Cyclic deformation and cracking behavior of 316LN stainless steel under thermomechanical and isothermal fatigue loadings

Isothermal fatigue (IF) tests at 250 degrees C, 350 degrees C and 450 degrees C, and thermomechanical fatigue (TMF) tests of 316LN stainless steel within the temperature range of 250-450 degrees C under in-phase (IP) and out-of-phase (OP) mechanical strain-temperature phase angles were carried out. Comparison of the cyclic deformation behavior, microcrack nucleation mechanisms, crack growth and failure behavior were performed between the IF test at 450 degrees C (IF-450 degrees C) and TMF tests. Results showed that the cyclic deformation behavior was governed by the combination of thermally activated recovery process, dynamic strain aging (DSA) and nitride precipitations for IF-450 degrees C and TMF tests. The occurrence of DSA and nitride precipitations including Z-phase and MX phase were found to promote the planar slip of dislocations and enhance the cyclic stress response. The occurrence of recovery process, however, offset the strengthening effects caused by DSA and precipitations, accelerating the softening stage. The intergranular cracking was attributed to the strong impingement of slip bands on severely oxidized grain boundaries, which can be found only on the specimen surface or near-surface regions. The transgranular cracking, resulting from the cyclic slip localization which was characterized by the formation of persistent slip markings (PSMs) in the form of surface intrusions and extrusions was enhanced by the presence of brittle oxide. The ultimate failure was still dominated by the transgranular propagation.