核电主管道取样管焊缝疲劳开裂案例分析

Case Analysis of Fatigue Cracking on the Weld of Sampling Connection of Main Coolant Lines

目的针对某核电机组主管道取样管焊缝水压试验后渗透检测时发现的超标线性显示,分析失效原因,探讨制造期间不常见的疲劳裂纹形成及扩展的机制,分析导致疲劳的交变应力源所在。方法通过宏观分析、金相观察、断口扫描电镜试验。结果材质未见异常,该线性显示为裂纹所致,是疲劳裂纹。结论取样管焊缝焊趾部位多源裂纹不是水压试验所产生的,为水压试验后机械加工去除堵头时,因切削不良使取样管侧焊趾附近承载的交变载荷过量引起的疲劳开裂;疲劳应力主要来自水压试验后取样管堵头切除加工工序,铣削不良是引发疲劳裂纹的关键因素。

Objective According to unacceptable linear indications by liquid penetrant examination on the weld of sampling connection of main coolant lines after hydrostatic test during manufacturing, the failure causes were analyzed, the formation and propagation mechanism for rarely seen fatigue cracks during manufacturing was explored, and the alternating stress source of fatigue was discussed. Methods In the paper, macroscopic analysis, optical microscope and scanning electron microscope were used. Results The results showed that the material quality was acceptable and the linear indication was caused by fatigue cracks. Conclusion The multi-source crack in the weld toe of the sampling pipe was not caused by hydrostatic test, but was fatigue cracking resulted from the excessive alternating current load near the weld toe of the sampling pipe caused by improper cutting during the removal of the plug via mechanical machining after the hydrostatic test. The fatigue stress came from the cutting process of plug after the hydrostatic test. Unreasonable milling was the key factor leading to fatigue crack.