Just going by what I read in the Brophy book on LC Smith plans and Specifications (Brownells 1981) I was surprised they showed AISI 1018-1020 mild steel for the hinge pin, but did indicate carburizing and an oil bath quench--You are correct, all steels need a minimum of 30 points of carbon content or more to take heat treatment, no matter the quenching medium used and the resultant Rockwell or DPH reading desired-- and I should also mention that in tool steels (way different in analysis that 1018 steel) there are three basic types: Oil hardening, Water hardening and Air hardening. Carpenter TK and P21-lusterdie were several I used to be familiar with-- 'back in the day'!!!
