CM;
Yes I understand that. I have been around case hardening, just not Color Hardening. In the machine shop where I worked we often actually did the carburization & the hardening in seperate operations. Other times all in one, depending upon circumstances. After that "Soak" though the "case" is no longer a Low Carbon Steel, but is now a High Carbon one. It now will respond to heat treating & this is what is done in the heating & quenching process. The core remains low carbon & does not respond to the quench to much extent & remains soft & ductile. Just the addition of carbon will of itself increase the hardness of the steel to some extent, but certainly not to that file hardness test unless heated & quenched. To obtain this result it must be above the critical temp for that steel & this would seem to drop but very little below 1400°F.
As you noted a 1hr soak at 1600° gives more penertration than a 4hr soak at 1425°. Drop much below that & my understanding is penertration will virtually halt. This too must be above that critical temp for effective penertration. From all data I have it would seem about 1350° would be about absolute bottom temp at the quench for effective hardening to take place.
In re-doing an action I am not certain it is necessary for any significant amount of penertration to take place. It would seem the carbon rich case is still there from the original carburization. Just enough surface penertrtion to provide the layer needed for the coloring, thus the higher temps (Or Long Times) necessary for deeper penertration would not be needed. As the part will have been annealed it will be necessary to re-harden & I can simply not see this taking place below 1350° bottom.
