Failure Analysis of Alloy Boron Steel Bolts in Steel Structure Assembly

The present work investigates the failure of three alloy boron steel A325M-8S type-1 grade bolts in steel structure racks. The bolts failure occurred almost 3 years after erection in three different locations within the period of approximately 4 months. The steel structure is subjected to mostly static loads with insignificant vibrations. The chemical composition and the mechanical properties of the failed bolts conform to the standard reference values. The bolts microstructure consists of tempered martensite, which is typically observed for heat-treated bolts. Visual inspection revealed the formation of a necking zone near the fracture area, with almost 30% area reduction at the fracture point. The fracture surface consisted of radial ridges forming a 45 degrees angle with the bolt axis. SEM examination revealed quasi-cleavage fracture mode which is often observed for high strength steels. Similar fracture surface morphology was observed after the tensile test of a new unused bolt from the same grade. It is accordingly suggested that the fracture happened due to tensile overloading during installation. The usage of lubricants during installation can facilitate such tensile overloading especially with insufficient quality measures to ensure the application of proper tightening torques. It is recommended to replace the failed bolts and ensure proper monitoring of the tightening torques during the installation.