A few years ago I gave at least a ton of hedge to a neighbor. He only burns hedge in his furnace. A lot of it was fence posts that I pulled out of the ground when removing a fence. The posts must have been in the ground well over 30 years and were hard as a rock when I pulled them up. After a few cuts with his chainsaw he'd pull out a file to resharpen it.
I have lots of the trees on my place. May need to get out the chainsaw and a good file.
Thanks for the information folks!