Case hardening (or surface hardening) as a dual-phase metallurgical process designed to create a high-hardness exterior "case" while preserving a ductile, shock-resistant interior "core." This technique is essential for engineering components like gears, shafts, and conduits that must withstand both intense surface abrasion and sudden mechanical impacts.
The process typically involves a thermochemical diffusion phase—such as carburizing or nitriding—where a low-carbon steel substrate is heated in a carbon or nitrogen-rich environment. At elevated temperatures (often between 850°C and 950°C), these elements diffuse into the surface lattice to a depth of roughly 0.1 mm to 2.0 mm. Following diffusion, the component is quenched, transforming the enriched surface into a wear-resistant martensitic structure, while the low-carbon core remains unaffected and tough.