Effect of Nitrogen on the Fatigue Crack Growth Behavior of 316L Austenitic Stainless Steels

Fatigue crack growth (FCG) behavior of 316L austenitic stainless steels (SSs) is studied as a function of nitrogen concentration and load ratios, R. Addition of nitrogen to austenitic SSs, in general, improves many of its properties. Austenitic SSs are known to undergo deformation-induced martensitic transformation (DIMT), which can influence their mechanical properties. DIMT occurring near the crack tip can improve the crack growth resistance under monotonic and cyclic loads. Nitrogen, however, stabilizes the austenite inhibiting or retarding DIMT, thereby reducing the toughness. The present detailed study was undertaken to evaluate the effect of nitrogen concentration on the FCG behavior of this steel at room temperature at different load ratios. The crack growth data are analyzed using the unified approach based on the two-parametric nature of fatigue, developed by the one of the authors. Crack growth trajectory maps were constructed using the above approach. These trajectory maps show how the material resistance to crack growth changes with increasing stress intensity factor and nitrogen content. The results are compared with the crack growth trajectories derived using the published crack growth data for 304 austenitic SSs known to show DIMT. The comparison indicates that the results of the present study can be explained with transformation toughening, albeit at a reduced rate compared with nitrogen-free alloys. Fractographic and transmission electron microscopy results are also consistent with the above conclusions.