Microstructure and properties of automatic welding process of X80 pipeline steel under extremely cold conditions

In order to study the effect of cold environment of −30 ℃ on the weldability of multi-layer and multi-pass welding of X80 pipeline steel, the microstructure, tensile property, microhardness and toughness of 22 mm thick welding joints were analyzed. The results show that the fusion zone is mainly composed of acicular ferrite and proeutectoid ferrite, and there are a large number of M/A particles in coarse grain zone. The microhardness distribution of welded joints is M-shaped, which is related to a large number of lath bainite ferrite. The average tensile strength of welded joints is 684 MPa, and it exhibits the typical characteristics of ductile fracture. The average impact absorption energy of fusion zone is 83 J. The cold environment improves the cooling rate and promotes the precipitation of acicular ferrite and M/A particles. Compared to normal environment, the tensile strength and microhardness of joints increase, but the toughness decreases dramatically. Meanwhile, it is easy to form porosity during welding in the cold environment.