Rohdetherm

Gas Nitriding

During gas nitriding, atomic nitrogen is separated at temperatures around 500°C and diffused into the surface of the steel. Ammonia (NH3) is used to generate this atomic nitrogen. The diffusion can be influenced by several factors:

  • Amount of atomic nitrogen available
  • Nitriding temperature
  • Chemical composition
  • Microstructure
  • Surface characteristics

Nitrided steels should only be used once hardened. In this condition the correct formation of the nitriding layer can be guaranteed.
This enables the core to achieve good strength and tenacity characteristics. Great care and attention must be paid to the surface of the workpiece. There must be no surface layers that are decarbonised, oxidised or rusty. They must be free of oil and grease.
The surface hardening has a negative influence on the formation of the nitriding layer. Nitriding leads to less distortion. However, stress-free annealing is recommended during the manufacturing in order to relieve internal stresses before nitriding. The annealing temperature must be higher than the subsequent nitriding temperature and lower than the tempering temperature of the previous hardening process.
Nitriding causes a small increase of the volume due to the inclusion of the nitrogen. A finishing treatment is not necessarily required. however, it is advisable to retain an allowance of at least 0.05 mm.
Gas nitrided parts offer:

  • Higher vibration resistance
  • Higher corrosion resistance
  • Higher fatigue fracture resistance
  • Higher resistance to passive rust formation than case hardened parts.