通过宏观形貌检查、化学成分分析、力学性能测试、显微组织观察和断口形貌分析等方法，探讨了某电厂2号锅炉过渡段水冷壁15CrMoG钢连接管在安装阶段发生环向开裂的原因。结果表明：对口焊接后，鳍片尾部角焊缝未焊透处存在很大的应力集中，同时该处出现了马氏体+贝氏体的淬硬组织，显微硬度非常高，在管系拘束应力、焊接残余应力的作用下，裂纹在此萌生；该部位母材的硬度大、塑性差，在横向拉应力作用下裂纹沿管子环向扩展，从而导致管子发生环向脆性开裂。

The circumferential cracking reason of 15CrMoG steel connecting tube of transition-section water wall during installation stage had been analyzed by macroscopic examination, chemical composition analysis, mechanical property testing, microstructure observation and fracture appearance analysis. The results show that the great stress concentration occurred in incomplete penetration of fillet weld at tail of fin after butt-weld, microstructure contained martensite+bainite hardening structure and hardness was extremely high. The pipeline restraint stress and welding residual stress caused the crack initiation from this location. Base metal of the area had high hardness and low plasticity and the crack expanded along the tube circumference under the transverse tension stress, which resulted in the brittle cracking of tube at the circumferential direction.