Microstructural development and cracking behavior of AISI 304 stainless steel tested in time dependent fatigue modes

Transmission and scanning electron microscopy were used to study the substructural development, crack initiation and crack propagation of AISI 304 stainless steel tested in low-cycle fatigue with various hold times. Tests at 5930C, a strain rate of 4 E-03 s-1 and strain ranges of 0.5, 1.0 and 2.0 percent were interrupted at various fractions of the fatigue life, both during rapid hardening and during saturation. Cells developed during the first few percent of the fatigue life, depending on a threshold stress value, below which cells were not observed. The cell size had a different dependence on the relaxed tensile stress during rapid hardening than during saturation. A work-hardening model relating the peak saturation stress to strain and cell size also applied during the late stages of rapid hardening. The number of cycles to initiate a crack of one grain diameter and to propagate the crack to failure were proportionally reduced for one minute tension holds. For a 60 minute tension hold, crack propagation occupied a much smaller fraction of the fatigue life