Performance and strain evolution of different strength matching girth welds of high Nb X80 pipeline

In order to investigate the fracture mechanism of pipeline girth weld, The microstructure and properties of high niobium X80 girth welds with different strength matching were tested and analyzed, and the strain behavior of the welded joints during tensile load was observed by digital image correlation (DIC) method. The results show that the FCAW-S girth welded joints of high Nb X80 pipe with low strength matching and high strength matching have good impact toughness, and the average Charpy impact absorption energy of the two joints is similar. The toughness dispersion in the heat-affected zone (HAZ) is mainly caused by the microstructure heterogeneity of coarse-grain zone. The carbon equivalent (CE_IIW)of high Nb X80 pipeline steel is low, and the HAZ of girth weld joint is softened to a certain extent. Under axial tensile load, regardless of low strength matching or high strength matching, strain concentration first appears in the root weld and heat affected zone of girth weld. With the increase of tensile load, the strain concentration of the high-strength matched girth weld joint gradually shifts from the root weld and HAZ to the base metal, and the ability of the pipe to bear axial load and deformation is greater than that of the low-strength matched girth weld joint. However, for low strength matching, even with good toughness, the girth welding joints will fracture due to the accumulation of plastic strain in the weld and HAZ.