Causes of cracking at the bottom of internal screwed teeth in drilling tool joint

The circumferential cracks appeared in a drilling tool joint after short-term use. The cracking causes were analyzed by chemical composition analysis, hardness test and metallographic examination. The results show that the crack of the drilling tool joint was fatigue crack, and the crack initiated at the bottom of the thread tooth profile closest to the internal thread joint and the rod body. Under the action of periodic fatigue load, the crack extended circumferentially along the bottom of the thread tooth profile, and at the same time extended outward, and finally the outer wall cracked.