It is known that the addition of nitrogen enhances the several mechanical properties of austenitic stainless steels. But most of the reported investigations for the effect of the nitrogen on the mechanical properties are studied under 600$^\circ$ The purpose of this study is to investigate the effect of nitrogen on creep-rupture behaviors of 18Cr-9Ni stainless steels with different nitrogen contents in varying stress and temperature. The influence of nitrogen on creep-rupture properties for 18Cr-9Ni stainless steels was investigated in air atmosphere at 700 and 800$^\circ$.
Under creep-rupture test, 18Cr-9Ni-0.14N steel had longer rupture lives than those of 18Cr-9Ni-0.08N steel at 700 and 800$^\circ$. From the observations of fracture surface, it can be known that the Cr-rich carbides are precipitated mainly at the grain boundary. Comparing the ratio of the linear density of the precipitate particles, 18Cr-9Ni-0.14N steel has lower grain boundary carbide density than 18Cr-9Ni-0.08N steel has, which has been known to provide the preferential site for the cavity nucleation. So, the reason why 18Cr-9Ni-0.14N steel has longer creep-rupture lives than those of 18Cr-9Ni-0.08N steel can be explained that nitrogen retards carbide nucleation at grain boundary by inhibiting diffusion of Cr to grain boundary, 18Cr-9Ni-0.14N steel has lower grain boundary carbide density, which results in retard cavity nucleation.
Finally, it can be said that the addition of nitrogen is beneficial for real structural materials which are subjected to complex loading at high temperature, because the addition of nitrogen can increase strength and decrease the carbide density to inhibit cavity nucleation.
