R'Man;
A most excellent & informative post. You have brought up a point which needs to be understood by all contemplating having this done. This is the fact that a carburizing cycle "Changes" the composition of the outer layer (The Case) of the steel. Indeed the depth of the case is controlled by how long a part stays at temp in the carbon mixture. Every time it goes back in the furnace for further heating a little more carbon is added to the case. Thus every time a part goes back in the furnace it will be slightly "Changed" from what it was previously in actual composition. Even alloy steel can experience a slight compositional change in heat treatment from certain alloying elements "Cooking Out" of them, from oxidation. These steels are often heated in a molten salt to minimize this.
The problem with the color case hardening is we cannot simply heat enough for "Just" the color without destroying the hardness of the case, & we cannot process in such a way as to preserve its hardness with out going up into the range where more carbon will be absorbed & the part then be quenched from above its critical temp, which is almost certain to result in "Some Amount" of dimensional change.
To me the Risks are simply not worth it for a little Vanity for the colors. The proper original hardness of the case itself has not been lost, only the color.
