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The problem is that a few of the green see-thru chisels have 10-15 percent
chips gone from its plastic striking surface. Since that day I am more careful.
I'm not sure whether the cause is the a 12-oz. iron hammer or the way I pound
them. What's the best way to repair these tool?

I don't know what the best way to repair them is, but I see a few
causes for your problem right offhand. First, a "plastic striking
surface" is not a striking surface at all- the chisel is most likely
designed for hand use only. If you can't push it by hand, it's
probably not sharp enough for what you're doing. Chisels that are
intended to be used with mallets usually have a steel shaft running
all the way through the tool, and a metal striking plate at the end or
a socket-type reciever for the handle, and (again) a metal striking
plate at the end. Asking plastic to hold up to repeated hammering it
probably unrealistic, unless you get some superb chisels.

Second, even when carving with a mallet, you probably want to use a
wooden mallet rather than an iron hammer.

Third- Chisels carve, they don't umm.. "pound". They're not nails,
after all. Try taking several shallower cuts, and sharpen your tools
more often!

Sounds like you got yourself a fairly cheap set of chisels to begin
with- it may just be easier to get another set than it is to fix them.
Aut inveniam viam aut faciam