Miller
Yes you are correct… in that annealing does not remove the carbon, but it does disperse the carbon a little more and a little deeper. A hypothetical example; SAE 1010 case hardening steel, case hardened, then annealed may change the carbon content of the steel from .10% to say .12%, which is still within the “safe zone” for case hardening steels (less than .25% carbon).
Steels with greater than .25% carbon are usually considered thru hardening steel, where there is enough carbon present for the steel to harden all the way thru.
For very thin parts this may be a problem, An example of this is if you have a very thin area say .005 thick of low carbon steel (SAE 1010, with .10% carbon) and you case harden to a depth of .002, then anneal the part, you have probably increased the carbon content so that piece may be equal to 1020 or 1030 with .20-.30% carbon, too much resident carbon to be safely case hardened.
Case hardening is certainly not a no risk endeavor. There are way to mitigate the effects such as reducing the soak time at critical temp, reducing the quench temp below the critical temp etc. But the bottom line is that the case hardener needs to have the experience to make that judgment call.
There have been several times that I’ve turned down work, and have refused to case harden a part, the risk just wasn’t worth it.
But on the OTOH, I have seen some folks that would color case just about anything.
