I just fried a 220v 20amp plug when it came in contact with a frame for a riveter. It's fed from a 30amp breaker.. can somebody please tell me why that happened!.

Well let's see - the breaker should have tripped at 30A, though it's not instantaneous. Hence for some period of time there were 30A (and probably more) of current flowing through a connector rated at 20A. So it fried.