Plasmanitriding

Plasmanitriding; for decades a proven and extremely distortion-free method of nitriding

above: changing pictures of typical plasma nitriding work-pieces

Short description

Plasmanitriding is the term for a nitriding technology that typically takes place in vacuum furnaces at temperatures of 420 °C to 500 °C. It uses the properties of an ionized gas to specifically change the work piece surface.
We offer other proven nitriding methods like gas nitriding and nitrocarburizing as well.

Salt bath nitriding respectively Tenifer® we substitute with short duration gas nitriding respectively short duration plasma nitriding. Please let us make you a specific offer. Changing to our nitriding methods will let you benefit from work pieces that are free from salt residues.

Advantages of Plasmanitriding

• least possible distortion and negligible dimensional changes, therefore no grinding after heat treatment and no straightening of work pieces required
• high reproducibility of nitration results with mass produced parts
• nitriding depth up to 1 mm, therefore case hardening can often be replaced by plasmanitriding
• all iron based materials can be treated
• no salt residues, therefore no removal expenses caused by those undesirable salt deposits on the work piece surface, in holes and threads required
• high ductility of the layer composition
• improvement of mechanical data of the basic material
• surface quality higher (less "roughness") than with other nitriding methods
• polishing ability remarkably high
• high corrosion resistance, protective post heat treatment methods like black-finishing, phosphating (and in special circumstances zinc galvanizing and nickel plating) may be substituted
• welding ability

Detailed Description of Plasmanitriding

Precipitation hardened chromium-nickel precision gearwheels

Plasmanitriding takes place in a vacuum furnace The process runs within the metastable form of a glow discharge. At high tension (600 to 1.000 Volts) and under low pressure process gas is being converted into a partially ionized, electrically conductive plasma.

Depending on the task the treatment temperature is between 400 °C and 580 °C. However, more typically is a treatment temperature of 420 °C to 500 °C.

Due to the low treatment temperatures employed, there is no re-crystallisation even under high plastic deformation of the basic structure, resulting in minor dimensional alterations. Mag = 1000:1

A typical treatment duration is between 10 minutes and 70 hours and depends on the material used, the layer composition desired and the layer thickness required. By a variation of the nitriding parameter the layer composition can be adopted to the individual application.

Ammonia, nitrogen, methane and hydrogen are the process gases we usually employ. Oxygen and carbodioxyd are the gases we use for the corrosion protective post-oxidation treatment. .
Apart from the type of gas the furnace pressure, temperature and the treatment duration are our parameter for plasmanitriding. That way the plasmanitriding result can be optimized to suit the kind of wear and tear to be taken care of.

Layer compositions produced in plasmanitriding consist of an inter-metallic compound of iron and nitrogen. They are up to 20 µm thick and posses a high degree of hardness. However, the white layer formation can be suppressed altogether. Below the white layer there is a diffusion zone that can be as deep as 0.8 mm. Here nitrogen is embedded into the existing iron lattice and it forms - together with alloy-elements such as Cr, Mo, Ti, Al, V special nitrides. Those special nitrides are of paramount importance for the high hardness and wear resistance of alloyed materials.

Plasmanitriding Pre-treatment and Post-treatment
Parts to be plasma nitrided can be finished to their finished state. Plasmanitriding is the heat treatment with the least distortion. Work pieces with extremely tight tolerances should be discussed with us before nitriding.

Surfaces to be treated must be completely free of residues of rust, paint and oils and they may not be black-finished as this would prevent the nitrogen to penetrate the work pieces surface.

Handling of the goods apart from the actual Plasmanitriding Process
We compare parts taken in for plasmanitriding with external data such as delivery notes, drawings, material descriptions and allocate an internal process number. That way we assure a clear identification of the parts and a process where all circumstances of the treatment are documented for later inspection, if needed..

The Nitrion GmbH cleans all parts before plasmanitriding in a CFC-free washing process.

Washing media: A diluted alkalic watery tensid solution
Washing temperature: 70 °C
Mechanical activation of washing process: High pressure jets below water level for hydrodynamic sonic waves.
Working pressure: 15 bar
Process duration: approx. 20 min.

Cleaning with demineralized water rounds the process up, followed by a high pressure clean air blast. Small parts get degreased with Isopropanol in a ultrasonic bath..
Cleaned parts are transferred directly into the plasmanitriding furnace or if not they will be subjected to ambient conditions for not longer than four hours.

Covering of threads and other sections that may not be nitrided
Threads are covered by fitting suitable screws respectively nuts. Surfaces are covered with a copper based paste to prevent nitriding. The Nitrion GmbH normally does not charge extra for such work..

Furnace Space Allocation and Process Duration
The furnaces get loaded in accordance with the written space allocation instructions prepared for the individual parts.
The process duration depends on the requested nitriding depth. We observe relevant instructions on your drawings.

