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.