JMO, but I think we've come a ways from the days of plunging a red hot sword into a virgin slave to caseharden it...as well as understanding the mechanism which hardens the steel.
Casehardening is not a mysterious process. However, the particular techniques by which 100 yr old guns were casehardened is somewhat less than desireable from a repeatability/reliability veiwpoint. That's why the millions of pounds of steels that get casehardened today don't use charcoal packing and water quenching by a person dumping it all out in an air atmosphere. Still, casehardening in the broader industrial applications, is still accomplished thru the same physical changes in the steel, in the same basic manner of elevating the temperature of the steel in a supersaturated carbon environment until carbon penetrates and is absorbed pretty much to the practical limits, then it is quenched.
There is some risk of over-carburizing from extended saturation in the carbon environment, which can lead to a brittle martensite on the surface and undesireable stresses in the surface layer. I think the cracked frames may have been partly due to this problem. But I think this risk is relatively low if the standard saturation times/temps are adhered to.
