The Study of Fatigue Behaviors and Dislocation Structures in Interstitial-Free Steel

There are three types of cyclic hardening for cyclically deformed interstitial-free (IF) steels. The magnitude of cyclic hardening was unobvious and dislocation cells smaller than 2 mu m were very hard to find when total strain amplitude (Delta epsilon/2) was controlled to within 0.1 pct. When Delta epsilon/2 is increased to 0.125 to 0.3 pct, secondary cyclic hardening takes place prior to fatigue failure. Delta epsilon/2 = 0.6 pct, following an initial rapid-hardening stage. Dislocation cells smaller than 2 mu m tend to develop near grain boundaries and triple junction of the grains while cycling just above Delta epsilon/2 = 0.125 pct. Such dislocation development results in secondary hardening. However, no failure occurs if cycling just below Delta epsilon/2 = 0.1 pct; hence, the fatigue limit for IF steel should be very close to Delta epsilon/2 = 0.1 pct.