Abstract
The addition of nitrogen has been widely recognized to significantly enhance the elevated-temperature strength of austenitic stainless steels. However, the effect of solute interstitial nitrogen remains unclear in many cases owing to nitride precipitation. This study investigated the influence of solute interstitial nitrogen on the strength of austenitic stainless steels at elevated temperatures under conditions where nitrides do not precipitate. The 0.2% proof stress and tensile strength at 1173 K increased with increasing solute nitrogen concentration. For the Fe-18%Cr-8%Ni-N alloy with up to 0.1% nitrogen, the strength increased linearly with the square root of the solute nitrogen concentration. However, at higher nitrogen concentrations, the strength deviated from this relationship and exhibited an even greater increase. Observations of the dislocation microstructures revealed distinct subgrains in the 0.1N steel, whereas subgrain formation was less pronounced in the 0.2N steel, suggesting different strengthening mechanisms for the 0.1N and 0.2N steels at elevated temperatures. A theoretical investigation of the evolution of deformation at elevated temperature using creep and stress-change tests suggested that the 0.1N steel exhibits recovery-controlled deformation, in which the climbing motion of edge dislocations dominates the deformation, whereas the 0.2N steel exhibits glide-controlled deformation, in which the dislocations are dragged by the solute atmosphere. This may stem from the sufficient segregation of the nitrogen and Cr-N atmospheres into the dislocations and stacking faults at higher nitrogen concentrations, which reduces the mobility of the dislocations.
