Nitriding of Austenitic Stainless Steel

Abstract

Stainless steel is a difficult material to nitride because a
passive oxide film is present on its surface. Hence it is essential to
renove this film by chemical or mechanical means before any surface
treatment can be carried out. This increases the cost of the treatment
substantially.

The cammonly used processes for the surface treatment of
stainless steel have been gas nitriding, salt bath nitrocarburising and
lately plasma nitriding. Plasma nitriding due to its operating
conditions, does not require any prior treatments for removal of the
passive oxide film. Plasma nitriding also provides the possibility of
treatment at low temperature, which is important to retain corrosion
resistance of stainless steel.

The fact that nitriding of stainless steel improves the wear
characteristics but impairs the corrosion resistance has been reported by
many researchers. However, very little work has been carried out to
study and analyse this problem. This research work has studied the
corrosion and wear properties of austenitic stainless steels using the
three different nitriding techniques mentioned above. It was also the
objective to study the optimum plasma nitriding conditions required to
achieve optimum wear and corrosion properties.

Corrosion has been studied by using an electro-chenical
technique. A microprocessor based Princeton Applied Research Model-350
Corrosion Measurenent Systen was used. Linear polarisation curves have
been plotted so that the comparison could be made between different
oprocesses, different grades of untreated austenitic stainless steel and
nitrided stainless steel. The evaluation of wear properties was carried
out by using the Falex testing technique.

It was concluded that the plasma nitriding of austenitic
stainless steel at 460°C followed by a post-oxidation treatment resulted
in better wear and corrosion properties than the other treatments and
would be suitable where wear requirements are more rigid than the
corrosion resistance. Plasma nitriding of the 321 grade stainless steel
at 380°C would be suitable where the corrosion resistance was a primary
requirement. Salt bath nitrocarburising of austenitic stainless steel
also resulted in better corrosion and wear properties than gas nitriding.
The results showed that the wear rates obtained by using the above
different techniques of nitriding, were different. However, the corrosion
rate was observed to be the sane over long periods.

Date of Award	1985
Original language	English