There are of course numerous processes which fall in the category of "Heat Treating", case hardening being one process in the broad category, IE it "IS" a heat treating process. In actuality, though often combined into a single operation, Case hardening is a two part process. The first step consists of "Carburizing" the low carbon steel to give the outer surface a higher carbon content. 2nd step is then to "Heat Treat" this carburized portion, thus the common name Case Harden.
The carburizing portion consists of bringing thr part to a temp above its critical temp & holding it there in the presence of a carbon rich atmophere for a time suitable to the desired penetration. The longer it is held the deeper the carbon penetrates though the deeper it gets the longer it takes for more depth, so there are practical limits which are not often exceeded.
The Heat treating portion is simply to quench the part in a suitable medium, usually water here, from a tamp above its critical temp. Since the part is already above this temp in the carburizing process it is quite common to simply quench directly from the carburizing, but this is not a requirement. The carburizing & heat treting can be done in seperate operations, though this would require bringing the part up to heat twice.
I totally agree with Chuck here, there simply is not a sharply defined "Line" to which the carbon penetrated & stopped but as he said it simply thins & eventually fades out as you go deeper from the surface. When the carbon content drops too low even though it may still be more than the original alloy as in the center it no longer hardens from the quench.
I believe this is basically what Dick was saying, but his statement seemed to indicate that casehardening was not heat treating, which it of course is.
