Case Hardening

Case hardening, also called carburization, is the process of introducing carbon into the surface of a low-carbon iron or steel object to harden its surface while leaving its interior comparatively soft. Steel with a carbon content above about 0.25 percent can be direct-hardened by heating it to around 600 degrees Celsius and then quenching it, typically in water or oil, a treatment that increases strength and wear resistance at the cost of making the metal more brittle and less malleable. To produce a hard skin on steel with less than about 0.2 percent carbon, the workpiece is instead heated in the presence of a carbon-rich substance, such as powdered charcoal or a hydrocarbon gas, which causes carbon to diffuse into its surface; a typical case depth reached this way is about half a millimeter, depending on exposure time. Once the carbon has diffused in, the steel is heated and quenched again to harden the newly carbon-rich skin, leaving the low-carbon core beneath it soft.