carburization

Carburization is a heat treatment process that introduces carbon into the surface layer of iron- and steel-based alloys to produce a hard, wear-resistant outer case while leaving the core relatively softer and tougher. The resulting gradient of carbon content enhances surface strength and fatigue resistance without compromising overall toughness.

The process relies on carbon diffusion into the surface at elevated temperatures. The carburized layer typically reaches higher carbon contents (often around 0.8–1.0 wt% C or more) than the core, with case depths that depend on time, temperature, alloy composition, and the chosen method. After carburizing, the part is usually quenched to transform the case to martensite, followed by tempering to adjust hardness and relieve stresses.

Common variants include gas carburizing, pack (solid) carburizing, and liquid (salt bath) carburizing. Gas carburizing uses carbon-rich atmospheres (for example hydrocarbon gases) in a controlled furnace; pack carburizing employs a solid carbon source enclosed with the workpiece; liquid carburizing uses molten salt baths. Each method offers different control, throughput, cost, and surface quality. Some processes are conducted under vacuum or protective atmospheres to minimize oxidation and decarburization.

Post-treatment typically involves quenching and tempering to achieve the desired hardness, toughness, and residual stress balance. The technique is widely used for tools, gears, bearings, camshafts, crankshafts, fasteners, and other components requiring a hard wearing surface with a tough interior.

Carburization is related to cementation and nitriding as surface-hardening strategies, but it specifically targets carbon diffusion to create a carbon-rich case. Quality control emphasizes case depth, surface carbon content, hardness profile, and dimensional stability.