Failure Analysis on 165.1 mm Drill Collar Box

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.