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Properties that are improved by nitrocarburizing include: fatigue strength (resistance to dynamic loading), friction and resistance to wear, and corrosion resistance.
Ferritic nitrocarburizing (FNC) is a process in which nitrogen and carbon diffuse into the surface of a part that is in a ferritic condition. The temperature must be below that at which austenite begins to form during heating – the Ac1 temperature – namely 723ºC (1333ºF). The process is typically performed between 550-595ºC (1025-1100°F).
Nitrocarburizing is commonly performed in an atmosphere of 50% endothermic gas + 50% ammonia, or 60% nitrogen + 35% ammonia + 5% carbon dioxide. Other atmospheres, such as 40% endothermic gas + 50% ammonia + 10% air, are also used. The presence of oxygen in the atmosphere activates the kinetics of nitrogen transfer. The thickness of the white, or compound, layer is a function of gas composition and gas volume (flow).
A complex sequence is involved in the formation of the nitrocarburized case. This process creates an epsilon (ε) nitride phase that improves wear and scuffing characteristics in most steels. This process can be applied to inexpensive steels to create high hardness in shallow cases.
The surface compound, or white layer, produced has excellent wear and anti-scuffing properties and is achieved with minimum distortion. The underlying diffusion zone, provided it is substantial enough, improves fatigue properties such as endurance limit, especially in carbon and low-alloy steels. Some of the increased hardness of the case is due to a diffusion zone beneath the compound layer, especially in the more highly alloyed steels with strong nitride formers.
It is not uncommon to observe porosity of the compound layer due to the presence of a carburizing reaction at the steel surface influencing the nitriding kinetics and, therefore, the degree and type of porosity at the surface of the epsilon (ε) layer. Three different types of layers can be produced: reduced porosity, sponge porosity and columnar porosity.
Some applications require deep nonporous layers. Others applications where, for example, optimum corrosion resistance is needed benefit from the presence of sponge porosity. Still others benefit from columnar porosity, where oil retention can enhance wear resistance.
Nitrocarburizing is often followed by an oxidizing treatment to both enhance corrosion resistance and surface appearance.
The claimed benefits of nitrocarburizing include:
- Exceptionally high surface hardness
- Resistance to wear and anti-galling properties – good in poor lubrication conditions
- A minimum of distortion and deformation (than, say, carburizing/hardening)
- Resistance to tempering (i.e. resistant to softening effect of heat) up to nitrocarburizing temperature at which conventional steels would soften
- Stability of the nitrocarburized case
- Improved fatigue life and other fatigue-related properties
- Reduction in notch sensitivity
- A marked resistance to corrosion in several common media (except for stainless steels)
- Small volumetric changes, although some growth does occur