Gasnitriding

A Nitriding method offered by Nitrion GmbH, Bavaria, Germany especially for mass-produced
Machine Parts

Short description
Gasnitriding is the term for a nitriding method that typically takes place at temperatures of between 500 °C and 590 °C. nitrogen diffuses into the steel with the result of a predetermined alteration of the work piece surface.

Apart from gasnitriding and nitrocarburizing (short duration gasnitriding) we offer plasmanitriding as a service. Please refer to our free Technical Reports for a summary of the advantages and disadvantages of those nitriding methods. Salt bath nitriding respectively Tenifer® we substitute with short duration gasnitriding or short duration plasmanitriding. Please let us send you a quotation with favorable conditions.
Switching to the nitriding methods offered by Nitrion GmbH would let you benefit from work pieces that are free from salt residues.


The Advantages of Gasnitriding

• least possible distortion and negligible dimensional changes, therefore no grinding after heat treatment and no straightening of work pieces required
• deep bores/holes and similar inner work piece contours may be nitrided
• high reproducibility of nitration results with mass produced parts
• nitriding depth up to 1 mm, therefore case hardening can often be replaced by plasmanitriding
• 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
• high corrosion resistance, protective post heat treatment methods like black-finishing, phosphating (and in special circumstances zinc galvanizing and nickel plating) may be substituted
• even colored work piece surface after nitriding
  

Brief description of Gasnitriding
Work pieces to be gas nitrided are placed hanging or - on batch holding devices - standing in a gas tight furnace (retort) and they are exposed to a nitrogenous gas (ammoniac NH3) while being warmed-up.

A typical treatment duration is between a few minutes and 100 hours and depends on the material used, the layer composition desired and the layer thickness/nitriding depth required. By a variation of the nitriding parameter the layer composition can be adopted to the individual application.

Layer compositions produced in gasnitriding 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.

Machining before Gasnitriding / Dimensional Changes during Gasnitriding
Parts to be nitrided can be finished to their final dimensions due to the low rate of distorting during gasnitriding . However, one needs to take into account a small increase in material volume ( = growth of outer dimensions). Gasnitriding brings with volume increases that are slightly above those of plasmanitriding. Work pieces with extremely tight tolerances should be discussed with us before nitriding.

Work pieces handed over to us for gasnitriding need to have a machined metallic surface. 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 Nitriding Process
We compare parts taken in for nitriding 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 nitriding 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
Where applicable, surfaces are covered by Nitrion GmbH staff with a special copper based paste to prevent nitriding.

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 Gas Nitriding

• Our quality control staff executes suitable check-ups and certifies the results. For more details please refer to our web page "Material Test Laboratory".
• Nitrided work pieces are ready for assembly right after the heat treatment.
• Unlike with salt bath nitriding there 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 gas nitrided parts such advice can easily be observed
• With superior demands regarding the surface roughness plasma nitriding parts can be lapped or polished.

The Cost of Gas nitriding versus the Cost of Plasmanitriding
Based on a concrete work piece of yours we will gladly advise on the most advantageous method of nitriding. Gasnitriding has got the advantage of allowing work pieces to be placed closer to each other in the furnace while plasmanitriding has the advantage of a more simplified method to keep surface sectors soft, if required.