Microstructure and mechanical properties of girth weld joint of X80 steel pipe

The microstructure and mechanical properties of X80 steel pipe girth weld joints at different welding positions were studied. The results show that there were no welding defects such as porosity, slag inclusion and lack of fusion in the welded joints of flat welding position, vertical welding position and overhead welding position. The microstructure of base metal was composed of granular bainite. The microstructure of weld seam was composed of intragranular nucleation acicular ferrite and a small amount of granular bainite and polygonal ferrite. The microstructure of coarse grain zone was composed of granular bainite, and the microstructure of fine grain zone was composed of polygonal ferrite and a small amount of martensite and austenite island structure. The tensile strength of the circumferential weld joint in the four quadrants was basically the same, and the value of yield strength and tensile strength of the full weld tensile specimens of flat welding position and overhead welding position were also very close. The average impact absorbed energy of vertical weld position and heat affected zone was lower than that of flat weld position and overhead weld position. The average hardness of vertical welding position seam and heat affected zone was higher than that of overhead welding position. The crack tip opening displacements of flat welding position, vertical welding position and overhead welding position weld were less than the heat affected zone, which were consistent with the impact test results.