22nd November, 2017

Plasma Ferritic Nitrocarburising with Post Oxidation (PlasOx)
A Single Stage Process

Metaltech have enhanced the plasma assisted ferritic nitrocarburising process with a steam oxidation cycle to produce a coating in one process that has the many advantages over conventional ferritic nitrocarburising and Tufftride processes.

Background:
Salt bath ferritic nitrocarburising is basically the processing of parts in a molten bath of an alkali cyanate salt. Tufftride is a trademarked name for one type of salt bath ferritic nitrocarburising process.

Tufftride salt bath nitrocarburising is carried out typically between 580°C and 610°C. The process of Tufftride produces a thin outer iron nitride compound layer, typically comprising of 7-9% nitrogen and approximately 1% carbon. A typical nitrocarburised case depth would be 10-20 microns. As the layer is not metallic in nature it can develop resistance to wear, seizure and corrosion and has high temperature cracking resistance to at least the processing temperature at which it was formed. Hardness values quoted range from 400HV to 1500HV depending on material used.

By giving the process a special quench – Tufftride Q- corrosion resistance can be enhanced with reported performance of up to 200 hours in a salt spray system.

Tufftride QPQ means quench, polish, quench and consist of salt bath nitrocarburising followed by oxidative cooling (quenching) mechanical polishing, lapping and re-immersion into the oxidative salt melt. The end result gives unalloyed steels a lustrous black finish with corrosion resistance.

Process Step Bath Temperature Time at Temperature
Preheat - 570°F -750 °F (300°C - 400°C) 30 mins – 1 hour
Nitriding Nitriding 1075°F ±15°F (580°C ± 10°C) 90mins – 2 hrs
Cooling Oxidising 650°F -750°F (340°C - 400°C) 10-15 mins
Rapid Cool Water Bath not to exceed 120°F (50°C) -
Mechanical Processing Surface mechanically processed using a technique appropriate to the desired surface finish
e.g. abrasive treatment or polishing / lapping.
Post-Oxidation Oxidising 650°F -750°F (340°C - 400°C) 10-30 mins
Rapid Cool Water Bath not to exceed 120°F (50°C) -
Cleaning Parts are to be thoroughly rinsed prior to immersion / coating with a rust preventative
e.g. Rust Veto MP2-, 4212 or equivalent.

Metaltech's Plasma assisted ferritic nitrocarburising achieves improved results over the salt bath and gaseous processes. The media is produced by the controlled ionisation of nitrogen, hydrogen and methane enabling the diffusion of nitrogen and carbon into the component surface. By a targeted and controlled oxidation phase into the compound layer produces by the initial plasma treatment (post oxidation) the corrosion resistance can be significantly enhanced. The treatment known as PLASOX is done in a single stage process unlike the Tufftride QPQ treatment. Therefore, it has an immediate advantage over the QPQ process in that no mechanical processing is required – it is essentially a one stop shop process.

Benefits of the PLASOX processes:
PLASOX oxide layer, white layer and diffusion zone

Improvements Achieved by PlasOx


Alloy 4140 – 24hrs salt fog corrosion test ASTM B117-3 Alloy 4140 – QGQ Tufftride- 48hrs salt fog corrosion test ASTM B117-3
 
Alloy 4140 – PlasOx - 72hrs salt fog corrosion test ASTM B117-3 Salt spray corrosion test to ASTM V117-3


48hrs salt fog corrosion test results (ASTM B117-3) – Alloy 4140 base metal, Tufftrided, Gas Nitrided, PlasOx (air), PlasOx (steam).
Clearly exhibiting the superior properties of the Metaltech PlasOx (steam) process over the comparative themochemical surface treatments.
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