Nitriding definition

Nitriding is a type of heat treating process in which the surface of steel is hardened by using nitrogen. It is not to be confused with nitration, the chemical process of the formation of nitrates and nitro compounds. Nitriding is distinguished from nitrocarburizing because it only uses nitrogen, instead of nitrogen and carbon.  

Areas of application

The majority of steel grades can be nitrided. Alloy steels are particularly suited because they achieve better results. Alloy steels that contain molybdenum, titanium, chromium or aluminium react better with nitrogen to form hard nitrides.

Nitriding low or unalloyed steel grades can lead to a brittle surface instead of a hard surface. Interstitially resolved nitrogen can set into the coherent grain boundary, leading to the reduction of toughness. Stress corrosion can thus be enabled.

Steel grades that are particularly fit for nitriding are also known as “nitrogen steels”. They contain high levels of reactive alloys, such as molybdenum, titanium, chromium or aluminium. Examples for nitrogen steels are:

  • 1.8519 (31CrMoV9)
  • 1.8515 (31CrMo12)
  • 1.8550 (34CrAINi7)
  • 1.8507 (34CrAlMo5)

Procedure

Nitriding takes place in a furnace at temperatures between 500 and 520 °C. The previously cleaned material will be heat treated for several hours, depending on its size, composition and desired hardness.

Gases containing nitrogen are directed continuously into the furnace. The nitrogen then reacts with the elements on the surface of the material. This is how iron-nitride phases and other alloy-nitrides are formed. The formation of austenites is prevented and the core of the material remains untouched.

Advantages of Nitriding

Advantages of nitriding include:

  • Improved surface hardness: Metallic nitrides form a very hard crystal structure, resulting in high hardness.
  • Improved resistance against corrosion: Due to the improved hardness, a protection layer against corrosion is formed. Iron cannot corrode anymore because the material is encased in a layer of nitrides. If desired, resistance against corrosion can be even more improved by forming an oxidation layer in a controlled environment.
  • Broad area of application: A large number of steel grades can be nitrided. While the heat treatment is better suited for some alloy steels, nitriding can be performed on almost any grade.
  • Tolerance against high temperatures: By nitriding a material, its tolerance against high temperatures can improve to up to 600°C.
    Improved wear properties: Since nitrogen steels are protected by a layer of nitrides, they are much slower to wear.