According to a letter from an engineer for L C Smith at Hunter Arms (written 1949) the procedure used there was to pack the parts in a crucible containing 3½ parts charcoal to 1 part charred bone. This was placed in a furnace, heated to 1600°F & held at heat for 2¼ hours. The crucible is then direct quenched into a tank of running, cold, soft water. I have in the past had access to a time/penertration chart for carburizing depths but do not have one here at home (retired). I would however expect this to give a minimum penertration of .010" to the case. Adding carbon to a low carbon steel of course does not reduce it's ultimate strength, but increases it. What can happen though is (as in the early 03 Springfields) the steel can be strong, yet brittle & unable to stand a "Shock". I am not up enough on this to say, if in "Re-Casing" penertration is accumulative, ie does it pick up where it left off, or does it have to start over. As this outer case is already carbon rich, I would assume it would immediately upon reaching temp begin to penertrate deeper, but this is simply a guess. I have always had great reservations about having a frame re-cased.
Incidently Cyaniding is normally limited to depth of less than .010".
Miller
