Nitriding or ferritic nitrocarburizing of steels and cast iron
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:
Enrichissement superficiel en azote et carbone éventuel dans le cas de nitrocarburation, par diffusion dans le domaine ferritique < T° Ac1 (450 à 620°C selon les procédés), afin d’obtenir une dureté superficielle élevée (450 à 1200 HV selon le matériau traité) associé à un gradient décroissant sur une profondeur donnée, ainsi qu’une couche de combinaison superficielle constitué de nitrures de fer.
The achievable diffusion depths are between 0.05 and 1.5 mm. The most common ones are between 0.1 and 0.8 mm (the treatment time increases strongly with the targeted depth, nitriding speed of about 0.03 to 0.01 mm/H). The thicknesses of the combination layers most commonly obtained during nitrocarburizing at temperatures between 550 and 620°C are between 5 and 30 μm depending on the processes and temperatures applied. The characteristics are obtained without quenching transformation.
CHARACTERIZATION:
Surface hardness (HV, HRA)
Thickness of the combination layer or white layer
Structure of the combination layer (single-phase epsilon or gamma ' )
Conventional depth of hard coat Ec or Dc (unless otherwise specified in NFA04-204)
Microstructure requirements
Residual stress rate in compression
APPLICATIONS :
Wear and tear
Fatigue reinforcement
Tribological properties of the combination layers (seizure inhibitor, low friction coefficient)
DIFFERENT PROCESSES:
Nitriding, gaseous nitrocarburizing: the treatment atmosphere established between 500 and 620°C after a heating phase under nitrogen, consists of ammonia (NH3), of an ammonia-nitrogen mixture to which can be added various gases: activator like nitrous oxide (N2O) carbon dioxide (CO2), air (one can speak about oxynitriding) -fuel: carbon dioxide (CO), endothermic gas, exothermic gas in the case of nitrocarburizing The kinetics are relatively slow. The current industrial processes are controlled and regulated.
There are reduced pressure versions of these processes implemented in vacuum purge furnaces under pressure of 200 to 300 mbar.
The gaseous emissions rich in ammonia must be burned before being released into the outside atmosphere.Nitrocarburizing in a salt bath: use of alkaline chloride and cyanate-based salts activated by air and additives (specific to the process developers) between 550 and 620°C (ideally 570-580°C). The main purpose of these processes is to obtain epsilon-type combination layers (Fe2-3N nitrides) with a thickness of 10 to 20μm and interesting tribological properties. They are not intended to obtain deep diffusion layers.
Their kinetics are very fast. The disadvantages are related to the use of molten salts (recovery of rejects and cleaning of treated parts).Plasma assisted nitriding, nitrocarburizing: (often known as ion nitriding): the reaction is established under a nitrogen-hydrogen plasma in a vacuum purge reactor equipped with a high frequency arc generator (opposite polarization of the parts and the furnace walls). The ionized molecules produce activated molecules by collision which improve the reaction kinetics. The advantages are the wide temperature spectrum possible, especially towards the lowest temperatures and the modularity of the results (presence and nature of the combination layer), the easy saving by metallic cover. The kinetics are fast for the lowest depths. The preparation of the charges is more delicate and less adapted to mass production than the other processes.
Oxidizing coatings: all processes producing a combination layer can receive an oxidation phase at the end of the nitriding cycle by maintaining in an oxidizing medium at a temperature close to 450°C developing a thin layer of conversion of nitrides into black Fe3O4 oxides. This layer contributes to an improvement of the corrosion resistance which can reach 450 to 600 hours in salt spray on layers free of microporosities. The harmful effect of the presence of these microporosities can be cancelled out by impregnating the surfaces with cold-curing organic products. In gaseous processes, the oxidation is carried out by the addition of an oxidizing gas (N2O, water vapor, air), in salt bath processes, the oxidation is carried out by sequentially holding the coating in an oxidizing salt bath (based on nitrates).
The nitrided layers, after removal of the white layer, can be used as a reinforcement underlay for coatings by thin layers of PVD and PACVD type (duplex treatment).
This family of treatments can be used on all ferrous alloys, with variable properties depending on the grade. The geometric variations are very small, the use of parts without rework after treatment is possible provided that the nitriding is carried out on a previously stabilized state. The mechanical finishing processes used are grinding (the white layer is eliminated, the depths must be planned accordingly), polishing with an emery strip on the surfaces of revolution, and the projection of glass balls.
APPLICATIONS OF CARBONITRIDING
NITRURATION
Nitriding treatments are most often carried out for the effectiveness of the nitrogen diffusion layer providing surface and depth hardness as well as a residual stress rate in compression particularly effective in fatigue resistance. The maintenance of these characteristics up to the treatment temperature is also exploited (case of the gears of helicopter gearboxes able to function after interruption of lubrication for some time).
However, ion nitriding producing single-phase gamma' combination layers is applied for the frictional qualities of this layer.
The applications of nitriding on alloy steels dedicated to this application (see EN NF 10085), previously hardened and tempered, are quite numerous in mechanical engineering on highly stressed components: gears, shafts, engine crankshafts, hydraulic components. The limits are those of the achievable depth, limited industrially to 0.8 mm exceptionally up to 1.4 mm. For these large depths the combination layer is removed by grinding or chemical dissolution.
The field of tooling also finds numerous applications, in particular for hot forging impressions (on X35CrMoV5 type steels) on plastic injection molds, on aluminum profile extrusion dies, etc.
NITROCARBURIZING
Nitrocarburizing treatments are mainly carried out for the qualities of the epsilon combination layer. The applications are extremely numerous, they concern all operating conditions with wear, risk of seizure combined possibly with an increased mechanical resistance.
The range of steels extends from unalloyed carbon steels to low and high alloyed steels. On low carbon non-alloyed steels, processes including rapid cooling after the nitrocarburizing phase allowing the maintenance of diffused nitrogen in solid solution and the formation of nitrogen martensite, lead to an increase in mechanical strength.
This treatment is with the hardening after surface heating is applicable to graphite cast iron (FGL and FGS), it is found in particular on the bodies of pumps, pistons, rings, crankshafts, engine liners, hydraulic distributors.
All applications requiring resistance to wear, resistance to thermal seizure, fretting corrosion, wear by small deflections are concerned.
Implementation
Main equipment (furnace, reactor, line, machine...)
Additional equipment (washing, degreasing, pickling, finishing, handling)
Ancillary equipment (pumps, turbines, resistances, burners, quenching tanks, filters, exchangers, refractories...)
Process control and monitoring ( pyrometry, atmosphere analyzers and probes, automatons, regulators, recorders...)
Energy and fluids (gases, chemicals, quenching liquids, salts...)
Tools and handling (assemblies, baskets, trays...)
Control of the result (hardness test, mechanical test, metallography, NDT, thickness measurement...)
Application materials (steels, cast iron, titanium alloys, aluminum alloys...)
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