Yes they're hardened but they are not high carbon steel. They're medium carbon content and I'm pretty sure that's not the problem. I blasted and cleaned them better this last time and they welded a lot better, but I still got some small pits. The biggest problem is when I try to weld up the socket hole and it's the hardest place to clean out thoroughly. I'm pretty sure it's related to cleanliness, not the steel. I did some research and most every source said welding a grade 8 bolt shouldn't be a problem.