Cyclic hardening and softening behaviors in SS400 structural steel weld joints subjected to fatigue loading

This study aims to explore the cyclic hardening and softening behavior of the heat-affected zone (HAZ) and weld metal (WM) in the SS400 weld joint by performing a series of experiments including low-cycle fatigue, nanoindentation tests, and optical microscope examinations. The experimental results indicate that the mechanical properties of the HAZ show a hardening behavior. Both yield stress and hardness of the HAZ increased with the further increase of loading rate and fatigue condition. Furthermore, the influences of fatigue conditions on the mechanical properties of HAZ were stronger at a low loading rate and became less pronounced when the loading rate increased. The hardening behavior of hardness of HAZ under loading rate and fatigue conditions was mathematically described using a new equation as \(\:H=\left[57.47{\dot{\epsilon\:}}^{m}-0.184\right]\left[-0.729{\epsilon\:}_{a}+42.811\right]\) . The impacts of the fatigue conditions on the mechanical properties of the WM region were very different compared with the HAZ. The experimental results show the cyclic softening behavior, whereas \(\:H\) and yield stress tended to decrease when the fatigue condition increased. A new mathematical equation was introduced to describe the softening behavior of hardness of the WM as \(\:H=\left[54.554{\dot{\epsilon\:}}^{m}-0.535\right]\left[0.498{\epsilon\:}_{a}+45.315\right]\) . The relationship between strain rate sensitivity index and fatigue conditions was also investigated and described using the proposed equation. The findings from this study provide insights for further understanding the dynamic plasticity of constituent phases in SS400 structural steel weld joints subjected to strain amplitude loads.