The Effect of Nitrogen Alloying on the Low Cycle Fatigue and Creep-Fatigue Interaction Behavior of 31LN Stainless Steel

摘要

Low cycle fatigue (LCF) and Creep-fatigue interaction (CFI) behavior of 316LN austenitic stainless steel alloyed with 0.07, 0.11, 0.14, .22 wt.% nitrogen i briefly discussed in this paper. The strain-life fatigue behavior of these steel is found to be dictated by not only cyclic plasticity but also by dynamic strain aging (DSA) and secondary cyclic hardening (SCH). The influence of the above phenomenon on cyclic stress response and fatigue life is evaluated in the present study. The above mentioned steels exhibited both ingle- and dual-lope train-life fatigue behavior depending on the test temperatures. Concomitant dislocation substructure evolution has revealed transition in substructure from planar to cell structure justifying the change in slope. The beneficial effect of nitrogen on LCF life is observed to be maximum for 316LN with nitrogen in the range 0.11 - 0.14 wt.%, for the tests conducted over a range of temperature (773-873 K) and at ±0.4 and 0.6 % train amplitudes at a strain rate of 3*10~(-3) s~(-1). A decrease in the applied train rate from 3*10~(-3) s~(-1) to 3*10~(-5) s~(-1) or increase in the test temperature from 773 to 873 K led to a peak in the LCF life at a nitrogen content of 0.07 wt.%. Similar results are obtained in CFI tests conducted with tensile hold period of 13 and 30 minutes. Fractography studies of low strain rate and hold time tested specimen revealed extensive intergranular cracking.