Microstructure and Impact Toughness of Heat Affected Zone of FB780 Steel Welded Joint with Different Welding Processes

The thermal cycle process of the heat affected zone (HAZ) of FB780 steel welded joint under single pass welding condition was simulated by the Gleeble-3500 thermomechanical simulator. The microstructures of HAZ at different welding peak temperatures and for different t_8/5 times (cooling time from 800℃ to 500℃) were studied, and the micro-hardness and impact toughness of HAZ under different welding conditions were tested. The results show that when the t_8/5 was the same, the microstructure of HAZ coarsened with the increase of peak temperature. When the peak temperature was constant, with the increase of t_8/5, the ferrite lath widened and gradually intergrated; the M/A component coarsened; the coarse ferrite grains were mixed with the fine quasi-polygonal ferrite grains; the metallographic homogeneity deteriorated. At the same peak temperature, the hardness and impact energy of HAZ decreased with the increase of t_8/5. At the peak temperature of 1 175℃ and t_8/5 of 6 s, the impact fracture morphology of HAZ at low temperature (-20℃) showed dimple pattern, and the dimple was relatively large; the toughness of HAZ at low temperature matched with that of the base metal; the hardness was relatively high; all indicated that the process was the best single pass welding process.