Nitriding of super-ferritic stainless steel by plasma immersion ion implantation in radio frequency and ECR-microwave plasma system

Stainless steel 470 Li-24 Cr and 460Li-21 Cr are two new generation super-ferritic nickel free grades, contains titanium (Ti), niobium (Nb) and very small percentage of carbon (C) and nitrogen (N). The Ti and Nb enhance the corrosion resistance while the low interstitial content of C and N leads to finer precipitates and greater ductility. These grades are economic alternative to 316L and 304 with corrosion resistance comparable or superior. However, as 316L and 304 can be nitrided efficiently to improve the mechanical surface properties like hardness and wear; it has to be asked whether these new generation super-ferritic grade stainless steels can be improved further by plasma nitriding too. Two sets of experiments have been carried out, one with capacitively coupled radio frequency Plasma at PUC Chile and the other is microwave plasma at IOM Leipzig, to investigate further improvements in the mechanical properties of 470 Li-24 Cr and 460Li-21 Cr steel. Nitrided and unnitrided substrates are further investigated using different surface characterization techniques including secondary ion mass spectroscopy, scanning electron microscopy, energy dispersive x-ray analysis, Vickers hardness, wear resistance, corrosion test etc. In most of the characterizations the nitrided 470 Li-24 Cr and 460Li-21 Cr shows similar results. Increase of surface roughness for higher treatment temperature is observed in both the experiments, independent of the steel types. The formation of chromium nitride compound takes place at treatment temperature around 400
^o
C-450
^o
C, and it increases with further increase in temperature. But at higher temperature corrosion property deteriorate. The characterization results show up to 25% of nitrogen, 4-5 times increased in hardness and diffusion of nitrogen layer up to 4-6 micron for different experimental conditions. The samples implanted with temperature higher than 400°C presented wear resistance values around two orders of magnitude higher than the others.