New possibilities for surface hardening of stainless steels

How to improve wear and seizure resistance of stainless steels while keeping their corrosion resistance?

Today, thanks to plasma assisted technologies, it is possible to answer to this always ongoing question.

By varying the temperature treatment, we can obtain different metallurgical structures:

1)	Under well known conventional plasma nitriding conditions, typically a temperature between 460-600°C, the hardening mechanism is due to chromium nitrides precipitation which leads to depletion of chromium in the austenite and thus the material loses its stainless features.(Surface hardness: better than 1000Hv, thickness: up to 0,4mm.)
2)	At low temperature, typically lower than 460°C, the hardening mechanism is due to compressive stresses developed by insertion of N or C in the lattice of the austenite.(surface hardness,: better than 1000Hv.) Chromium nitrides precipitations are avoided and the corrosion resistance remains very good. But the thickness of the layer remains thin, about 10 to 40 microns.
3)	For some applications, we need thicker layers, it’s why high temperature (above 900°C) plasma or low pressure carburizing processes were also developed. For austenitic stainless steels, the hardening mechanism is due to a very fine and homogeneously distributed carbides precipitation.( surface hardness: 700Hv, and thickness: up to 0,4mm.) Non magnetic and corrosion behaviour remain good.

Main metallurgical results after low temperature processes with Carbon and/or Nitrogen:
When we use carbon only, we obtain a very pure solid solution of carbon in the austenite, called C “S Phase”. Fig.n°1

Fig.n°1

With carbon and nitrogen simultaneously introduced, a solid solution of carbon and nitrogen is obtained, called C+N ”S PHASE”, Fig. n° 2. The typical carbon and nitrogen distribution in the layer is shown on Fig n° 3. Carbon presents a specific profile with two peaks, one just below the surface, and one in front of the layer.

Fig.n°2

Fig.n°3

The rate wear improvement is better than a factor of 700 for Nitrogen and Nitrogen+carbone solid solution, and a factor of 70 for Carbon solid solution.

The corrosion resistance is perfect with Carbon and Carbon+Nitrogen solid solution, and decreases a little with only Nitrogen solid solution.

New possibilities, now available for surface hardening of stainless steels, are summarised in Fig.n°4

Reference	Treatment characteristics	Materials
Nitrogen .”S Phase”. Temp< 420°C	Best wear and friction behavior improvement. Very slight loss of corrosion resistance at the extreme surface. Thickness: 5-35μm	Austenitic and martensitic stainless steels
Carbon “S Phase” Temp.< 450°C	Best corrosion resistance: no loss of corrosion resistance; good friction and wear resistance, but slightly inferior to N “S phase”; thickness 5-40μm	Only austenitic Stainless steels
Carbon and Nitrogen “S Phase”. Temp<420°C	Best compromise between wear and corrosion resistance .thickness 5-40μm	Austenitic and martensitic stainless steels
Conventional plasma nitriding 460°C<600°C	excellent wear resistance. Loss of corrosion resistance, but local treatment of friction areas. Thickness: 20 to 300μm	Austenitic and martensitic stainless steels
Carbon base plasma or low pressure carburizing Temp.>900°C	Improve wear resistance, good corrosion resistance, Thickness: 100-800μm. Risk of distortions.	Austenitic and martensitic stainless steels

*Fig.n°4*
1)	If we are looking for the best wear and friction improvement, Nitrogen “S Phase” offer the best results, but, with a slight loss of corrosion resistance at the extreme surface.
2)	If we are looking for the best corrosion behaviour, Carbon “S Phase” is the best treatment, but only on austenitic stainless steels and a wear resistance lower than Nitrogen “S phase”.
3)	Finally, the best compromise between wear and corrosion resistance is offered by Carbon and Nitrogen “S Phase”, on either austenitic or martensitic stainless steels.
4)	The thicknesses of the different “S Phases” are between 5 and 40 microns. When we need deeper layers, we can use conventional plasma nitriding on stainless steels with localised treatments (thicknesses up to 300 microns); we have a loss of the corrosion resistance, but only on the treated areas.
5)	If we need an important thickness and a good corrosion resistance everywhere, the only solution consists in high temperature carburising on austenitic as well as martensitic stainless steels. The thicknesses available are in the range of 100 to 800 microns. But, we have to be careful about the risk of distortions due to the high temperatures (900 to 1100 C°)

Advantages of low temperature processes: No distortion due to treatment-No skin growth-Preserved surface finish-

Possible dimensions of the parts to be treated: Length: from 1mm to 6m.-Diameter: from 1mm to 2.5m- Weight: from 1gr to 6 tons.

Applications: Nuclear, chemical, and food industries-Biomedical (implants, surgical instruments)-Spatial and aeronautic industries-Components for valves and pumps-nuts and Bolts......

Author: Dr.-Ing. Jean-Paul Lebrun