Ok as far as heat treatment, steels fall into several categories: low carbon, med carbon and high carbon. Low carbon steels are generally steels with less than .25% carbon, these steels cannot self-harden. Steels such as 1095, 4140 with more than .25% carbon will self-harden, meaning when heated to a certain temperature and quenched in the appropriate medium, have enough resident carbon to harden all the way thru.
Low carbon steels do not have this capacity, thus carbon is added thru the use of Bone/Wood, above a certain temperature the charcoal off gasses carbon in the form of carbon dioxide and carbon monoxide which is adsorbed by the steel at these elevated temps.
Case hardened parts, have a hard outer layer .001 -.003 and a soft inner core, which make it ideal for gun parts. Hard outer shell to provide wear resistance, soft inner core to absorb shock.
Steel can only absorb carbon, it cannot desorb carbon. Annealing does not remove carbon, it only allows it to spread within the steel, diluting it somewhat. IF a part is case hardened enough times, it can change the steel from a low carbon steel to a medium carbon steel; so from a case hardening steel to a through hardening steel. Thin parts are especially susceptible to this. If you have a thin piece of steel say .030 thick, it will quickly absorb enough carbon to become thru hardening.. Remember, unless blocked, carbon is absorbed from all sides top, bottom, front back.
I have had parts (done by someone else) that were so hard that the owner snapped a butt plate attempting to install it. He sent me all the CCH parts, and I could immediately tell what had happened. Quenched at too high a temperature and quenched in brine water; which greatly increases coolingmakes water wetter. I annealed the parts, welded the butt plate and re-cased properly.
