某核电厂高压缸抽汽管线疏水管道气动调节阀后的弯头焊缝发生泄漏，通过宏观形貌和显微组织观察、断口形貌分析、振动与应变测试等方法对焊缝进行了失效分析。结果表明：气动调节阀的间歇性开启与关闭使焊缝产生较大的交变应力，应力超过焊缝的疲劳强度，导致疲劳开裂，焊缝中的残余拉应力促进了裂纹扩展；在气动调节阀回路上外加一条疏水管道旁路，将间歇性疏水改为连续疏水，可有效避免此类失效的发生。

The leakage of the weld of an elbow in the sequence after pneumatic control valve of drain pipe of high pressure cylinder extraction line occurred in a nuclear power plant. The failure analysis of the weld was carried out by means of the macroscopic morphology and microstructure observation, fracture morphology analysis, and vibration and strain measurement. The results show that the intermittent opening and closing of the pneumatic control valve resulted in the large alternating stresses in the weld, which was greater than the fatigue strength and led to fatigue cracking. The residual tensile stress existing in the weld promoted the extension of the cracks. Turning the intermittent drain into continuous drain by adding one bypass drain pipe on the pneumatic control valve return road could avoid the occurrence of such failure effectively.