用Gleeble-3500型热模拟试验机模拟单道次焊接条件下FB780钢焊接接头热影响区（HAZ）的热循环过程，研究了在不同焊接峰值温度及不同t_8/5（由800℃冷却到500℃的时间）下HAZ的显微组织，并测试了不同焊接条件下HAZ的显微硬度和冲击韧性。结果表明：在t_8/5相同的条件下，随着峰值温度的升高，HAZ组织粗化；当峰值温度一定时，随着t_8/5的增加，铁素体板条变宽并逐渐融合，M/A组元粗化，粗大铁素体晶粒与细小准多边形铁素体晶粒混合，组织的均匀性恶化；在相同的峰值温度条件下，HAZ的硬度和冲击功均随着t_8/5的增加而降低；在峰值温度为1 175℃，t_8/5为6 s条件下，HAZ的低温（-20℃）冲击断口形貌为韧窝花样，韧窝尺寸较大，HAZ的低温韧性与母材的相匹配，同时硬度较高，该工艺为最佳的单道次焊接工艺。

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.