某飞机用1Cr18Ni9Ti不锈钢管在进行弯曲疲劳试验时焊接接头发生断裂。通过宏观分析、微观分析、金相检验等方法，对焊接接头的断裂原因进行了分析。结果表明：焊接结构不合理（焊接热影响区处于截面过渡位置），是导致该1Cr18Ni9Ti不锈钢管焊接接头疲劳寿命降低的主要原因；焊接区域存在未焊透情况，需要对焊接工艺进行优化。

The welded joint of 1Cr18Ni9Ti stainless steel pipe used in an aircraft broke during bending fatigue test. By means of macro analysis, micro analysis and metallographic examination, the fracture causes of the welded joint were analyzed. The results show that the unreasonable welded structure (the welded heat affected zone was in the transition position of the cross section) was the main reason for the decrease of fatigue life of the 1Cr18Ni9Ti stainless steel pipe welded joint. There was incanplete penetration in the welding area, so it is necessary to optimize the welding process.