If conventional nitriding is used to nitride stainless – be it gas, salt or plasma – the passive layer of stainless is detroyed and the chromium nitrides which form rob the matrix of the chromium that is essential to maintain a passive state. The result is the formation of black surface layer that has poor tribological propeties and very poor corrosion resistance.

S-Phase on Stainless Steel

However, if the nitriding temperatures are kept below a critical level, the formation of the deletrious phases are avoided and Sub-Critical Nitriding (SCN) does exaclty that. The result is the formation of a nitrogen-enriched surface layer which shows good tribological properties and good corrosion resistance. This nitrogen rich layer is also known as the S-phase.

The S-phase is of high hardness (70-72 HRc) with a diffusion depth of typically 20 – 30 µm deep on austenitics, but layers up 150 µm can be formed on certain alloys. The S-phase does not negatively affect the corrosion resistance (as a matter of fact it can enhance it) and, importantly, it creates a surface that is scratch resistant and highly resistant to galling – even on soft stainless steels (304/316/ etc.). SCN can be performed on several stainless' including austenitic – , martensitic – and duplex stainless steels. The low temperatures used in SCN also do not alter the austenite-ferrite ratio of duplex alloys.

SCN treated parts have shown good performance in applications ranging from sea water hydraulic systems, injection moulding inserts (rotating and sliding cores), parts used in the food and beverage industry, pump components, ball valves, fittings and machine parts.

Contact us regarding the SCN treatment for your applications.