Fatigue crack non-propagation assisted by nitrogen-enhanced dislocation planarity in austenitic stainless steels

Rotating bending fatigue tests were conducted to assess the fatigue crack propagation behavior of the Fe-25Cr-1N and Fe-18Cr-14Ni austenitic steels in terms of the microstructure, crack propagation paths, and non-propagating fatigue crack characteristics. The Fe-25Cr-1N steel exhibited a non-propagating fatigue crack at the fatigue limit (310 MPa), but this did not occur in the Fe-18Cr-14Ni steel at the fatigue limit (110 MPa). The non-propagating fatigue crack observed in the Fe-25Cr-1N steel was produced by roughness-induced crack closure. This phenomenon was caused by the enhanced planar dislocation and high dislocation pile-up stress resulting from the suppression of cross-slip, which inhibited the dislocation emission from the crack tip. The Fe-25Cr-1N steel exhibited a lower fatigue crack growth rate than the Fe-18Cr-14Ni steel because of the enhanced dislocation planarity produced by the Cr-N interaction. The Cr-N interaction affected the fatigue crack growth behavior as follows. The short crack region exhibited a planar glide dislocation pattern, but multiple slip systems were activated as the crack lengthened. As the dislocation pattern remained planar on each slip plane, the crack propagation occurred along the {1 1 1}_γ slip planes, even in the long crack. Moreover, the dislocation pile-up at the grain boundaries caused grain boundary subcracks, which can induce crack toughening through mechanisms such as stress redistribution. These positive effects contributed to the lower fatigue crack growth rate in the Fe-25Cr-1N steel than the Fe-18Cr-14Ni steel.