You can't void your warranty with a K-series CPU, or at least not that I know of.
If you ruin your CPU due to overclocking, then you can't get a refund or a new one.
But you can still overclock, and if anything goes wrong (non-overclock related) then you'd be alright.
So let's say I have my 4670k overclocked to 4.5Ghz and through a fault of Intel somehow my CPU cores die out - and it was just a faulty CPU in general, I can get a new one. But if my cores die out by overclocking, I'm not covered - even if I have a warranty.