bla bla bla.I play on EU and i tell you a set +7 without failed its practic imposibil.Only item i do +7 without any failed its my gs.. and was pure luck

Anyway now its more fun with the new Korean update `cause you can upgrade your item to +9 .
Basically each type of core adds a little %, max is 50%, perfect core brings it to 50%, there's a chance of failure and delevel of item and also chance item is destroyed. So you can't use the upgrade system unless your item starts out at +7.
Capped at 50% Success 25% Destroy
Based at 10% Success 45% Destroy
Fail +8 and up equal auto +7.