so that with subsequent annealing the
crystals again are forced to break down and recrystalize into smaller
crystals. This will require more work hardening than you might get
with a bit of gentle flexing or banging on the metal with a soft
mallet or something. You really do need to change the shape somehow.