After I struggled with mine, I discovered why some of them were so brutal to get out. Sometimes the holes of the attached pieces were not exactly the same size and sometimes they were not perfectly aligned, when assembled at the factory. The result was that the hot rivet just morphed to the various shapes of the void and thus they didn't want to just slide out. I also had one head fracture in shear, just like a piece of cast iron. I think the hardness varies from rivet to rivet.
