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Carbon is the element that makes steel harden. All steel has some percentage of Carbon. Tool Steels and other high carbon steels have enough (above .3%) Carbon and alloying elements to make them harden by heating to a specified temperature and quenching either in water, oil, or air, depending on the grade and type. After hardening, they become very brittle so they have to be tempered (drawn). Drawing temperature and time determine the final hardness. Since low carbon steels (mild steel) don't have enough Carbon to harden this way, they have to have Carbon added. We call this process case hardening because of the hard case that forms on the outer few thousandths of the steel. The technical term for it is Carborizing. There are a couple of ways commonly used to carborize steel. The steel can be heated and rolled in the Kasenite that john d mentioned. This gives a shallow case of about .003-.004 depth. The other way is to pack the steel in iron filings or other Carbon rich medium and heating it for a longer period of time, sometimes several hours. It will absorb carbon to a depth of maybe 1/16, depending on the time it is left in the heat. Water quenching is used with both these methods. Tempering isn't necessary because the center is still soft. Kasenite has a big advantage in the home or farm shop. Since it is just a process of heat and roll, it can be done with a torch. With the packing method, the steel needs to be kept at a high temperature for a long time, so you would need a furnace to take full advantage of it. Hope this helps
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