After Plasmanitriding

• Our quality control staff executes suitable check-ups and certifies the results. For more details please refer to our web page "Material Test Laboratory".
• Plasma nitrided work pieces are ready for assembly right after the heat treatment.
• Unlike with salt bath nitriding the are no salt residues at the part surface and inside threads!
• Machining such as grinding after nitriding is the exception - and due to the remarkable characteristics of the layer composition - ought to be discouraged. With the dimensional stability of plasma nitrided parts such advice can easily be observed
• With superior demands regarding the surface roughness plasmanitrided parts can be lapped or polished.

More Details regarding the advantages of Plasmanitriding

Low Distortion
With conventional nitriding processes work pieces get heated up by convection, direct heat exposure or in salt baths. Work pieces with varying cross sections react with tension buildup as a result of such those heating methods. Distortion may be the result and consequently renewed machining can be required, sometimes the parts need even be scrapped.
Because of the process taking place in a vacuum atmosphere without quenching the distortion observed - if any - is normally due to intrinsic tension caused by machining before the heat treatment.
Therefore, tension reduced work pieces are an important precondition for low distortion plasmanitriding results, not unlike with other heat treatment methods.

Plasma nitrided surface with well defined grain boundary e.g. no minor cracks leading to material failure under alternate-direction-loads. Mag. =1000:1

Dimensional Stability and Surface Quality
Dimensional changes after plasmanitriding often are negligible. Work pieces can normally be machined to the final dimension.
Those are the dimensional changes in question:

Distortion
While both the increase in surface roughness and in volume are just about negligible, there may be minor distortion due to material tension induced during machining.
To release such tension before nitriding we suggest strees-release-annealing prior to the final dimension-controlling machining.
Annealing temperature and annealing duration are depended on the parts geometry.

Increase of Surface Roughness
Crystal growth of the iron nitride layer at the surface of the work piece may cause an increase in surface roughness. Depending on the material and type of treatment used, the surface roughness "Ra" in the polished state may increase by approx. 0.1 to 0.6 µm.

The surface roughness increases with the duration of the heat treatment. It should be noted that gas nitriding takes longer than plasma nitriding and results in a slightly more rough surface. Surface roughness may be reduced to its initial value by subsequent grinding or lapping, e.g. by lapp-blasting, a procedure offered by Nitrion GmbH.

Volume Increase
Volume increases in plasma nitriding are subject to the same rules as in other nitriding processes. The increase in diameter depends on the amount of nitrogen embedded, and this can be exactly calculated and allowed for.
In the plasma nitriding of a low alloy steel to a nitriding depth of 0.4 to 0.6 mm, the measured increase of shaft is 15 to 20 µm.

Even complex contours have a uniform nitriding result. Mag = 100:1

Improvement of Mechanical Properties
Steel, cast iron, and sintered steel materials may be plasma nitrided. Major improvements in material properties can be achieved with low alloy construction and case hardened steels, heat treatable and nitriding steels as well as high alloy steels and sintered materials.
By varying treatment parameters, surface layers can be generated that are optimal for the specific type of demand on the individual component.
The generation of nitride layers at low temperatures permits treatment of annealing sensitive materials.
The white (nitride) layers generated in plasma nitriding are denser and free of pores at the surface due to the close control of nitrogen activity in this process. Moreover, treated parts show improvements in wear resistance, durability, and fatigue strength under reversed bending stresses.

Toughness (Ductility)
Plasma nitrided boundary layers possess relatively high levels of toughness, especially in higher-alloy materials. There are two main reasons for this: first it is feasible to generate a mono phase white layer of nitrides by closely controlling treatment conditions, and second the white layer can be kept very thin or completely suppressed.
Highest ductility, and therefore toughness, is attained with thin diffusion zones and no white layer.

Leveling of major roughness: before: R2D = 7.00; after: R2D = 4.5. With gray cast iron, the graphite lamellae are completely covered by a white layer of nitrides

Corrosion Resistance
The corrosion properties of non-alloy and low-alloy steels are improved by the thin white layer of stable nitrides.
Corrosion resistance may be increased even further by post-oxidation as a part of the nitriding treatment.

It ought to be noted that the corrosion resistance of the basic material returns to its normal value after the white layer is worn off.
The corrosion resistance of corrosion resistant and acid resistant steel is significantly reduced by the formation of a white layer. The formation of a white layer should therefore be avoided. This can be done by a process called "blank nitriding", by which only part of the corrosion protective alloy elements are converted to nitrides.

SUMMARY
Apart from machining saved after plasmanitriding, the process is most beneficial when problems with corrosion, wear and tear and material strength need to be solved as well.

Extensive positive results based on the high process repeatability can be documented with mass-produced parts such as gearbox synchronizing rings, precision gears for high performance gear drives, valves, ejector pins and hydraulic pistons. All parts mentioned are ready for assembly after plasmanitriding.

Seal for armored gearing, outer diameter = 580 mm, tolerance ± 0.01 mm