Variations in DBTT and CTOD within weld heat-affected zone of API X65 pipeline steel

The fracture resistance of heat-affected zones (HAZs) in girth welded joint of API X65 steel pipeline was systematically investigated. While the change in Charpy impact energy has been typically evaluated in previous studies, here the variations in ductile-to-brittle temperature (DBTT) and crack-tip opening displacement (CTOD) within HAZ were explored. A series of experiments revealed that both values vary dramatically (i.e., DBTT increases and CTOD decreases) as the location approaches the fusion line (FL) and thus the region adjacent to FL exhibited the lowest CTOD and highest DBTT, possibly due to the increasing portion of coarse-grained HAZ. Interestingly, however, even the FL regions still showed moderate toughness at −40 °C ∼ room temperature. Microstructural analysis and additional impact tests using simulated HAZ specimens suggested a possibility that fine-grained HAZs with higher toughness may suppress the brittle fracture from neighboring coarse-grained region.