Low carbon steel or "mild steel" containing less than 0.3 per cent carbon is used to manufacture a variety of automotive components. These include suspension parts, exhausts, brackets and a number of engine components. Mild steel is a tough, but malleable material that cannot be hardened by heating and quenching without the addition of extra carbon. However, when extra carbon is added and mild steel is case hardened, it has the advantage of having a hard, wear-resistant outer surface with a tough, flexible inner core.
Heat the section of mild steel to be hardened evenly with an oxyacetylene torch until it is a bright cherry-red colour.
Immerse the hot steel in a container of case hardening compound. Cover the steel and let it cool down. Due to the insulating properties of coarse powdery case hardening compound, this may take an hour or two.
Remove the steel from the hardening compound and reheat it to a cherry-red colour. Quench the steel immediately by immersing it in a bucket of cold, clean water.
Clean off the mill-scale with a wire brush and test the hardness of the material by rubbing a file over the surface. Repeat the above hardening sequence to increase both the hardness and depth of the layer of case hardened steel.
Do not use a cyanide-based case hardening compound without wearing a respirator mask and welder's gloves. Wear welder's gloves and handle hot steel with tongs or a large pair of pliers.