WAY beyond my knowledge, but as SDH explained, the steel may determine the color case hardening method.
We know prior to WWII U.S. maker's actions were AISI 1020, which with 1018 is easily pack hardened.

Hopefully Bro. Tom will elaborate as I couldn't find my reference, but Hunter Arms was using cyanide for some parts. I believe Charles Jerred was briefly blinded by an accident in the Heat Treating Dept.

8620 seems to be the modern choice
http://www.suppliersonline.com/propertypages/8620.asp

4140 can be color cased but apparently with more difficulty, and requires an oil rather than water quench with careful attention to the temperature used.

What appears to be a 2-fer; cyanide + torch? frown


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