I just recently broke a hellforge with a Bone Pickaxe which has 50% pickaxe power whereas the wiki states that you need 60% pickaxe power.
Nope, I have a Bone Pickaxe too, and it can't break Hellforges, not even the tiles they're on. My Bone Pickaxe can break Dungeon Bricks, however, yet it takes a bit of time without a mining potion.