The cyclic stress–strain behaviour of two microalloyed steels with different microstructures has
been characterised at room temperature under strain controlled low cycle fatigue. The cyclic
stress–strain curve in the double logarithmic plot shows a linear relation for both steels. A
transition of the cyclic stress–strain curve from softening to hardening with increasing strain
amplitude has been observed with respect to the corresponding tensile curve. The strain
amplitude for the onset of cyclic softening to hardening transition has been found to be
dependent on grain size. The strain lifetime behaviour, estimated from modified universal slopes
equation, shows similar trends as Nb or V bearing microalloyed steels. The cyclic characteristics
of the two microalloyed steels have been compared with corresponding predeformed state
carried out under stress controlled conditions. While, cyclic saturation was observed in case
where the extent of predeformation was within the Lüders strain, cyclic softening occurred when
it exceeded the Lüders strain. It has been attempted to provide a mechanistic understanding
of the differences in the cyclic behaviour of the two steels owing to the microstructure and
predeformation.
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