Microstructure and Impact Property of Welding Joints of Ultra Low-Carbon Micro-Alloyed Pipeline Steel

Using optical microscopy, impact test and scanning electron microscopy, the microstructure and toughness of welding joints of an ultra low-carbon micro-alloyed pipeline steel were studied. The results show that the microstructure of the ultra low-carbon micro-alloyed pipeline steel mainly consists of granular bainite and bainite ferrite. The coarse grained region of heat-affected zone is composed of bainite ferrite, granular bainite, a small amount of polygon ferrite and fine M-A island, and the grains are obviously coarse. The nucleation of acicular ferrites in weld line is induced by composite oxides containing Ti, Al, Si and Mn, and the acicular ferrites distribute as interlaced basket woven structure. The impact energy of the joints was 198 J at -10 ℃. And the fracture mode of the joints in impacting is ductile fracture.