Fracture mechanism and toughness of the welding heat-affected zone in structural steel under static and dynamic loading

Due to the influence of the welding thermal cycle, the toughness of structural steel generally degenerates. Recently, the intercritically reheated coarse-grained heat-affected zone (IC CG HAZ) was found to demonstrate the worst toughness in welded joint, which was associated with its fracture mechanism. In this article, two IC CG HAZs of a structural steel were prepared by welding thermal-cycle simulation techniques. For the two IC CG HAZs, the static and dynamic fracture toughness were evaluated; the fracture mechanism was also studied. Under both static and dynamic loading, cracks in the IC CG HAZ were found to initiate at the intersection of bainitic ferrite α _B ^/0 packets with different orientations, followed by propagation in cleavage. In some crack propagation regions, adjacent cleavage facets are connected by shear, thus producing dimple zones. Though the brittle fracture initiation mechanism remains unchanged, the cleavage facet size, the proportion of the dimple zones between facets, and the distance from the cracking initiation site to the crack tip vary with loading speed and welding conditions. These changes were found to be related to the variations caused by strain rate and welding conditions in fracture toughness for the IC CG HAZs.