Case hardening is a heat treatment that combines the ductility of low carbon materials with the strength of high carbon through the creation of a tough outer coating which surrounds a softer more malleable core. Unlike other coating and plating techniques, case hardening actually penetrates the surface of the base material providing a gradual transition from high carbon content to low.
Also known as surface hardening, case hardening can be applied in various applications. Industries such as construction, fencing, machining, grating and metalworking among others commonly utilize case hardened steel and iron components as they provide reliable parts and improve product longevity even under extreme stresses. Screws, grates, bolts, engine camshafts, firing pins, theft prevention systems, chains, metal panels and doors undergo this particular type of heat treating to resist cutting or shearing while remaining less brittle than untreated high carbon materials. Most often these parts are shaped before hardening as it reduces machining opportunities due to the heightened strength and rigidity. While carburization, or the diffusion of carbon into a metal, is the most commonly used type of case hardening, nitriding and boriding are also used and involve the use of nitrogen and boron diffusion respectively. With each technique high temperatures are used to diffuse the material, after which the surface layer is treated to attain the desired hardness.
Although the results of nitriding and boriding heat treatments may differ slightly from that of carburizing, the processes are quite similar. The steel or iron worked piece is placed inside what is known as a carburizing pack. Essentially this is an encasement tightly filled with carbon-based compounds. The part and its pack are then put inside a hot furnace and heated to very high temperatures, usually between 482 and 955 °C ( 900-1,750 °F) depending upon materials, thickness, desired hardness and corrosion resistance. The duration and temperature of this heating determines the depth to which the hardening extends beyond the surface of the substrate. Typical depths are around 1.5 mm at which point carbon content tapers down. While this is the traditional and perhaps most common technique used for case hardening, it is not the only one. Alternative methods abound and are even growing in popularity. One such technique is to heat the parts in a carbon-rich atmosphere, such as a methane-rich furnace. The carbon from the air will create the thin protective layer. Additionally, steel and iron components can be heated repeatedly with a torch and then quenched in a carbon rich medium, though this is most effective for smaller parts and is often capable of less penetration than furnace based methods. As homogeneous steels with low or high carbon content are now more readily available, case hardening is used less frequently, though these uniform metals cannot match the combination of extreme hardness and extreme toughness provided by case hardened parts and materials.