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There is a lot of misinformation in this thread.
"Case hardening" involves carburizing the surface of a soft steel. It only affects the surface, is extremely hard and wear resistant.
4140 steel will not go to 60 RC. There is not enough carbon in 4140 (.4%) to get that hard. 4140 is a carbon steel that requires quenching in water or oil to harden. It will not air-harden after welding.
Steel begins to soften, from it's fully hardened state, at around 400 degrees. Complete annealing occurs arounf 1400 degrees.
I'm not an expert on gun heat treating, but it has been my experience that a lot of gun parts are not hardened at all. I do know a little bit about carbon steel, though.
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