Cyclic Deformation Behavior and Failure Mechanism of 316LN Stainless Steel under Creep-Fatigue Loading at 550 °C

A comparative study on the low-cycle fatigue properties of 316LN austenitic stainless steel with and without a hold period at the maximum tensile strain was performed. Different hold periods (0 s, 60 s, 120 s, 300 s, 1800 s) at a constant strain amplitude of +/- 0.8% were employed in creep-fatigue tests at 550 degrees C. Dynamic strain aging (DSA), creep and thermal recovery dominated the deformation and failure behavior. The evolution process of DSA activity and stress relaxation caused by creep presented an opposite trend during cycling. The amount of stress relaxation increased while the associated inelastic strain rate decreased with increasing dwell time, which promoted the accumulation of intergranular damage per cycle and facilitated the formation of dislocation cells. The enhanced process of cross-slip attenuated the DSA activity, which compensated for the increased creep damage; thus, the fatigue life remained almost constant after the introduction of a hold period of 300 s. Interestingly, the fatigue life showed a slight increase when the hold period increased to 1800 s due to the increased thermal recovery process.