Microstructure and toughness of heat-affected zone in girth welding of X80 steel pipe with different Nb content

In order to study the effect of Nb content on the microstructure and properties of heat affected zone (HAZ), the 0.055%Nb and 0.075%Nb content X80 steel pipes were girth welded by Gas metal arc welding (GMAW) and shielded metal arc welding (SMAW). Charpy impact test and metallographic analysis were used to study the impact toughness and microstructure differences in the HAZ. The effects of the microstructure of X80 pipe body with different Nb content on the properties of the heat-affected zone were analyzed by scanning electron microscope and confocal laser microscope with high temperature. The results show that at 0 ℃ and −20 ℃, 0.075%Nb and 0.055%Nb X80 steel pipes have high impact toughness in the HAZ of GMAW girth welded joints, and their average impact absorbed energy are higher than 150J. However, the Charpy impact absorbed energy in the HAZ of GMAW girth welded joint of X80 steel pipe with 0.055% Nb is higher than that of X80 steel pipe with 0.075%Nb. When GMAW is used, the ductile-brittle transition temperature (DBTT) of X80 girth welded joint with 0.055%Nb is lower than that of X80 with 0.075%Nb because of the low welding heat input. When SMAW is used, due to higher heat input, the coarse grain heat affected zone (CGHAZ) of X80 girth welding joint with 0.075%Nb has higher impact absorbed energy on the upper-shelf and lower temperature of upper-shelf zone and DBTT, and its low temperature toughness is better. It is also discussed that the impact toughness of the HAZ of X80 girth welded joint is not only related to the heat input and the shape, size and distribution of M-A, but also genetically influenced by the content of Nb in the pipe body and the original strength, toughness and microstructure state.