Everyone has a different risk tolerance.
Investments to old guns, we are all different.
This is from the horses mouth. I trust him with my life.
Carbon is a solute in the steel solvent. It migrates above a certain temperature along a concentration gradient. If the temp is too low it doesn't migrate from the carbon rich pack through the carbon poor steel. As temp is raised, it migrates along a curve that looks like a hockey stick. Relatively fast for the first .025 and then ever more slowly across the cross section.
At 1650 degrees F, (his most common carburizing temp) he said we are talking hours and hours of soak time to
allow,
not push the carbon atom to migrate only a little further into the steel. Thousandths, not inches.
When asked about the effect of multiple annealings and carburizings, he said " The carbon equalizes above temp with the surrounding environment. So, if packed in a carbon poor environment for annealing, carbon atoms migrate toward the areas of lowest concentration from highest. Both a little deeper (as soak time permits) but mostly outward toward the low concentration packing chips."..."This leaves residual carbon atoms in the case, but they cannot be driven deeper until critical temp is reached in a carbon rich environment. There is not enough carbon in the surface to create the gradient necessary to allow the migration deeper. They will go "out" just as likely as "in" in this case. Leaving a piece of steel with a very low concentration of carbon in it. (Soft)
But wait, there's more.
The time gradient is different for thick or thin parts.
Here is the common ground for the bickerers.
Small parts can much more easily be hardened through with successive heat treatings. Leaving them brittle. They are easier to allow carbon to migrate through, and the distances are smaller. So, less time to equilibrate.
How small parts are packed and processed, he believes, should be different that the larger forgings.
Well, what about an action that was repeatedly heat treated?
In the annealing, the equilibration lowers carbon content to a low level. The next heating in a carbon rich environment slowly raises the carbon content from the outside in, and starts the gradient migration all over. It would take hours and hours to add a slight increase in depth from the prior state.
So, first the annealed part has to reattain where it was before, and then follow the time gradient to let carbon migrate a tiny bit deeper. Up until that moment, carbon atoms are as likely to migrate outward, as inward. The gradient favors outward until the previous percentage is attained.
Now here is where I blend in what Dr. Gaddy told me.
"Color hardening is very very thin."
"Microns thin."
That suggests to me a thin case on the part, and a much shorter time of soak than what Smith has been talking about.
Achieving a color case for decorative purposes does not allow carbon atoms to migrate very deeply into an action. Even if done multiple times. The times at temp are either too short, or temps too low.
Again, deeper for thin parts.
So, as a guy that doesn't have a kiln, I am guessing that most colorers bring a part up to a temp pretty quickly, soak it a relatively short time, and then quench it. Leaving a micron thick film of oxides, which is more decorative, than strengthening. A wear surface. Not strengthening.
The implied conditions followed don't allow for very deep penetrations. But, for all I know, you guys are soaking these actions for days and days, just in case. It doesn't seem to be necessary in order to create a decorative skin.
So, what do you think? Am I full of it?
You can always cut up an old action and look at it under a loop to determine case depth necessary. But I suspect that information is already out there.
