Machinery's Handbook;
Casehardening;
In order to harden low-carbon steel it is necessary to increase the carbon content of the surface of the steel so that a thin outer "Case" can be hardened by heating the steel to the hardening temperature and then quenching it. The process, therefore, involves two separate operations. The first is the carburizing operation for impregnating the outer surface with sufficient carbon, and the second operation is that of heat-treating the carburized parts so as to obtain a hard outer case and, at the same time, give the "Core" the required physical properties. The term "Casehardening" is ordinarily used to indicate the complete process of carburizing and hardening.
To absorb the carbon the iron or iron alloy material is heated above its transformation temperature in the presence of a carbonaceous material which may be solid, liquid or gas. The longer the part is kept at heat the deeper will be the case. Cyanide is normally used for only shallow cases while the pack process is normally used when the deeper cases are desired. Depths of up to around .030" are quite practical with the pack (Solid) hardening process.
Note also the carburizing & hardening process can be done as separate operations or combined into one where the part is immediately quenched from the carburizing box.
Also note traditional Rockwell testers are normally useless on case hardened parts. The "Penetrator" will break through the case & give a false reading. The usual method of testing the hardness of the case is to use a "Shore Scleroscope". This instrument drops a diamond tipped hammer (Not pointed) of about 40 grains from a height of about 10" onto the metal. The rebound or bounce is then recorded, the harder the metal the higher the bounce. On hardened steel it may bounce to a height of about 6Ľ".
Case hardened steel, even when drawn at the low temp which is usual will be too hard for a file to cut in, it simply skates over the surface. This degree of hardness is impossible to obtain with a steel containing 30 points of carbon. A file normally has a carbon content in excess of 1% or 100 points. A case hardened part should have a carbon content up in that range.
Even though freely admitting the soft steel frame has the strength to withstand the firing of properly loaded shells, even the proof loads, I personally am Totally & Absolutely Un-Convinced that the case hardening process adds No Strength to the frame. I believe that it makes the frame both stronger as well as more wear resistant.
