If you approach from the point of view that it has a DANGEROUS fault that causes the phone to heat up to the point that it MELTS AND BURNS PLASTIC and discolours the screen, and it's still under warranty, I'm sure they'll replace it for free, before you sue the bollocks off them for their equipment, through no fault of your own, injuring you.
Just say to the operator... "Look, it was in my POCKET and SMOKE started coming out of the damn thing... I assume this was poisonous given the warning labels on the battery and phone about safe disposal and not disposing of them in fire and the Lithium content in the battery... I had to pull my pants off in the middle of the street."