Nitrokarbourisering

Nitrokarbourisering is a diffusion-based surface hardening treatment for steel in which nitrogen and carbon are introduced into the surface. The process forms a hard compound layer on the exterior and a diffusion zone beneath, resulting in increased surface hardness, wear resistance, and often improved fatigue strength, while preserving core toughness.

The main implementations are gas nitrocarbourizing (GNC) and salt bath nitrocarbourizing. Gas nitrocarbourizing exposes the workpiece to a furnace atmosphere containing ammonia and a hydrocarbon source to supply nitrogen and carbon, typically at temperatures around 540–580°C. Salt bath nitrocarbourizing uses a molten salt bath with nitrogen- and carbon-bearing species, which can shorten cycle times and provide uniform diffusion. Variants include oxidizing nitrocarbourizing, which can enhance surface corrosion resistance, and neutral salt baths. Post-treatment tempering is commonly employed to adjust hardness and relieve residual stresses.

Microstructure-wise, a top compound layer, often based on epsilon-Fe2-3N (and sometimes gamma' Fe4N), forms at the surface, over a diffusion zone where nitrides and carbonitrides develop. This structure yields high surface hardness and compressive residual stresses, reducing wear and improving fatigue life. Corrosion behavior depends on the environment and on subsequent finishing or lubricating conditions.

Applications are widespread in components subject to wear and fatigue, such as gears, camshafts, crankshafts, valves, and bearings in automotive, agricultural, and industrial machinery. Nitrokarbourisering offers relatively short processing times compared with some other surface hardening methods and can be tailored through process parameters to balance hardness, depth, and core properties.