Traditional charcoal & bone case color

[Spin Drift]

Carbon Steels usuaaly start with low carbon like 1008 and go up to 1030, medium carbon are 1040 to 1060 and high carbon is 1070 to 1090. Note the last two numbers refer to the decimal percent carbon content. These are simple unalloyed steels. Low carbon is not heat treatable per se but can be case hardened. The case depth is dependent on the process used and can be as little as .005 for nitriding and .010 for carbon packing and furnace heating. Gas carburizing can even go deeper.

Case hardening only makes the surface harder for low carbon steels. It does not make it stronger. What is the outcome of the process is a hard outer surface with a non-brittle tough core. Note that this is a very old process done with low tech steels.