Failure analysis of cracking on side tooth gingivae of a percussive drill bit

Failures of percussive drill bits have been attributed to the external mechanical force and structural design by engineers and researchers for a long time. Few attentions have been focused on the material itself. Generally, it is the unqualified material, induced by the improper metallurgy and heat treatment process, which makes the drill bits fail soon after they are put into service. Reasons of the crackings on side tooth gingivae of a drill bit, which failed prematurely at the beginning of the service, were analyzed comparatively. Scanning electron microscope, metallographic observation, mechanical performance test and chemical composition analysis were adopted for the failure analysis. Results show that microstructure, chemical composition and mechanic properties of the substrate were excluded from the reasons of the cracking. It is mainly the excessive inclusions and defects in the carburized layer which induced the inhomogeneity of material organization and performance. Those inclusions and defects act as stress concentrators and leading to the crack initiation. In addition, toughness of the carburized layer is also decreased due to its over-carburization. It makescracks more easily initiate from the carburized layer and then propagate into the substrate under the stress of interference fit.