Nitriding and nitrocarburizing: materials used and application qualities

Industrial processes for nitriding and nitrocarburizing iron-carbon alloys are described in [M 1 227] . They comprise a family of treatments whose effect is to convert the surface into iron nitrides (or iron carbonitrides) over a variable thickness of 2 to 40 µm, depending on the treatment parameters (nitriding potential, temperature, time), thus constituting the combination layer, or white layer, whose theoretical formation mechanisms were described in [M 1 224] . This layer presents particularly interesting characteristics, including hardness and tribological properties. We have also seen that it is possible to control the crystalline structure of the combination layer, which is either face-centered cubic (γ′) or face-centered hexagonal (ε), with the entire layer being able to be single-phase or two-phase. The nature of the structure is very important for obtaining the desired performance. The control of these results depends on the nitriding or nitrocarburizing potential of the process, the adjustment range being the widest in gas nitriding and nitrocarburizing, hence the introduction of the coefficients KN related to the nitriding activity, and KC related to the associated carburizing reaction.

In addition to this surface reaction, which is limited in thickness, the various processes feature a thermochemical reaction, mainly involving nitrogen diffusion, which introduces a gradient of hardness in depth, strongly dependent on the nature and concentration of alloying elements in the treated material. The laws of diffusion, or Fick's laws, define the diffusion coefficient, a function of temperature and alloy composition, and the diffusion rate. The theoretical aspects of this mechanism were defined at [M 1 227] . This hardness gradient is associated with a compressive stress gradient, which has a major influence on...