Highly doubt Norinco warranties anything, be it rifles, pistols, tanks, or MiG ripoffs.
Luckily, the stuff is usually overbuilt and reliable. You have to deal with the supplier for when it's not. Here's how I think it works: A supplier will warranty the product. If it's coming from a company like Norinco, the supplier is going to have to do all the work themselves in the event a customer has an issue. If too many customers have too many issues, the supplier will eventually lose both money fixing them all, as well as losing confidence in the product. When it comes time for them to order more of the Noinco Break-a-Tron 4500, they won't do it, as they're not convinced it's a reliable product for both their customers and them. If a lot of suppliers do this, Norinco will eventually figure out that they need to put a bit more work into the Break-a-Tron in order for anyone to buy it from them.
A possible example of this is many of the first M305's imported into the states in the late 80's and early 90's were dangerously unsafe. Apparently they liked to go full auto and crack parts with alarming regularity (although the receivers were always excellent and are still desirable down there.) This was eventually fixed by Norinco, obviously. News of this spread like wildfire at the time in the US and is one of the reasons why a lot of people there believe that Chinese guns are junk, because they haven't been able to see the new M305's, CQA M4geries, and the like, but that's getting off topic.