Study of microstructure of low-temperature plasma-nitrided AISI 304 stainless steel

The microstructure of the low-temperature plasma-nitrided layer on AISI 304 austenitic stainless steel was studied by transmission electron microscopy (TEM). The results show that the surface of the layer consists of a supersaturated solid solution (gamma'(N)) based on the gamma'-Fe4N phase whose electron diffraction pattern (EDP) has a strong diffuse scattering effect resulting from supersaturating nitrogen (above 20 at, pct) and [110] streaks arising from matrix elastic strain due to the formation of paired or clustered Cr-N. The latter is due to the N above the 20 at. pct gamma'-Fe4N-phase value and leads to a lattice parameter that is greater than that of the gamma'-Fe4N phase. The subsurface of the layer is composed of a supersaturated solid solution based on gamma-austenite, which is an expanded austenite, gamma(N). Its morphology shows the basketweave or "tweedlike" contrast consisting of so-called stacking fault precipitates having twin relationships with the matrix whose EDP shows diffuse scattering streaks with certain directions. The epsilon martensite transformation was observed in the subsurface of the layer. The increase in stacking faults compared with the original stainless steel and formation of epsilon martensite in the subsurface of the layer indicate that nitrogen lowers the stacking fault energy of austenite.