Nitriding
N.B.: The information contained in this sheet comes from reliable sources. Nevertheless, it is provided without any guarantee, express or implied, of its accuracy.
Principle
Nitriding is a family of surface treatments involving a nitrogen enrichment phase on the surface of a steel to allow hardening by the formation of nitrides. Other elements such as carbon, oxygen or sulfur can be added to modify the properties.
The treatment is generally carried out in the ferritic phase at a temperature below 590°C followed by slow cooling. But a variant, not detailed here, is performed in the austenitic phase up to temperatures of 680 to 700°C. The holding time is between 1 hour and several days.
The nitriding treatment forms a combination layer (or white layer) composed of epsilon ε (Fe2N1-x) or gamma prime ϒ' (Fe4N) nitrides or a mixture of the 2, and a diffusion layer where finely dispersed nitrides are formed which, depending on their composition, generate a strong increase in hardness.
A combination layer composed of ε-nitride has interesting properties. The coefficient of friction and corrosion resistance are improved. To promote its formation, a small amount of carbon is added to the nitriding medium. This is called nitrocarburizing.
Oxygen can be introduced during the treatment or after treatment to form a layer of iron oxide (Fe3O4) to help resist corrosion. This treatment is called Oxynitriding or Oxynitrocarburizing. If the layer is worn away by abrasion, the protection against corrosion is that of a classic nitriding.
Sulfur can be introduced into the nitriding medium to improve the frictional properties by improving the accommodation of the surfaces. This treatment is called Sulfonitriding or Sulfonitrocarburizing.
When carbon, oxygen and sulfur are added to the nitriding medium, the treatment is called Oxysulfonitrocarburation. This treatment improves both the friction properties and the corrosion resistance by modifying the composition and morphology of the combination layer.
The hardening of the diffusion layer depends on the presence of chemical elements such as chromium, aluminum, molybdenum or vanadium in the alloy. The hardness of the diffusion layer of a non-alloyed steel does not exceed 350HV. Pre-treatment by quenching and tempering is favorable because the availability of the additive elements is favored by the presence of much smaller carbides than after annealing.
Nitriding can be performed :
In liquid medium, in salt baths;
In a gas mixture at atmospheric pressure (gas nitriding) or at reduced pressure (low pressure nitriding);
In a plasma formed mainly of nitrogen ions (ionic nitriding).
The treated steels can be :
Carbon steels or cast iron. The hardness is only present in the combination layer (800 to 1000HV).
Structural steels such as 42CrMo4 steel or the 1100MPa pretreated 40CrMnMo8 steel. The surface hardness is in the range of 600 to 800HV.
Steels with high affinity to nitrogen such as 33CrMoV13 or 41CrAlMo7-10. The surface hardness can reach values above 1000HV.
Tool steels such as X37CrMoV5-1 steel used to make hot forging dies and punches or plastic injection mold cavities. The hardness can exceed 1200HV.
Austenitic or ferritic stainless steels to improve hot friction properties. The surface hardness exceeds 1000HV. It should be noted that nitriding of these steels under conventional conditions degrades the corrosion resistance.
Typical treatment features
The hardness of the combination layer is about 1000HV;
The thickness of the combination layer varies from 0 to 40µm;
The hardness of the diffusion layer varies from 350HV to over 1200HV if additive elements such as chromium, aluminum, molybdenum and vanadium are present.
The residual surface stresses are in compression of the order of -500MPa which increases the resistance to mechanical fatigue;
The targeted processing depth is between 0.03 and 0.5 mm in common cases. In some extreme applications, depths of up to 1 mm are targeted;
The surface fatigue strength is increased because the stressed surface area has a high yield point. The treatment depth must be sufficient to cover the stressed area;
The corrosion resistance is improved by the formation of a surface layer of iron oxide and by an impregnation after treatment with resin, wax or oil.
The friction properties are mainly improved by the formation of the combination layer;
The deformations after treatment are very low because the nitriding temperature is chosen to be 50°C lower than the tempering temperature. However, a slight volume expansion of a few microns is noted due to the introduction of nitrogen;
In some cases, savings are possible (easier in ion nitriding by masking) and allow localized treatments.
Applications
Tooling parts to limit friction and resist abrasion (molds, cavities, dies...)
Standard mechanical parts (axles, shafts, rods, pistons, pinions, crankshaft...) for wear resistance
Parts that can withstand a corrosive atmosphere for a short time (storage)