The thickness of the "case" depends on the time and the carbon availability while at high temperature. Typically the low carbon steel piece to be cased is packed or submerged in something with excess carbon. Typical case thickness is a few thousandths. After soaking up extra carbon the case/skin becomes a high carbon steel alloy and the piece is quenched in "contaminated" water. Colors form from the oxides and some extra mineral "stuff" from the packing material and the water. Colors can be formed without hardening and a piece can be hardened without forming colors. Certain modern alloys can be hardened all the way through.
I believe engravers will tell you that cutting cased steel is not a fun thing to do; possible, but not conducive to good work and lower cost. Some modern alloys are also tough to cut , even before hardening.
