Low cycle fatigue behaviour study of a nano precipitate strengthened ferrite-bainite 780 steel

Low-cycle fatigue behaviour of a newly developed ferrite-bainite 780 steel with the ferrite phase, strengthened by nanosized precipitates was investigated. The LCF tests were conducted in fully reversed strain-controlled mode to determine the cyclic stress response and strain life. The steel showed predominantly cyclic softening behaviour at lower strain amplitudes (up to 0.25%). However, at higher strain amplitudes (& GE; 0.35%), cyclic hardening was observed with increase in the number of loading cycles. EBSD study done on the fatigue tested samples revealed grain fragmentation and texture weakening during cyclic deformation. TEM study elucidated the evolution of dislocation structure responsible for cyclic softening/hardening behaviour during cyclic deformation. At lower strain amplitude (0.2%), concurrent presence of random arrangement of dislocations and dislocation tangles were observed with some grains having weakly developed persistent slip bands (PSBs) and veins. At high strain amplitude (0.55%), subgrain structure was found to be the dominant feature along with occurrence of PSBs and veins at few locations. Further to this the role of nano-precipitates in influencing the fatigue behaviour was also investigated.