Microstructure and toughness of coarse grain heat-affected zone for Nb-microalloyed X80 pipeline steel

Based on welding thermal simulation on Nb-microaUoyed XSO pipeline stee! using Gleeble-3500 thermal simulation equipment, microstlttcture and impact toughness in coarse grain heat-affected zone （CGHAZ） under different welding parameters were investigated in this paper. The results show that high heat inputs with low preheats or low heat inputs with high preheats should be applied to achieve high impact toughness. Coarse original austenite grains may lower impact toughness. CGHAZ microstructure is mostly composed of upper bainite, granular bainite and lath bainite. The phase composition of microstructure and the quantity, size, shape of M/A constituents both have effects on impact toughness.