Drawing down the hardness: wrap the bolt in a stainless steel bag to keep down the oxidation. Heat the bolt at 900F for 1.5 hours and cool. You end up with a tough bolt that is machineable. I did a lot of experimenting to find this out. All Rc measurements were done on a calibrated machine. Some people are thoroughly softening these bolts and then trying to reharden them. That carries the risk of cracking from thermal shock if the temperature and the quench medium are improper.