Butlersrangers has reported that the case hardening was done by the pack method with organic carbon bearing material and special areas such as the receiver and bolt lugs receiving a cyanide maybe in a paste form applied for deeper carbon penetration. I have never seen any hardness data published for Krags. These were made in the early days, not sure if the Rockwell hardness system was even developed then. Krags probably varied quite a bit from year of manufacture, which shift, maybe which season and which operator was involved.
If  you tested a number of part and paid the most attention to strength and wear areas you might get some clues on what is right. I am not sure if anyone has done this till now.
Thin areas of the receiver would have gotten carbon penetration from both sides and maybe quite hard and perhaps brittle. Maybe a little extra tempering in those spots would prevent cracking.
If you are looking to get nice cased hardening colors, I think I would ask for just enough carbon in the pack to match what would be lost in the treatment then temper a little more on areas like the bottom of the magazine box that might get brittle. Hope your can work it out with the heat treater the necessary details as most if not all the original trial and error secrets have been lost.
If anyone can give specific hardness numbers for Krag parts and different areas of the bolt and receivers I am all ears.
A few of the parts like the extractor body maybe a higher carbon spring steel and completely different from most of the low carbon cased parts.

Chuck beat me in typing a reply but his question WHY is a good one. I  personally do not see the risks involved as worth the effort to get nice case colors. Trying to get more hardness or strength would probably bring negative unintended consequences.