Surface hardening of Austenitic Stainless Steels with Nitrogen
It is well known that Austenitic Stainless Steels cannot be through hardened, so whilst they remain a preferred choice of Stainless Steel for many applications they are very susceptible to wear and galling.
One common treatment used to increase the surface hardness of such steels and to minimise galling is to Nitride the steel by Plasma or Salt Bath Nitriding. This provides a very hard (>1000Hv) surface however there is an associated loss of corrosion resistance in the nitride layer.
When Stainless Steels are treated with traditional nitriding a surface layer is created which consists of a diffusion zone and sometimes also a compound layer. Characteristic to these traditional methods of treatment is the formation of Chromium Nitride (CrN) in this layer, which improves the surface hardness and wear resistance but distinctly reduces the corrosion resistance.
Stainihard® and Stainitec treatments
However, nitriding treatments are available which will provide a very hard, wear resistant, anti galling surface and still maintain the good corrosion resistance of the steel. Stainitec, provided by TTI Group in the UK and Stainihard®, available from sister company H&ST in Eindhoven, Holland are two such treatments.
Both these processes are used to harden the surface of Austenitic Stainless Steels without reducing the corrosion resistance – in fact, in some cases the corrosion resistance is even improved. This provides anti galling, scratch resistance and high surface hardness.
Stainihard® NC is a gaseous thermochemical process suitable for processing individual components or large batch volumes. The Stainihard® NC process enriches the steel surface with Nitrogen and Carbon to provide a hard, wear resistant layer at the surface. The layer formed is known as ‘S-phase’.
The Stainitec process produces a similar extremely hard surface ‘S-phase’ layer by a plasma treatment. The surface hardness of the layer can reach up to 1400Hv and is always over 1000Hv.
The depths of layers created with these treatments depends on the type of stainless steel that is used, the amount of work hardening in steel surface as well as the specific treatment applied.
It is also possible to apply a PVD hard coating such as TiN on top of these layers to further enhance wear resistance, anti galling and friction characteristics.
In both the Stainihard® and Stainitec treatments the formation of Chromium Nitride (CrN) or Chromium Carbide (CrC) is suppressed and a so called ‘S-phase’ or ‘Supersaturation phase’ is created. This layer consists of Stainless Steel which is supersaturated with Nitrogen and / or Carbon at the surface. The saturation creates high internal stresses in the layer and the hardness is increased significantly, without reducing the corrosion resistance. In fact the corrosion resistance is often enhanced also.
Examples of suited austenitic Stainless Steels
301, 303, 304, 304L, 304Ti, 310, 316, 316L, 316Ti, 321, 347, 904L
1.4301, 1.4305, 1.4307, 1.4310, 1.4401, 1.4435, 1.4539, 1.4541, 1.4550