Fatigue Behaviour of New Duplex Stainless Steels Upgraded by Nitrogen Alloying

The fatigue behaviour of two duplex stainless steels (DSS) alloyed with different nitrogen contents (0.24 and 0.4 wt%) has been studied in air and in a NaCl-containing water solution. Both of the alloys exhibit a cyclic softening preceded by a cyclic hardening at high strain amplitude as reported in the literature for lower nitrogen grades. In air, the fatigue lives are nearly the same for both of the DSS for any given strain amplitude. In the corrosive environment, the fatigue life was systematically reduced by a factor two for the 0.24% N-containing DSS in the strain range studied in contrast with the 0.4% N-containing DSS which appeared to be all the more insensitive as long as the strain amplitude was small. The fatigue resistance and the cyclic accommodation of these DSS are strongly controlled by the volume fraction of α and γ phases. It is shown that DSS with a high fraction of austenite present a good combination of fatigue resistance and cyclic softening. Alloying with nitrogen appears to be a promising way to master an optimised microstructure leading to high mechanical resistant DSS.