Effect of Cu-Ni components in X80 pipeline and heat input on discretization of toughness in CGHAZ

In recent years, the strength and toughness of pipeline steels were considered by aiming for setting alloy and branching out thickness. However, welding would worsen its microstructure and toughness in coarse grain heat affect zone (CGHAZ) on microalloyed pipeline steel with much lower cost, especially. In this paper, the microallyed X80 pipeline steels were chosen, and single welding thermal-cycles to simulating two-wires longitudinal submerged arc welding (SAW) were carried out by Gleeble-1 500 to study the correlation of microstructure and toughness in simulated CGHAZ. The values of toughness of the simulated CGHAZ in different Cu-Ni components and dissimilar welding heat input were tested, and corresponding evolution characteristics of microstructure in the same area were investigated by OM, SEM, TEM and EBSD. The results showed that the average grain size in simulated CGHAZ changed indistinctively with the decrease of austenite-stability alloys, but the discretization of grain size in CGHAZ increased. Moreover, the transition temperature from prior austenite to bainite was decreased. At the same time, the cementite was precipitated between grain boundaries during cooling process. Otherwise, the misorientation between granular bainite grains was small, i.e., it would decrease the density of high angle boundary (≥15°) in case of much lower Cu-Ni components. Furthermore, the shape of M/A has been changed from block-like structure to strip-like one during the decrease of austenite-stability alloys. Based on the mentioned results above, it has come true that discretization of toughness in stimulated CGHAZ of the X80 pipeline steel was increased. Oppositely, the discretization of grain size in stimulated CGHAZ decreased and the density of high angle boundary (≥15°) increased while the welding heat input was declining, which made the stimulated CGHAZ toughness of X80 pipeline steels stabilized.