LOW-CYCLE FATIGUE BEHAVIOR OF A NEW TYPE OF STAINLESS-STEEL

Specimens of an Fe-Cr-Mn-N dual-phase stainless steel were solution treated at 1050 and 1250-degrees-C, yielding grain sizes of 10 and 32-mu-m respectively. Low cycle fatigue tests at constant total strain amplitude ranging from 4 x 10(-3) to 1.2 x 10(-2) were carried out at room temperature in air. The cyclic stress response, hysteresis loss per cycle and dislocation structure were investigated in the course of the cyclic deformation. No significant differences were noted in either the cyclic behaviour or the dislocation structure between the two heat treatment conditions. Cyclic hardening occurred in the very early stage of fatigue life. After passing through a maximum of hardening, cyclic softening set in until fracture. The strain amplitude affects the initial hardening rate and hysteresis loss. Dislocation tangles and planar arrays of dislocation were observed after the first few cycles in the initial hardening stage, while the cyclic softening stage is characterized by the formation of irregular bands or wall structures. In addition, grain boundaries were found to be effective barriers to slip dislocations during the initial hardening stage, while increased relaxation activities at grain and twin boundaries are evident during the cyclic softening stage. The present observations suggest that the dislocation activities within the grains and at or near the grain boundaries are both responsible for the observed cyclic hardening and softening of the material under study. The failure mode for the specimens solution treated at 1050-degrees-C was transgranular while that at 1250-degrees-C was intergranular. The former has a fatigue life about twice that of the latter under identical total strain amplitude.