Effect of Heat Input on Microstructure and Impact Toughness of Coarse Grain Heat Affected Zone of Welded Joint of X120 Pipeline Steel

The welding thermal simulation tests of X120 pipeline steel were conducted with a Gleeble-3800 type thermal simulator, and the microstructure and impact toughness of the coarse grain heat-affected zone (CGHAZ) of welded joint of X120 pipeline steel under different heat input conditions were investigated. The results show that when the heat input was 10 kJ·cm^-1, the microstructure of CGHAZ was mainly composed of bainite and a small amount of low-carbon martensite with small-angle grain boundaries. As the heat input was 20 kJ·cm^-1, the microstructure of CGHAZ consisted of fine lath bainite with large-angle grain boundaries. With the increase of heat input, the microstructure of CGHAZ was composed of coarse bainite and the proportion of large-angle grain boundaries decreased. The impact toughness of CGHAZ increased first and then decreased with the heat input increasing from 10 kJ·cm^-1 to 40 kJ·cm^-1. When the heat input was 20 kJ·cm^-1, the impact toughness was the best and the fracture mode was ductile fracture. The combination fracture with ductile fracture and brittle cleavage fracture occurred when the heat input was 30 kJ·cm^-1 and the brittle cleavage fracture occurred when the heat input was 10, 40 kJ·cm^-1.