Metallurgical Failure Investigation of Quench Cracking in Hexagonal Bolt Head Fasteners

A particular engine user experienced the repeated fracture of new fixing bolts in combustion chambers upon first tensioning. The heads of the bolts fractured from their shafts upon torque-controlled tensioning. Two affected screws were received from site by the original equipment manufacturer's (OEM) laboratory to perform a metallurgical failure investigation and to determine the metallurgical root cause of failure. From the findings of the failure investigation described in this paper it is concluded that the metallurgical cause of failure in both cases was quench cracking, i. e. the imperfections leading to failure were induced upon manufacture and pre-existed in the bolts, i. e. they did not develop in service. These pre-existing cracks eventually caused failure by torsional overload upon first tensioning in assembly.