Microstructural and Mechanical Properties of X80 Girth-Welded Joints under Different High-Arc-Energy Welding Conditions

High-arc-energy welding undoubtedly increases the tendency for both local softening and embrittlement to occur in the coarse-grained heat-affected zone (CGHAZ) of line pipe girth-welded joints. In this paper, the CGHAZs of X80 welded joints with series of arc energies of 20.3 kJ/cm, 29.9 kJ/cm and 40.2 kJ/cm were prepared by automatic submerged arc welding (SAW), and the correlations between the microstructural evolution and mechanical properties, i.e., strength, hardness and impact toughness, were systematically investigated. The results showed that the microstructure of the CGHAZ was composed of lath bainite and granular bainite (GB), the arc energy increased from 20 to 40 kJ/cm, the area fraction of the GB increased from 17.3 to 37.4%, the grain size increased from 39.8 to 79.0 μm, and the high-area fraction of the GB decreased the number fractions of high-angle grain boundaries (HAGBs). In addition, the M-A constituents obviously coarsened as the arc energy increased, the proportion of island M-A constituents decreased from 85.2 to 76.2%, and the amounts of massive M-A constituents increased from 0.4 to 15.9%. The mechanical properties showed that obvious softening and embrittlement occurred in the CGHAZ, the joint strength decreased from 708 to 602.5 MPa, and the impact energy decreased from 218 to 92 J. The significant increases in the grain size and area fraction of the GB were the fundamental reasons for the softening of the joint at high arc energy, while the significant decrease in the number fraction of the HAGB and the substantial increase in the content of coarse-stringer and massive M-A constituents seriously deteriorated the impact toughness of the CGHAZ.