Failure analysis of 22Si2MnCrNi2MoA drilling rods

The fracture of a 22Si2MnCrNi2MoA drilling rod near the internal thread was analyzed in term of the microstructure, inclusions and fractograph by means of optical microscopy, scanning electron microscopy and energy dispersive spectroscopy. Fatigue crack initiation and propagation in the drilling rod were studied. It is found that the fracture of the drilling rod is not induced by the abnormal microstructure and inclusions. Instead, because the wall thickness of the drilling rod near the internal thread is obviously uneven, the area with a thinner wall thickness tends to be the weak load-bearing point under the continuous stress of high frequency, serious outside consumption and internal friction existing simultaneously. Thus fatigue cracks are likely to emerge in the area preferentially. After that, the drilling rod ruptures in the area and eventually fractures. Furthermore, this fatigue failure is originated from the inside surface of the drilling rod and it belongs to the fatigue fracture with many crack sources. The morphology of the zones around the crack sources is dimple fracture, but the morphology of the zones of crack propagation is a mixed type with dimple and intergranular fracture.