Three specimens of X80 pipeline steel welded joint regions were in situ TEM tensile test by observing initiation and propagation of cracks in each region, the micro mechanical behaviors were studied and analyzed. The results show that, in the base metal zone, the crack initiation and extension are mainly composed of multiple crack, in the mode of intergranular extension, finally cause shear brittle fracture under the action of shear stress. In the heat affected zone, the main crack extension is in the direction of 45°, the whole expansion process including the main crack blunting, dislocation emission, formation of dislocation free zone in front of the main crack, microcrack nucleation, connection of main crack with microcracks, the multiscale process constantly occurrence, finally leads to quasi cleavage transgranular fracture. In the weld zone, cracks branch off from the main crack which extend to the direction of 45°, in the mixed fracture mode of intergranular and crystalline, finally lead to the transgranular brittle cleavage fracture.
