I kind of like to improve things myself, but messing with the heat treat of a Krag receiver and bolt would be problematic.

It has been discussed here recently that cyanide was used in casing Krag receivers. So the depth of carbon infiltration is deeper than I would have guessed. Originally I would thought at the time these were made only organic sources of carbon were used (leather and bone). The base metal on both is a relatively soft "machinery" alloy.
Anyway the biggest area of concern to me is the varying thicknesses of the receiver. The thin areas will get carbon all the way through and become even more brittle. I have heard of some taking a hammer to Krag receivers and breaking them to pieces, probably the thin areas around the magazine. The heavier areas would have not done that as the untreated mass would have been too soft.
Masking the thin areas so no additional carbon was absorbed could be done, I guess. I am not sure how many receivers you would have to go through to get the process down with a positively safe result.
The bolts would not be a difficult as the there are less extreme differences from the heavy to thin sections. Aesthetically I like to see a polished bolt not color case hardened, but that is me.
If I had all the time in the world I would like to build a Krag receiver out of L-6 tool steel then I could load to Weatherby pressures.