Microstructure characteristics and fatigue properties of welded HSLA with and without buffer layer

The microstructure variation and its subsequent influence on the fatigue behavior and Vickers hardness of an extensively weld-repaired high strength low alloy (HSLA) were investigated in this study. Microstructures of the as-received HSLA, or parent metal (PM), welded HSLA without buffer layer (BL), and welded HSLA with 4 or 10 mm BL between the PM and weld metal (WM) were examined and compared with the corresponding fatigue behavior, hardness measurement, and fatigue-fractured surface. It was found that the fatigue resistance and hardness in part of the WM and in the melted parent metal (MPM) during welding of the welded HSLA without BL were lower than those of PM. However, the welded HSLA with a 4-mm-BL produced a marked increase in fatigue resistance and hardness, especially within the WM, within the mixed area of BL and MPM (BL + MPM). This was because the WM and MPM of welded HSLA without BL had block ferrite and coarse columnar grain structures. Furthermore, no obvious differences were found in fatigue property, hardness and microstructure between the MPM of welded HSLA without BL and the BL + MPM of welded HSLA with 10-mm-BL. In addition, optical microscopy and SEM examinations of the corresponding regions revealed that block ferrite and non-metallic particles were the main reason of poor fatigue resistance and low hardness.