Mechanism of suppressing HAZ softening in friction stir welded martensitic steel through hardening phenomenon initiated by adding vanadium

In this study, heat-affected zone (HAZ) softening occurred in friction stir welding (FSW) joints of martensitic steels is suppressed through the short-time secondary hardening phenomenon initiated by adding V. Adding V significantly suppresses the HAZ softening owing to the precipitation of the extremely fine and high-density V carbides in HAZ. Vickers hardness (HV) profiles of the FSW joints were measured to evaluate the degree of HAZ softening. The degree of inhibition in HAZ softening was found to be related to the amount of V present and the Ac1 temperature of the steels, which can be controlled by the quantity of Mn. These relationships were elucidated quantitatively via transmission electron microscope (TEM) and 3D atom probe-tomography (3DAPT) together with hardening theory after investigating multiple areas of the V carbides in FSW joints containing different V and Mn contents.