Saltbadkarburisering

Saltbadkarburisering, or salt bath carburizing, is a metallurgical heat treatment used to increase the hardness and wear resistance of the surface of steel by diffusing carbon from a molten salt bath into the surface layer. The process is a form of carburizing conducted by immersing the workpieces in a molten salt medium at elevated temperatures, commonly in the range of about 800 to 1000°C (roughly 1400–1800°F). After carburizing, the parts are typically quenched and tempered to develop a hard martensitic surface.

Bath compositions historically relied on cyanide-containing salts, such as sodium or potassium cyanide, sometimes in combination with other salts and fluxing agents. The carbon source in the bath enables carbon diffusion into the steel surface without relying on a gaseous carbon atmosphere. Because cyanide-based baths are highly toxic and generate hazardous waste, handling, control, and disposal require strict safety measures. In modern practice, salt bath carburizing has declined in many industries in favor of gas carburizing, vacuum carburizing, or plasma-assisted methods, which offer safer environments and easier control.

The resulting microstructure is a carburized case with a high carbon concentration near the surface and a gradient of decreasing carbon inward. Case depths vary with time, temperature, and bath composition, typically ranging from fractions of a millimeter to a few millimeters. The process enhances surface hardness and fatigue resistance while preserving a tougher, ductile core. Saltbadkarburisering remains of historical interest and is still used in some niche applications where its particular diffusion characteristics are advantageous, albeit less common today.