A dislocation mechanics constitutive model for effects of welding-induced microstructural transformation on cyclic plasticity and low-cycle fatigue for X100Q bainitic steel

This paper presents a physically-based cyclic viscoplasticity model to capture the influence of welding-induced microstructural transformation on the fatigue response of the bainitic high-strength low-alloy steel, X100Q. The model incorporates the strengthening effects of dislocations, microstructural boundaries and precipitates, and the softening effects of microstructural degradation and early-life fatigue damage on yield strength and nonlinear cyclic-plastic response. The model is applied to predict the constitutive responses of X100Q parent material, physically-simulated intercritical heat affected zone (HAZ) and fine-grained HAZ, based on differences in bainitic hierarchical microstructure between the materials. A refined bainitic block structure is shown to be the primary microstructural feature contributing to monotonic and cyclic strength in the materials, whereas dislocation annihilation and the concomitant coarsening of the bainitic lath structure with cyclic-plastic deformation leads to cyclic softening behaviour.