Mechanisms of cyclic softening and cyclic creep in low carbon steel

Abstract

In order to understand better the mechanism of cyclic softening, a low carbon annealed steel has been first prestrained and then cycled in load control with zero mean stress. Both mechanical results and transmission electron microscopy studies of the defect structures were explored. Cyclic softening and creep were observed to occur in parallel. A new softening parameter, based on the changes in plastic strain amplitude with cycling and designed to discount any superposed cyclic hardening, was defined to describe the softening process and its kinetics. A softening threshold stress was discovered by extrapolating softening results of high load amplitudes to zero softening and was found to be 0.44 of the flow stress of the prestrained metal. This is suggested to be a constant for wavy slip metal and is related to the fatigue limit. The changes in defect structure with softening consisted of (1) reductions in the dislocation content of cell interiors and cell walls, with a consequent sharpening of the walls, (2) changes in the diffuseness of the initial cells into elongated cells or checkerboard cells, without change in cell size, and (3) enlargement of the cell size and development of typical fatigue cells as saturation was attained. The cyclic creep behavior is interpreted by the Bauschinger effect and by back stresses from dislocation clusters having excess dislocations of similar sign. The softening behavior is associated with mutual annihilation of dislocations paired with respect to sign, and rearrangements of the residual dislocations into low energy structures.