对内螺纹接头失效的钻铤进行了化学成分、力学性能和显微组织分析,并采用扫描电镜和X射线能谱仪对断口形貌及夹杂物成分进行了分析,找出了钻铤内螺纹接头失效的原因。结果表明：该钻铤内螺纹接头失效为早期疲劳失效；导致其疲劳失效的主要原因有钻铤内螺纹根部加工不光滑，使该处在交变载荷作用下应力集中严重,首先萌生微裂纹成为疲劳源；钻铤内螺纹型号和内外径尺寸不合标准要求以及材料中含有数量较多的MnS夹杂物也都大大降低了其疲劳性能，加速了疲劳裂纹的萌生和扩展。

The failure reason of the drill collar box was investigated by means of chemical compositions analysis, mechanical properties test, microstructure examination, SEM observation and EDS analysis. The results show that the failure mode of the drill collar box was early fatigue failure. The thread root of the drill collar box was not machined smoothly resulted in heavy stress concentration and initiation fatigue cracks under the effect of alternating stresses. The wrong internal thread type of the drill collar box and much MnS inclusions in the material both decreased the fatigue strength and fastened the expansion of the fatigue cracks.