某超超临界锅炉水冷壁D层吹灰器密封盒角焊缝在运行约59 159 h后发生开裂并产生泄漏，采用宏观检查、化学成分检测、显微组织和断口形貌观察、硬度测试、能谱分析等方法分析了该角焊缝的开裂原因。结果表明：角焊缝发生了疲劳断裂。在运行过程中，角焊缝承受着较大的轴向应力，同时角焊缝的焊脚尺寸小于标准要求、焊趾处未圆滑过渡，导致轴向应力增大而萌生裂纹；在锅炉启停、负荷波动过程中，周期性变化的轴向应力促使裂纹扩展而形成疲劳开裂；由于D层温度最高，D层吹灰器密封盒角焊缝最先开裂。

In an ultra-supercritical boiler, the fillet weld between water wall and soot blower seal box cracked and leaked after 59 159 h service. Reason for cracking of the fillet weld was investigated by macroscopic inspection, chemical composition testing, microstructure and fracture morphology observation, hardness measurement and energy dispersive spectroscopy. The results show that the fillet weld failed by fatigue fracture. The fillet weld was subjected to relatively large axial stresses. The fillet weld leg size was smaller than the standard requirement and the weld toe transition was not smooth, leading to the increase of axial stress, and then cracks initiated. During the process of the boiler start-stop and load fluctuation, the periodical change of axial stresses caused propagation of cracks and formed fatigue cracks. The temperature of the D layer was the highest, resulting in the first cracking of the fillet weld of the D-layer soot blower seal box.