The process which adds the carbon to the surface of low carbon steel is "Carburizing". This carburizing process itself does not add hardness other than the small amount of higher carbon steel over low carbon steel. The subsequet quench from an elevated temperature produces the hardness just as it does in a higher alloy through hardening steel. The carburized low carbon steel will harden only to the extent its carbon has been raised high enough to be effected by the ""Heat Treatment". Steels with a higher carbon allooy do not need (in fact is is normally not desirable) the carburizing step, but the heating & quenching part is essentially the same heat treatment as given to the other. Exact alloy content determines the quench medium & thus we have water hardening, oil hardening & air hardening steels.
The hard outer surface over a soft core has led to the term "Case Hardened" being a generally accepted term, but technically it is carburized & hardened, the hardening being done by heat treating. As previously stated it is most often combined into a single operation. Never-the-less the heating & quenching is nothing but plain old heat treating.
When carburizing & hardening parts in the machine shops I worked in we always drew the hardening to a temp of around 300°-350°F after being quenched. This left the surface at virtually maximum hardness but was done to prevent surface checking of the surface. I saw one piece on which this step was omitted & the following day its surface was full of small cracks. I do not recall ever seeing this mentioned in relation to CCH of gun frames & have often wondered why.
Even on a through hardening alloy steel if it has much thickness there will be some gradient to the hardness, as the center simply cannot cool as fast as the surface, resulting in some loss of hardness to the core, though certainly not to the same extent as a case hardened part.
As a general rule parts carburized by the molten
Cyanide process do not acheive as deep penertration of the carbon as those carburiced by the charcoal "Pack" process. I have never worked where the Cyanide process was in use, but used numerous which had been so treated.
