某电厂T91钢主蒸汽压力取样管的焊接接头在运行过程中发生开裂。采用宏观观察、化学成分分析、硬度测试、金相检验以及管系检查等方法，对该焊接接头的开裂原因进行了分析。结果表明：该焊接接头开裂符合冷裂纹（延迟裂纹）的特征，其形成原因为焊接接头中存在未回火的马氏体，使管子膨胀受阻，焊接残余应力未消除，产生了应力集中。建议采用合适的工艺措施，合理安排支吊架的状态，以减少焊接接头的应力集中。

The welding joint of main steam pressure sampling tube of T91 steel in a power plant cracked during operation. The cracking reasons of the welded joint were analyzed by macroscopic observation, chemical composition analysis, hardness test, metallographic examination and pipe inspection. The results show that the cracking of the welded joint conformed to the characteristics of cold crack (delayed crack), the formation reason was that there was untempered martensite in the welded joint, the expansion of the pipe was hindered, and the welding residual stress was not eliminated, resulting in stress concentration. It was suggested that appropriate process measures and arrange the state of support and hanger reasonably to reduce the stress concentration of welded joints.