Case hardening work is a manufacturing process involving changes in composition of metallic objects. It is employed to strengthen the surface layer of the material. Generally, low carbon steel will be given added strength by applying material that makes for a stronger alloy. This is of particular value in components that are subject to high levels of stress along their surface areas. Examples of this would include gun barrels and high pressure pipelines. In the case of firearms, other parts that constitute points of high impact such as firing pins, triggers, and bolt action mechanisms may also be hardened in this fashion. Bullets as well may also be treated this way. For artillery shells, performance and capabilities may be enhanced by case hardening particular portions of the assembly such as the nose in order to give the shell armor piercing capability.
Parts may more easily be fabricated using low carbon steel since they are more malleable allowing them to be formed into the desired shape using less energy. Case hardening companies will therefore create a component initially with this material since it is easy to work. They will then lay down a layer of steel with higher carbon content atop which while stronger is also more brittle and difficult to form without fracturing. Inserting the material into a carbon source and then applying heat will allow the carbon to enter into the metal. The longer it is held there and the higher the heat, the more carbon will enter and the deeper it will suffuse through the surface.
Since case hardening work requires carbon to enter into metal, the carbon must be presented in some sort of permeable form. In its natural state, carbon is a solid at room temperature that will remain inert on contact with metal. The carbon is therefore usually transformed into a variety of gaseous compounds. Oxygen will be added while heating to create carbon dioxide. Barium and other elements may be added that allow the carbon dioxide to break down more easily into its elemental constituents.